SOME PHYSIOLOGICAL STUDIES OF THE PHOTOSYNTHETIC
AND DARK METABOLISM OF PURPLE SULFUR BACTERIA
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy In the Graduate School of the Ohio State University
by
JOHN JACOB TAYLOR, B, Sc., M. Sc.
The Ohio State University 1957
Approved byi
-4 ^ {■ ■/[ CL-t'-H.'&C Adviser Department of Bacteriology ACKNOWLEDGMENT
I wish to express my appreciation and gratitude to
Dr. Chester I. Randles for his assistance and timely guidance throughout this investigation.
ii TABLE OP CONTENTS
INTRODUCTION ...... 1
LITERATURE R E V I E W ...... 4
MATERIALS AND METHODS
O r g a n i s m s ...... 11
Medium ...... oil
Cultures ...... 12
Sulfate Determination * ...... 14
Patty Acid Determination ...... 14
Determination of Sulfhydryl Compounds ..... 15
Determination of Carbon Dioxide Uptake .... 16
RESULTS
Characterization Studies:
Oxidation of Intracellular sulfur .... 19
Nitrogen-fixing cultures ...... 23
Fatty acid analysis of filtrates ..... 23
Growth on sulfur compounds ...... 24
Carbon Dioxide Fixation (Illuminated) ..... 27
Carbon Dioxide Fixation (Non-Illuminated) . . . 33
DISCUSSION ...... 38
SUMMARY ...... 59
HI BLIOGRAPHY ...... 63
iii INTRODUCTION
With the definition and elaboration of new methods
and techniques for studying the physiology of micro
organisms hes come an increasingly detailed characteri
zation of the pnotosynthetic process. Before the begin
ning of the pest decade, investigations of the photosyn
thetic mechanisms had been more vigorous and perhaps
more fruitful for the plant physiologist and the algolo-
gist then for the bacterial physiologist.
The plant physiologist hes at his disposal a clearly
defined, photochemically active, and at present conveni
ently isolated chloroplast which he may remove from the
green plant cell and study quite independently of other
cell constituents. The source of these chloroplasts is,
with the exception of the fungi, as widespread and as
diversified as the plant kingdom. But despite this
diversity of source, all these chloroplasts contain
identical photosynthetic pigments which are responsible
for identical photo-autotrophic processes.
On the contrary, the bacterial physiologist hes no
such convenient means of isolating in an active form
the chlorophylls from the photosynthetic bacteria since chloroplasts do not occur in these organisms. In addi-
1 tlon, all those bacteria known to possess a photosynthet
ic mechanism may be classified, according to our present
knowledge, in less than a dozen genera. Yet in this
limited classification are included such divergent types
of organisms as: strict autotrophs which must be cul
tured anaerobically in the light; heterotrophs which may be cultured aerobically in the dark; and organisms with requirements between these two apparent extremes. Indeed,
the photosynthetic bacteria may well provide a broad
spectrum of assimilatory activities from the strict auto trophy of the green sulfur bacteria to the obligate het erotrophy found in the non-sulfur purple bacteria which, after repeated culture in the dark, have temporarily lost their ability to photosynthesize.
Intermediate between the green and the non-sulfur purple bacteria are the purple sulfur bacteria. These organisms assimilate carbon dioxide in the light at the expense either of Incompletely oxidized inorganic sulfur
compounds, or of certain simple organic acids. Their tolerance of oxygen varies among strains, although most are strict anaerobes. There has never been an unequiv ocal demonstration of growth In the dark, regardless of the composition of the medium or of the atmosphere above it.
The non-sulfur purple bacteria, which are more readily isolated, grown, and retained in pure culture LITERATURE REVIEW
Since the time of Lankester (1873) and Winogradsky
(1889), accounts of the photosynthetic bacteria, and
especially the purple bacteria, have contributed to
conflicting ideas of the taxonomy and physiology of this group of organisms. The taxonomic status of the photo synthetic purple sulfur bacteria during the first quar
ter of the twentieth century was based primarily on some what dubious reports of observations made of organisms morphologically different from those already recorded In the literature at the time. Many of the early descrip tions were the results of the chance appearance of an apparently new genus or new species in a sample of sul- fide-rieh lake water. Pure cultures of the organisms were often difficult if not impossible to prepare, and when success did attend the culture, the conditions, results, and the nature of the organisms obtained often varied considerably among investigators.
The photosynthetic bacteria were even less clearly characterized physiologically. They were known to occur frequently, but not always, in environments in which sul fides were present. While some investigators were work ing with what appeared to them to be aerobic, autotrophic bacteria which evolved molecular oxygen during photosyn thesis, other workers studied anaerobic, heterotrophic
4 than the purple sulfur bacteria, have contributed much to our knowledge of bacterial photosynthesis. Eut their requirements for organic substrates and growth factors have made studies of their photosyntbetic metabolism somewhat tedious, even though the use of radioactive iso topes has gone far in overcoming the difficulties encoun tered in detecting the primary stages of carbon dioxide assimilation.
In contrast, the purple sulfur bacteria will grow on strictly inorganic media. This ability has inclined many investigators to suggest these organisms as the more suitable subject for. a study of bacterial photosynthesis.
Ebt during the past ten years, the purple sulfur bacteria have been largely neglected in such studies.
The present investigation was initiated to deter mine the possible roles of various organic sulfur com pounds in the photosynthetic metabolism of the purple sulfur bacteria. The initial results obtained with sodium thioglycollate indicated that further work should be done with this compound alone. Therefore, an Inten sive study of the effects of thioglycollate on both the photosynthetic and the dark metabolism was undertaken. 5
bacteria which did not liberate molecular oxygen during
illumination.
This was the somewhat formidable view of the photo
synthetic bacteria at the time that van J^iel (1931) pub
lished his extensive review and monograph of the photo
synthetic purple and green sulfur bacteria. Through his investigations and observations, van Niel qualified the
contrasting reports of earlier workers by clearly delin
eating the sulfur free the non-sulfur purple bacteria,
and by defining the former group morphologically and physiologically.
Three groups of purple sulfur bacteria were recog nized on the basis of morphology: the large cylindrical to ellipsoidal Chromatium-type. the spherical ThlocystiS' type, and the small bsciliary Pseudomonas-type. All types were either motile or non-motile, and all but the last type contained sulfur droplets within the cells if grown on media containing sulfide. The sizes and shapes of the three types varied considerably if the pH and/or sulfide concentration of the culture medium differed from those concentrations providing optimum growth at
25 C. In fact, the Pseudomonas-type appeared as the typical small bacillus, as a spiral of one-half to one turn, or as a coccus about one micron In diameter, each form depending on the pH and the amount of sulfide present. Through an anaerobic light-dependent process, the purple sulfur bacteria were shown to be capable of oxi dizing sulfide with concomitant incorporation of carbon dioxide into cellular substance according to the follow ing equations:
HgS ♦ 2 H20 ♦ 2 C02 --*-2 (CHgO) ♦ H2S04 (1)
Sulfur is available for storage as droplets following an incomplete oxidation of sulfide sulfur:
2 H g S ♦ C02 --- *(CH20) ♦ H20 ♦ 2 S (2)
When the sulfide was depleted from the medium in which the cells were grown, intracellular sulfur droplets were further oxidized to sulfate: (3)
2 S ♦ 8 H 20 ♦ 3 C G 2 -- * 3 (CHgO) + 3 HgO ♦ 2 H 2S 0 4
Van Niel (1931) suggested the following scheme for the complete oxidation of sulfide:
\ H SO,
Although he found that sulfite and thiosulfate substi tuted for sulfide in the culture medium, sulfur droplets could never be found in cells grown under these condi tions. Neither was he able to demonstrate in his cul tures any of the Intermediates postulated in the above scheme. 7
In 1933, Muller reported that he was ahle to cul
ture both the Chromatium- and the Thlocystls-type on
acetic, propionic, lactic, pyruvic, succinic, fumaric,
and malic acids anaerobically in the light. The Thio-
cyatls-type also grew slightly on butyric acid, and a
Chromatium-type "did not grow on glucose very well."
The results of the carbon balance studies with these
acids indicated that all of the added substrate was con verted into cell substance and a very small amount of
carbon dioxide, an observation which seemingly contra
dicted the usual anaerobic metabolism Involving an
Incorporation of only about ten per cent of the sub strate into cell material.
Van Kiel (1935) concluded that the simple organic acids which permitted assimilation of carbon dioxide in the light were completely dehydrogenated to water and carbon dioxide, just as sulfide, sulfur, sulfite, and thiosulfate were completely dehydrogenated to sulfate.
He also added that intermediates in the oxidation of these organic acids may have been directly assimilated by the photosynthesizing cells. The presence of more or less highly reduced materials, such as pigments, within the cells during illumination indicated to him the pos sible occurrence of hydrogen acceptors other than carbon dioxide in the photosynthetic process.
In a more recent review of bacterial photosynthesis, van Niel (1943) states that the transfer of hydrogen to
carbon dioxide is the only light-dependent reaction.
In the dark, or In the absence of an oxidizable substrate
in the light, carbon dioxide reduction does not occur.
But studies with algae indicate that the only light-
dependent reaction is a splitting of the water molecule
Into hydroxyl and "activated" hydrogen, the latter then functioning in the attachment of carbon dioxide to an
acceptor. That the reducing effect of the "activated" hydrogen may not be light-dependent has been indicated by the demonstration that illuminated green algae con
tinue to fix carbon dioxide after Illumination has
ceased. (For reviews see Gaffron, 1954, and Bassham and Calvin, 1956.)
Gaffron (1934) demonstrated the production of car bon dioxide, hydrogen sulfide, and organic acids in cultures of purple sulfur bacteria stored in the dark.
He concluded that sulfate, accumulating in the light, was reduced to sulfide while endogenous materials were oxidized. The sulfide, carbon dioxide, and organic acids could, in turn, be used In the photosynthetic process when the cells were subsequently illuminated.
However, Roelofsen (van Niel, 1935) found that the cul tures used by Gaffron contained sulfate-reducing bac teria and that pure cultures of purple sulfur bacteria in the dark failed to indicate a disappearance of sulfate from the medium, an observation later confirmed
by van Niel (1941).
Hendley (1955) suggested that the conflicting data
arose from the differences in the ages of the cultures
studied by the earlier investigators. Hydrogen sulfide
was found to increase in cultures stored in the dark,
but only during the initial 24 to 36 hours, and only if
the cells contained sulfur granules. Cells lacking sul
fur produced molecular hydrogen in the dark. Hendley
concluded, therefore, that hydrogen sulfide was pro
duced as a result of the concomitant dehydrogenation
of endogenous materials (perhaps pyruvate or malate)
and reduction of stored sulfur. Thus, when sulfur was not present to function as an acceptor, the hydrogen was evolved in molecular form from the cells. This process may also explain the evolution of molecular hydrogen during the anaerobic decomposition of formate, glucose, pyruvate, glycerol, and glycerophosphate by
Chromatlurn minutisslmum in the dark (Nakamura, 1939).
However, van Niel (1941) questions whether the
"normal" dark metabolism of the purple sulfur bacteria is really fermentative rather than oxidative, with the tolerance of oxygen being the limiting factor in the growth of these normally anaerobic organisms. 10
Prom tha various attempts to grow the purple sul fur bacteria on a variety of media in the dark, no clear-cut results have been obtained (van Niel, 1931), although a few reports have claimed successful dark cultures (see van Niel, 1941). MATERIALS AND METHODS
Organisms - Tha organisms were recovered from
samples of mud and water taken from Miller’s Blue Hole,
an artesian spring-fed pond in Erie County, near Cas-
talia, Ohio, and from several ponds or streams on South,
Middle, and North Bass and Pelee Islands in Lake Erie,
All the organisms were classified as purple sulfur
bacteria by their color, their ability to grow only
when illuminated, and their storage of intracellular
sulfur granules when grown on media containing sulfide.
They varied in morphology from nearly spherical to quite
elongated bacilli from 2 to 5 microns wide by 5 to 15
microns long. The elongated forms were usually motile
while ellipsoidal cells apparently lacked motility.
The characteristic red pigment was often visible micro
scopically within the larger forms when young cells
were examined.
Since van Niel (1931) described the morphology of
the purple sulfur bacteria under varying conditions of pH and sulfide concentration, it was possible to clas
sify the organisms used in the present studies as the
Chromatium-type.
Medium - The medium used was that of Larsen (1952) and Newton and Wilson (1953) and was of the following composition:
11 12
n h 4c i 0.1 per cent KHgP04 . HgO 0.1 !l it Na2S • 9 H20 0.1 II 11 MgClg • 6 H20 0.05 II tf NaHC03 0.2 If ft Agar (Difco) 1.5 If W
The first three salts were dissolved in lake water or
In tap water, agar was added when a solid medium was
desired, and the preparation was autoclaved at fifteen pounds pressure for twenty minutes. After the medium
had been allowed to cool to about 55 C, sodium sulfide
and sodium bicarbonate solutions, which had been ster
ilized separately by filtration, were added aseptically.
The final pH was about 7.4 and no further adjustments were made. Since the medium could not be heated after the addition of sulfide and bicarbonate, all Inocula tions were made before the agar was allowed to solidify,
When used, sodium thiosulfate (NagSsOj * 5 H2O) was Incorporated into the medium at a concentration of
0.1 per cent. Organic sulfur compounds were added In amounts which contained sulfur equivalent to one gram of sodium sulfide per liter (Table 1). Unless other wise indicated, only one sulfur compound, organic or
Inorganic, was present in a single culture medium.
Cultures - Pure cultures were prepared according to the procedures used by van Niel (1951). The cooled 13
liquid agar medium was inoculated with mud or water to
a concentration of about 10 ml per 100 ml of medium.
Prescription bottles having a capacity of one to four
ounces were completely filled with this mixture, sealed
tightly with plastic screw caps, and placed about two
feet beneath a 100-watt incandescent bulb in a reflector
desk lamp* The incubation time varied from three or
four days to two weeks, after which time bottles showing
no typical red colonies were discarded.
When colonies of the purple sulfur bacteria did
develop, they were withdrawn with a capillary pipette
from the bottle and mixed with about 10 ml of liquid
cooled agar medium. The mixture was drawn into a piece
of glass tubing twelve to eighteen inches long and
allowed to solidify. Both ends of the glass tube were
sealed with a one-inch layer of vaseline before the
tube was illuminated.
Colonies that appeared in these cultures were
easily removed by scratching the glass tubing with a
file at a point near the colony, breaking the glass,
and picking out the colony with a sterile needle. The
cells thus obtained were used to inoculate a second
bottle culture and the isolation procedures were re peated. Usually the colonies growing out in the second glass tube culture could be picked free from contami nants and were used to prepare stock cultures. 14
Cultures for chromatographic examination or for
reapirometric determinations were prepared in media
from which agar had been omitted.
Attempts to isolate the purple sulfur bacteria
from illuminated streak plateB under an atmosphere of
five per cent carbon dioxide in nitrogen were unsuc
cessful.
Sulfate Determination - A one per cent solution of
barium chloride in 0.01 N HC1 was used for the determin
ation of sulfate. Five ml of the reagent were added to
an equal volume of the solution to be tested in a tube.
The tube was immersed in a boiling water bath for one minute then removed and allowed to cool to room temper
ature. The optical density of the precipitated barium
sulfate was determined at 620 millimicrons with a Beck man Model 3JJ Spectrophotometer. This value was compared with optical densities obtained with known solutions of sulfuric a$id from which sulfate had been similarly pre cipitated as the barium salt.
Fatty Acid Determination - After the cells had been harvested by centrifugation from liquid sulfide and thio- sulfate cultures, the filtrates were placed in a steam distillation apparatus. A solution containing 25 micro moles per ml of each of the to Cg fatty acids was treated in the same manner as the filtrates and served as a control. After the distillation had begun, the pH of the culture filtrate or control was lowered to
about 2.0 by the addition of sulfuric acid to the dis
tillation flask. When the distillate amounted to about
one-fourth of the original volume of the filtrate, the
steam flow was stopped and the distillate removed. The method of Block, Durrum and Zweig (1955) was used to prepare methyl esters and hydroxamates from the distil
late and the hydroxamate solutions were spotted on
Whatman number one filter paper. Amyl alcohol-formic acid-water (75:25:75 v/v) was used as the developer and five per cent ferric chloride in methanol-acetone (4:3 v/v) as the indicator for the C-^ to Cg fatty acids
(Feigl, 1943).
Determination of Sulfhydryl Compounds - Compounds containing the sulfhydryl radical were determined by the method of Kimball, Kramer and Reid as modified by
Siggia (1949), One or two ml of 0.01 N iodine solution were added to one ml of the sample to be tested and the excess iodine determined by titration with 0.01 N sodium thiosulfate. Sulfide was determined in ppm with the equation of Siggia. Thioglycollate was determined with the same equation after substituting the iodine- equivalent weight of thioglycollate for that of sulfide.
When both sulfide and thioglycollate occurred in the same sample and both were to be determined, the sample was first titrated as above, then boiled at pH 16
3.0, cooled, restored to original volume, adjusted to
pH 7.0, and titrated again. The titration before
boiling Indicated both sulfide and thioglycollate pre
sent, while the titration after boiling indicated only
the thioglycollate. The difference between the titra
tions of the boiled and the unboiled sample represented
the decrease in iodine equivalent to the sulfide lost
in boiling (see Hendley, 1955). When thioglycollate
alone was to be determined, all samples were boiled at pH 3.0 before titration.
Determination of Carbon Dioxide Uptake - Cells were harvested at 5 C from liquid cultures in an International
Equipment Company refrigerated centrifuge and washed three times with 0.01 M pH 7.4 phosphate buffer. Active cell suspensions could be obtained without taking pre cautions to avoid contact of the cells with atmospheric oxygen and without adding three per cent sodium chloride during the washing process (Newton and Wilson, 1953).
Manometrie measurements at 25 C were made using a conventional Warburg respirometer. Double sidearm flasks were used throughout. The main compartment contained
0.5 ml of a 10 per cent suspension by volume of washed cells in pH 7,2 phosphate buffer, and 0.5 ml of 0.033 M sodium bicarbonate solution which provided a pH of 7.2 during the experiment. One sidearm contained 0.5 ml of substrate and/or buffer at pH 7.2. The second sidearm 17
contained 0.5 ml of 5 N sulfuric acid.
All flasks were gassed for ten minutes with a mix
ture of five per cent carbon dioxide in nitrogen. When
the vessels had reached equilibrium with the gas atmos
phere and the temperature of the water bath, they were
closed and the first manometer reading recorded. The
substance under investigation was tipped into the main
compartment and the pressure changes during four hours
were recorded. At the end of the experimental period,
sulfuric acid was tipped in and the carbon dioxide
remaining as carbonate and carbon dioxide in solution
was determined.
Controls consisted of flasks prepared in the same
manner as the experimental vessels, but the carbon
dioxide initially present was determined by tipping in
sulfuric acid immediately after the addition of the
compound being Investigated.
Carbon dioxide uptake or production during the
experimental period was recorded as the difference
between total carbon dioxide concentration determined
at the beginning and at the end of the experiment by
acid evolution at pH 2.0 (Umbreit jet al., 1949). The fixation of atmospheric nitrogen was not considered
as a significant item in the gas pressure changes,
since resting cells of the purple sulfur bacteria are not known to possess this activity. Photoproduction 18 of molecular hydrogen was also assumed to be insignif icant in the atmosphere of five per cent carbon dioxide and 95 per cent nitrogen (Newton and Wilson, 1953).
The flasks were illuminated by two 100-watt incan descent bulbs located about one foot above the water bath. When necessary, the vessels were darkened either by wrapping the individual flasks with aluminum foil, or by covering the entire water bath with heavy black cloth. RESULTS
Characterization Studies; Oxidation of Intra
cellular sulfur - The results of experiments designed
to follow the rate of oxidation of the sulfur granules
within cells grown in sulfide medium are recorded In
Figure 1. The sulfate concentration, as I^SO^, in illuminated suspensions increased over a period of four weeks from an Initial level of less than one ppm
to more than 38 ppm in a total volume of 32 ml. The amount of sulfate produced in the suspensions of resting
cells was equivalent to about 1.1 x 10“® moles of sulfur or 3.2 x 10“4 grams. On the basis of the 0.05 ml of packed cells used in manometric determinations of carbon dioxide uptake, this would be equivalent to about 0.64 per cent sulfur in the cells. The total amount of sul fur in each flask would be about 320 micrograms or ten micromoles. According to equation 3, page 6, the molar ratio of carbon dioxide to sulfur is 1.5 to 1. Thus,
15 micromoles of carbon dioxide could be assimilated through the oxidation of ten micromoles of sulfur.
This amount of carbon dioxide is equivalent to about
340 microliters. On this basis, there was always sufficient sulfur in the cells to account for the total carbonate assimilated in the manometric experiments reported below.
19 20
-r 40
30
20
pH ppm S04* 10 as h 2s o 4
Inoculated Cdark) and Boiled cells (dark}; pH « ►Inoculated (light}; pH *—*- Boiled cells (light); pH «— ®- Inoculated (light); sulfate e— a - Inoculated (dark) and Boiled cells (light and dark); sulfate
12 3 4
Time (weeks)
Figure 1» Oxidation of endogenous sulfur in washed cells suspended in unbuffered saline. Controls consisted of boiled cells in saline and saline alaae. Total volume of each preparation was 32 ml. All prep arations incubated anaerobically at room temperature. 21
As the sulfate concentration Increased in the sus
pensions, there was a concurrent decrease in pH to about
6.8 after four weeks. At this time, the cell suspensions
began to turn yellow. When they were examined micro
scopically, only an occasional cell still gave evidence
of intracellular sulfur. After six weeks, nearly all
the cells had lysed. Since the optimum pH for these
organisms has been estimated to be about 8.0, lysis
was probably an effect of the acid produced.
The 3ulfate concentration of suspensions stored in
the dark remained less than one ppm during the entire
experimental period. This observation was in agreement with the results of Roelofsen (van Niel, 1935) and
Hendley (1955). The pH of these suspensions also re mained constant.
After this initial experiment with washed cell
suspensions, two one-liter liquid sulfide cultures were prepared. The sulfide concentration and the pH of these
cultures were determined daily. During the first five days, when the cultures were illuminated, the sulfide concentration decreased about 100 ppm as HgS. A sim ilar rate of biological sulfide oxidation was observed during the experiment summarized in figure 2. The decrease in sulfide was accompanied by a reduction in pH from 7.58 to 7.40, probably a result of the HgSO^ produced. 22
140r
120
100 T3T3 •H
80
60 Inoculated Uninoculated
2 4 6 8 10
Time (days)
Figure 2. The oxidation of sulfide by illuminated cultures of purple sulfur bacteria. Two liters of medium were prepared and transferred to two 1-liter bottles. One bottle inoculated with 10 ml of 4-day sulfide culture; the other was left uninoculated and served as a control. Both bottles incubated about 20 in from two 100-watt incandescent bulbs for ten days. (About 192 mg HgS were oxidized biologically during this period.) 23
Whenever the sulfide concentration fell below 60 ppm the cells clumped in gelatinous films or masses.
Microscopic examination revealed large amounts of cap
sular mucus. Cells growing after the sulfide concen
tration was increased above 60 ppm were not clumped and were readily suspended by a slight agitation of the
cultures.
After the cultures were placed in the dark, the sulfide concentration gradually increased, but the increase amounted to little more than ten ppm after ten days. The pH during the dark period decreased 0.2 and 0.3 in the two cultures. The sulfate concentration during the same period decreased from 353 to 340 ppm as
HgSO^. When compared with standards prepared in dupli cate, this change was within the experimental error for these concentrations.
HItrogen-fixlng cultures - A medium for the demon stration of nitrogen fixation was prepared and inocu lated according to the method of Newton and Wilson
(1953). The only source of nitrogen in these cultures was molecular nitrogen in the gas atmosphere above the medium. Heavy growth of all six strains of purple sul fur bacteria tested indicated that nitrogen fixation had taken place.
Analysis of filtrates for fatty acids - Chromato grams never revealed the presence of any of the normal 24
to Cg fatty acids In culture filtrates. This was
true of both the thiosulfate and the sulfide cultures
examined. If any of the acids were present in the fil
trates. they were present in concentrations below the
limit of sensitivity for the test, which would have been
less than about five micromoles of acid per ml of fil
trate.
TABLE 1
Growth of purple sulfur bacteria on sulfur compounds.
Substrates on grams per Substrates on which grams which growth liter of growth did not occur per occurred medium liter
sulfide •» 1.00 sulfite* 1.10 thiosulfate 1.00 bisulfite 0.90 dithionate 2.10 metablsulfite 1.58 sulfhydrate 0.47 pyrosulfate 1.93 thiocyanate 0.68 thioglycollate 0.96 ethyl sulfide 0.75 methyl bisulfide 0.79 thloacetate 0.64 thioacetamide 0.63 thiourea 0.64 thiouracil 1.10 thlomalate 1.29 cysteine 2.80 cystine 2.03 xanthogenate, K salt 1.35 diaminoethyl di sulfide 2 HC1 0.96
* All salts were the sodium salts unless otherwise specified.
Growth on sulfur compounds - The results of growth studies with cultures in which sulfide was replaced by another inorganic or by an organic sulfur compound are 25 presented in table 1.
A total of twenty-two strains were tested, twelve of whioh were inoculated in all the various media. Six of the eight strains which grew the most rapidly and extensively on thioglycollate had been originally iso lated from the same sample of mud and may well be iden tical organisms. When the activities of all the strains were examined, they were found to be quite uniform in respect to substrates permitting growth, although the rapidity and extent of growth sometimes varied.
Intracellular sulfur granules were never observed in cells cultured on 0.1 per cent sodium thiosulfate.
In media containing 0.3 per cent of thi3 salt, elemen tary sulfur was deposited within the cells, but only after an incubation period of nearly three weeks.
Hendley (1955) reported the occurrence of intracellular sulfur granules In Chromatlum strain D after two days in 0.3 per cent sodium thiosulfate medium.
The fact that all strains tested grew well on sodium sulfhydrate may be explained on the basis of the ionization of this compound. In solution, both sulfide and sulfhydrate ionize to form SH“ which, In each case, is the ion functioning In the photosynthetic process.
Since sodium sulfhydrate was incorporated Into the medium at a level equivalent to the amount of sodium 26
sulfide normally used, the media were nearly Identical.
The utilization of dithionate by the purple sulfur
bacteria has not been reported previously, although
Vishniac (1952) observed the slow oxidation of dithi-
onste in cultures of Thlobaclllus thloparus.
Likewise, there has been no published account of
thioglycollate utilization by these organisms. In order
to confirm the observation of growth on thioglycollate
and to rule out the possibility of contamination, tubes
of brain-heart infusion broth (Difco) were Inoculated
from the original thioglycollate cultures and incubated
aerobically and anaerobically In the dark. No growth
was evident In the broth tubes even after twenty days
at room temperature. Organisms from the thioglycollate
cultures were also passed through three serial thio
sulfate agar shake tube cultures and reisolated. When
the organisms were returned to thioglycollate medium,
growth again occurred. Four serial transfers through
thioglycollate medium yielded growth within three to five days after each transfer.
A series of five tube cultures, containing thio glycollate In concentrations from 0*01 to 0.15 M, was prepared In duplicate and inoculated. One series was
Incubated anaerobically in the light, while the dupli
cate series was Incubated anaerobically In the dark.
Growth occurred within three days in illuminated 27
cultures containing 0.01 or 0.02 M thioglycollate. No
growth was apparent at higher concentrations. None of
the cultures incubated in the dark gave evidence of
growth.
With growth obtained on thioglycollate, it became
pertinent to test certain non-sulfur containing acids
that might be intermediates in thioglycollate utiliza
tion. Growth never occurred in media containing from
0.01 to 0.3 per cent glycolic acid, regardless of
illumination. Media containing 0.1 per cent concen
trations of acetic, propionic, succinic, or malic acid
supported growth in the light but not in the dark.
Factors Affecting Carbon Id.oxide Assimilation by
Illuminated Suspensions - Illuminated suspensions of
cells containing intracellular sulfur granules fixed
carbon dioxide at a rate averaging 45 microliters per hour. When sodium thioglycollate, pH 7.2, was added to the suspensions, the rate of carbon dioxide assimilation was reduced. The Inhibitory effect of the thioglycollate increased in proportion to Its concentration (figure 3).
When the molarity of the thioglycollate added reached
0.3 M, carbon dioxide uptake ceased.. No significant change in the thioglycollate concentration In the sus pensions could be detected by titration.
An Inhibition of carbonate assimilation almost
Identical with that produced by thioglycollate was 28
50
40
f-t g XI * 3°- ® ft © .M OD +S ft 3 2 0 - w o o rH
1 0 -
005 .01 .02 .03 .05
Molarity of Thioglycollate Added or of Glycolic Acid Figure 3. Inhibition of carbon dioxide uptake by sodium thioglycollate and glycolic acid In suspensions of illuminated sulfide-grown cells. (See text for a description of the manometric methods.) 29
observed when equimolar concentrations of glycolic acid
adjusted to pH 7.2 with ten per cent NaOH, replaced
thioglycollate (figure 3). It seemed possible that
thioglycollate could have been interferring with the
oxidation of sulfur, or, by structural similarity, with
glycolaldehyde, a suggested acceptor for carbon dioxide.
But the addition of sulfide to a concentration of 0.002
M, which is equivalent to about 0.5 g N a 2S • 9 H 20 per
liter, did not overcome the inhibitory effect of thio glycollate. Nor was the inhibition suppressed by 0.01 M glycolaldehyde (table 2).
TABLE 2
The Influence of 0.002 M sulfide and of 0.01 M glycolaldehyde on the inhibitory effect of thioglycollate on carbon dioxide uptake.
Molarity of Thiogly Thioglycol Thioglycol thioglycollate collate late and sul late and gly alone fide colaldehyde
0.000 (45.6) * 0.005 26.8 ----- 0.010 23.0 22.8 * 23.5 * 0.025 16.5 __ __ 0.050 10.0 9.2 11.0 0.100 3.2 4.5 3.7 0.300 0.0 0.0 0.0 Values represent microliters CG2 uptake per hour over a 5| to 5 hour period.
Cells harvested from 0,1 per cent sodium thiosul fate or from 0.01 M thioglycollate never contained sul fur granules. Iffhen illuminated, suspensions of these cells did not fix carbon dioxide unless a sulfur com
pound was present. Thiosulfa te-grown cells assimilated
carbon dioxide at an average rate of 24.6 microllters
per hour when they were suspended in 0.1 per cent sodium
thiosulfate. In 0.05 per cent thiosulfate, the cells
assimilated an average of 17.5 microllters carbon dioxide
per hour. When, in addition, thioglycollate was added to
the suspension, the rate of carbonate assimilation again
decreased nearly proportional to the concentration of
thioglycollate added (figure 4). Sulfur-free, thiosul-
fate-grown cells did not fix carbon dioxide In the
presence of thioglycollate alone.
When increasing amounts of thioglycollate were
added to illuminated suspensions of sulfur-free thio-
glycollate-grown cells, carbon dioxide was assimilated
(figure 5). The rate of carbon dioxide uptake increased
in proportion to the thioglycollate concentration until
about 0.025 M thioglycollate had been added. This sug
gested that adaptation (or selection) to the light
metabolism of thioglycollate may have occurred. The
maximum concentration of thioglycollate supporting
growth was 0.02 M (page 27). With concentrations of
thioglycollate above about 0.02 M, carbon dioxide fix
ation decreased, but was still evident at 0.05 and 0.10
M even though growth In these concentrations never
occurred. y houft-rw cls upne i tislae and thiosulfate in suspended cells thiosulfate-grown by eea cnetain o thioglycollate. of concentrations several /XI COg uptake per hour 10 30 40 Figure 4. The inhibition of carbon dioxide uptake dioxide carbon of inhibition The 4. Figure .#N thiosulfate Na 0.1# .5 a thiosulfate Na 0.05$ oaiy f holclae Added Thioglycollate of Molarity 0 .01 005
05 31
09 05 uptake by suspensions m 2 025
.01 Molarity of thioglycollate added 005 Non-Illuminated Suspensions Illuminated Suspensions Figure 5. Figure 5. The effect of thioglycollate on the CO of of thioglycollate-grown cells. text for (See description methods.) of 10 15 20 /*1 COg uptake per hour 53
Factors Affectlng Carbon Dioxide Assimilation by
Non-Illumlnated Suspensions - In the dark, suspensions
of sulfide-grown cells containing intracellular sulfur
did not fix carbon dioxide. In fact, there was usually
an Increase of about 15 microllters in the total carbon
dioxide in the darkened flasks after four hours. When
thioglycollate was added to the non-illuminated sus pensions, carbonate disappeared (figure 6), The rate
of carbon dioxide uptake increased with Increasing con
centrations of thioglycollate up to about 0.07 M. At higher concentrations, the rate of uptake dropped off rapidly. Inhibition of the dark metabolism was nearly complete with 0.3 M thioglycollate. Tltrimetric analy ses of those suspensions In which there had been maxi mum carbonate assimilation in the dark revealed that one mole of thioglycollate disappeared for every 3.5 to
4.0 moles of carbon dioxide assimilated (table 3).
TABLE 3
The stoichiometric relationship between carbon dioxide fixed and thioglycollate utilized In the dark.
i co2 Moles COo ppm thio Moles thio Thioglycol (x i o s r glycollate glycollate late : COg (x 106 ) 1 4 3 . 8 * 5.72 9 6 . 9 * 1.7 1 : 3.5 139.9 5.56 79.9 1.4 1 : 3.9 133.0 5.50 85.0 1.5 1 : 3.7 157.4 6.40 90.0 1.6 1 : 4.0 158.2 6.22 98.2 1.7 1 : 3.6 * Values represent the average of duplicate samples. 34
5C>
30
-p
H
10
005 .01 025 05
Molarity of Thioglycollate Added
Figure 6. The uptake of carbon dioxide by sulfide- grown cells in the dark and in the presence of thiogly collate. (See text for description of manometric m e t h o d s .) 35
When equimolar concentrations of glycolic acid, pH
7.2, were substituted for thioglycollate, no carbon di
oxide uptake occurred. This result was in agreement
with the inability of the organisms to grow in glycolic
acid media.
There was no uptake of carbon dioxide in non-lllu-
minated suspensions of sulfide-grown cells to which
0.002 M sulfide alone had been added. But this concen
tration of sulfide completely inhibited the dark metab
olism of thioglycollate by similar suspensions. Like
wise, the dark assimilation, as measured by carbon
dioxide uptake, was almost completely inhibited by
0.01 M glycolaldehyde.
Neither thiosulfate- nor thioglycollate-grown
cells assimilated carbon dioxide in the dark in the absence of sulfur compounds. Nor could an uptake of carbon dioxide be observed when various concentrations of thioglycollate were added to suspensions of thio sulf a te-grown cells.
However, when 0.01 to 0.10 M thioglycollate was added to suspensions of thioglycollate-grown cells in the dark, a small amount of carbon dioxide was fixed.
The uptake was greatest with about 0.03 to 0.05 M thio glycollate, then dropped rapidly to 0 with 0.10 M
(figure 5).
Suspensions of thiosulfate-grown cells in 0.1 or 36
50
40-
Cells In 0.1$ Na thiosulfate Cells in 0.05$ Na thiosulfate
30"
+>
0 20 -
10
005 .01 05
Molarity of Thioglycollate Added
Figure 7. The uptake of carbon dioxide by sodium thiosulfate-grown cells In the dark and in the presence of thioglycollate. (See text for description of mano- metric methods.) 37
0.05 per cent sodium thiosulfate produced from five to ten microliters of carbon dioxide. (This observation resembled that seen with non-illuminated sulfide-grown cells containing intracellular sulfur granules.) When thioglycollate was also added to these suspensions, there was an uptake of carbon dioxide (figure 7). The extent of this uptake was nearly the same as that ob served in suspensions of sulfide-grown cells in similar concentrations of thioglycollate. DISCUSSION
On sulfide and thiosulfate media, the strains of
purple sulfur bacteria used throughout these experiments
have yielded results in close agreement with the activi
ties of photosynthetic sulfur bacteria recorded in the
literature. In the presence of sulfide, or 0.3 per cent
sodium thiosulfate, cells deposited elementary sulfur
internally as droplets or granules. When sulfide was
depleted from the medium, the Intracellular sulfur was
oxidized to sulfuric acid. In the dark, cells contain
ing sulfur granules excreted hydrogen sulfide into the
medium.
Anaerobically in the light and in the presence of
malate and thiosulfate, suspensions of cells fixed
atmospheric nitrogen when ammonium ion was lacking.
But several observations have indicated that the
organisms studied here differ in certain respects from
those strains which have been described in the liter
ature.
The first such observed difference was the pro duction of mucus in cultures in which the sulfide con
centration had fallen below about 60 ppm as HgS. Most of the detailed work done in the past has been with strains Isolated in Europe, chief among these being
Chromatium strain D (Delft). The selection of this
38 39
organism for study was based in many instances on its
growth in liquid culture without the production of mucus (Hendley, 1955). Although it is evident from
van Niel's experiments (1931) that mucus-forming strains were among those he isolated, detailed work with such
organisms has never been reported.
A second difference observed was that the strains of bacteria used in these investigations remained viable in handling more readily than strains reported in the literature. The addition of sulfide to thiosulfate or thioglycollate cultures to remove the last traces of oxygen was not necessary, in contrast to the experience of van Niel (1931). Nor was there any indication that cellular activity was affected by contact of the cells with air during normal harvesting and suspending pro cedures. Sodium chloride was never a necessary con stituent of either the culture medium or of the buffer solution used to wash and suspend cells. This is in marked contrast to the conditions under which Chroma- tium-D had to be studied by Newton and Wilson (1953).
The salinity required by their strain may be a reflec tion of its normally marine habitat.
The third, and perhaps the most significant char acteristic of our strains is their ability to assimilate carbon dioxide photosynthetically at the expense of thioglycollate. This is unique in that the incorporation 40
into media of organic sulfur compounds as hydrogen
donors for the photosynthetic metabolism of the purple
sulfur bacteria has never been reported.
Thioglycollate was found to have two quite oppo
site effects on the photosynthetic mechanism. With
small amounts of this compound, carbon dioxide was
assimilated in the light. But as the concentration of
thioglycollate was increased above about 0.025 M, photo
synthesis, as measured by carbon dioxide uptake and
growth, was inhibited. These two effects will be dis
cussed separately.
Van Niel has studied and resolved most of the sul fur oxidations occurring in the purple sulfur bacteria
during normal photosynthesis (1941). These have been listed below in order of increasing efficiency, i.e., moles of carbon dioxide assimilated per mole of sulfur
compound oxidized.
(1) 1 C02 + 2 EgS----- KCHgO) + 2 S + HgO (2) 2 C02 + 4 HgS03 + 4 HgO— >2 (CHgO) + 2 HgS04 + 2 H20 (3) 3 C02 + 2 S + 8 HgO-- *3 (CHgO) + 2 HgS04 ♦ 3 HgO (4) 4 COg + 2 Ha2S203 + 10 HgO *■ 4 (CHgO) + 4 NaHS04 +• 4 HgO (5) 4 COg + 2 HgS + 6 H20 — >4 (CH20) + 2 H2S04 ♦ 2 HgO
However, if the sulfhydryl compound of equation 5 is not limited to sulfide alone, a more generalized equation 41
may be postulated:
(6) 4 COg ♦ 2 RSH + 6 HgO *4 (CHgO) + 2 HgS04 + 2 ROH
Thus, when R is the hydrogen Ion, RSH becomes HgS, ROH
becomes HgO, and equations 5 and 6 are identical. With
sodium sulfhydrate, ROH becomes NaOH, and
2 HgS04 ♦ 2 N aOH------» 2 NaHS04 + 2 HgO.
Since low concentrations of thioglycollate permitted
growth, then thioglycollate must be assumed to have func
tioned as sulfide or other sulfur compound in carbon
dioxide uptake and growth of the organisms. The fate of
thioglycollate during photosynthesis must be assumed to have been fundamentally the same as that of sulfide.
This means, of course, that thioglycollate must be oxidized during the photosynthetic process. However, several possibilities other than the direct substitution of thioglycollate for RSH in equation 6 exist for such an oxidation.
One possibility would involve the reduction of thioglycollate to acetate and hydrogen sulfide, fol lowed by an oxidation of the sulfide to sulfate. Since acetate alone supported growth of the organisms in the light, it would be expected that acetate derived from the reduction of thioglycollate would also be oxidized.
The assimilation of thioglycollate could then be assumed 42
to proceed as follows:
H2C(SH)C00H ♦ 2 H--- ►H 3C-COOH + HgS
HgS + 4 HgO >H2S04 ♦ 8 H
H3C-C00H + 2 HgO ► 2 C02 ♦ 8 H
The oxidation of one molecule of hydrogen sulfide to
sulfuric acid would liberate eight (H). But since
two (H) are required for the initial reduction of thio glycollate, the net gain would be only six (H). The complete oxidation of one molecule of acetate would yield two molecules of carbon dioxide and eight (H).
(2 CHgO) + 2 HgO >2 COg + 8 H
However, all the hydrogen would be required for the reassimilation of the carbon dioxide produced during dehydrogenation and no net gain could result.
Thus, in such a scheme for the utilisation of thioglycollate, the oxidation of the sulfhydryl alone can result in net carbon dioxide uptake* Stoichio- metrleally, one mole of thioglycollate could account for no more than 1.5 moles of carbon dioxide. This is significantly different from the observed values of
3.5 to 4.0 moles of carbon dioxide per mole of thio glycollate.
A second possibility is the dehydrogenation and condensation of two molecules of thioglycollate to form 43
the disulfide, dithioglycollate.
H H 2 H2C(SH)C00H------^HO-£-C-S-S-C-C-OH ♦ 2 H 0 H H 0
Stoichlometrically, at least four moles of thioglycol
late would be required for the reduction of one mole
of carbon dioxide. Furthermore, the dithioglycollate
formed would probably be stable to further sulfur oxi
dation since methyl disulfide, HjC-S-S-CH^, and cys
tine, which have similar sulfur to carbon bonds, did
not permit growth of the organisms in the light.
Since Fromageot (1951) found that certain bacteria
catalyze a desulfhydration of cysteine to alpha-keto
acid, ammonia, and hydrogen sulfide, a third possibility
for the oxidation of thioglycollate may be its desulf hydration to glycolic acid and hydrogen sulfide.
E2C(SH)e00E + HgO s-HgC(OH)COOH + HgS
But glycolic acid alone neither supported growth of the purple sulfur bacteria nor initiated carbon dioxide up take. Therefore, the oxidation of hydrogen sulfide alone would have to account for the total carbonate assimilation. The ratio of thioglycollate to sulfide would be 1 : 1 and the stoichiometry for the reaction one mole of thioglycollate per two moles of carbon dioxide, compared with 1 : 1.5 for a pathway through acetate.
A fourth possibility, and one more strongly sup
ported through comparative biochemistry, is similar to
the desulfhydratlon mechanism just discussed. The dif
ference is that in this fourth scheme, oxidation of the
sulfhydryl radical takes place before the cleavage of
sulfur from the carbon chain.
Pirle (1934) and Fromageot (1948) have studied this
mechanism in the oxidation of cysteine in liver and kid ney sections. Based on their findings, the following
sequence of reactions may be proposed for thioglycollate
- 2 H - 2 H desulfinase (« 2 H) RSH > RSOH > RSOgH ? -----► (R” )------» ROH - 2 H - 6 H (- 2 H) - 2 H — ...... ► RS03H ------— ^S03----- >S04 where R la the radical -H2C-COOH.
Several observations lend favor to this mechanism.
First, in the oxidation of cysteine, all of the inter mediates with the exception of the unstable sulfenic acid (RSOH) have been identified. And, although the exact mechanism of the desulfinase reaction has not as yet been clarified, all of the enzymes participating in the reactions have also been identified.
Second, the intermediates in the oxidation of cys teine are analogous to those proposed but not identified 45
by van Nlel for the oxidation of sulfide (page 6). Sulf
hydrate, thiosulfate, sulfite, and dithionate may also
be fitted into the scheme. The total dehydrogenation of
thioglycollate would involve eight (H), including one or
the other pair of hydrogens in parentheses depending on
the number of water molecules entering the reaction.
And again, the stoichiometry of the reaction would be
one mole of thioglycollate per two moles of carbon di
oxide reduced.
Third, the mechanism above may also account for the
lack of sulfur within the cells, since hydrogen sulfide
would not be produced as an intermediate. However, this
cannot be taken as conclusive evidence for the scheme
since the possibility still exists that sulfide may have
appeared during the oxidation, but was produced consid erably slower than it was oxidized.
And finally, this scheme is compatible with the generalized equation for the oxidation of sulfhydryl compounds (equation 6, page 41).
Mention should be made of the position of stoichio metry in a discussion of thioglycollate assimilation.
At no time can a mechanism like those on page 40 account for the amount of carbon dioxide which was assimilated either in the light or in the dark during respirometric experiments with thioglycollate. Three possibilities exist to account for such a discrepancy. 46
First, the values calculated from experimental
observations may have been inaccurate. However, such
inaccuracies could not be expected to yield the same
range of stoichiometric ratios in repeated experiments
both in the light and in the dark.
Second, thioglycollate may have had some catalytic
or stimulatory effect on processes leading to the assim
ilation of carbon dioxide. If this were the case, how
ever, thioglycollate would not be expected to be inhib
itory at the same time* And such a stimulation would
have to be considered non-specific, since carbonate
assimilation in the light may proceed through mechanisms
different from those operating in the dark.
The third and most likely possibility for the
greater ratio of carbonate assimilated is that endoge nous materials may have been utilized concomitant with the oxidation of thioglycollate. Any one or more of the following functions may be proposed for endogenous m aterials.
First, they may be oxidized to yield hydrogen and energy. Fixation of carbon dioxide through such an endogenous oxidation may be represented thus:
reduced endogenous _oxidized endogenous substance 47
However, there are experimental results which argue
against this mechanism. First, a strict relationship
between carbon dioxide fixation and thioglycollate
utilization would not be likely to result. And second,
the above mechanism should occur Independently of thio
glycollate, but no such activity was demonstrable in
the absence of thioglycollate.
These two arguments favor a second possibility for
the function of endogenous materials, similar perhaps
to the above, but involving a more intimate association
of thioglycollate and the endogenous substrates in car
bon dioxide fixation. The function of the endogenous
substance may be thought to be dependent on the presence
or function of thioglycollate. Thus, not only could a
stoichiometric relationship exist between carbon dioxide
and thioglycollate, but carbonate would not be assim
ilated in the absence of thioglycollate.
For example, endogenous materials may function
fundamentally as carbon dioxide acceptors in non-reduc-
tive carboxylations requiring only energy.
R-H ♦ ~ P 0 i4
>-R-GOOH CO,2
In our experiments, the required energy could be sup plied through the oxidation of thioglycollate, a mech
anism for which has already been suggested, that is, 48
(7) thloglycollate---- *-glycolic acid + HgS04 ♦ 4 ~P04
(8) 4 R-H ♦ 4 C02 + 4~P04 --- >-4 R-COOH
In equation 8, R-H represents the carbon dioxide accep
tor and requires at least one high energy phosphate for
the fixation reaction. The nature of the acceptor is
unknown although several possibilities exist. Siegel
(1954) found that resting cells of Rhodopseudomonas
gelatlnosa. a non-sulfur purple photosynthetic bacterium,
carboxylated acetone to acetoacetate
0 0 h C02 H « HgO h 3 c -c - c h 3 >»h o o c - c - c - c h 3 > 2 h 3c -c o o h
by a light- or oxygen-Induced reaction. The hydrolytic
formation of acetate from acetoacetate was spontaneous,
Larsen (1951) found that Chlorobium thlosulfatophllum.
a green photosynthetic sulfur bacterium, assimilated
carbon dioxide in the presence of propionate in the
light.
COg
- 4 H 2 H ♦ 2 H propiona t e ------► pyruva te — ^ --► mala te --- >• succinate
Both light and a source of hydrogen donors were re
quired for the reaction. However, growth of the organ isms did not occur under these conditions.
But, for reasons which will be discussed more fully
In relation to dark metabolism, a more likely carbon dioxide acceptor is pentose phosphate which has been
suggested by Bassham et jal. (1954) for green plant
photosynthesis.
4 rlbulose +4 ATP 4 ribulose +4 COR 8 phospho- phosphate ^diphosphate glycerate
This scheme is also oompatible with equations 7 and 8.
The combined mechanisms would provide the greatest
ratio of carbonate to thioglycollate, which was the
ratio actually obtained in respirometric experiments,
that is, four moles of carbon dioxide per mole of thio
glycollate utilized.
& discussion of the utilization of thioglycollate
in the dark may be introduced to advantage by a con
sideration of the normal dark metabolism of the purple
sulfur bacteria. The following scheme for the endoge nous activity of this group of organisms under anaerobic
conditions may be proposed on the basis of the existing literature:
Succinate Propionate \ -2H | -2H Fumarate Lactate
' 2H Malate1 Activities represented by that portion of the diagram
below the broken line are substantiated by Hendley
(1955). Carbon dioxide and acetate were found to be the
chief end products of the endogenous metabolism of Chro
ma tlum strain D. When sulfur was present in the cell to
function as a hydrogen acceptor, hydrogen sulfide was
liberated. Without intracellular sulfur, molecular
hydrogen appeared. The quantities of these end products
could be considerably increased by the addition of malate
or pyruvate to the culture. That cell syntheses were
proceeding concomitant with endogenous respiration was
Indicated by the increase in carbon dioxide, acetate,
and hydrogen which Hendley observed after the addition
of 2,4-dlnitrophenol to these cultures.
Since, in the present investigations, sulfur was
still observed within cells after they had been stored
in the dark for ten days, the hydrogen donors rather
than acceptors seemed to be the limiting factor in endogenous metabolism. When metabolism is dependent on endogenous materials, the rate of formation of the res piratory products should decrease significantly within a short time. Hendley observed such a decrease within
Mseveral hours" (1955).
To increase the rate of cell synthesis, the crea tion of high energy phosphate through the transfer of hydrogen to a suitable acceptor must continue, and a suitable hydrogen donor must be added. This is sup
ported by the observation that neither thioglycollate,
a hydrogen donor, nor carbon dioxide were assimilated
in the dark unless sulfur, acting as a hydrogen acceptor,
was present. Cells with intracellular sulfur granules
were more active than those lacking sulfur, but which
had been supplied sodium thiosulfate in the suspension.
This difference may indicate that (1) the normal photo
synthetic oxidation of sulfide may not proceed through
thiosulfate as an intermediate, or (2) if thiosulfate ia an intermediate in the oxidation, the reaction is not completely reversible, or (3) for synthetic reactions thiosulfate does not function as a hydrogen acceptor as efficiently as sulfur.
It was also apparent that thioglycollate must function in the dark the same as it does in the light, that is, as the hydrogen donor in some mechanism for the assimilation of carbon dioxide. The metabolism of thio
glycollate In the light and the observation that gly colic acid could not replace thioglycollate during the dark assimilation of carbonate In the present investi gation are indicative of certain possibilities.
First, evidence presented here is In accord with our present knowledge of the bacterial photosyntheses, in which hydrogen donors for the process are dehydro genated by reactions Independent of light ("dark” 52
reactions). And second, the mechanisms for dehydro
genation are probably the same whether the cells are
illuminated or not. The major difference between the
light and the dark metabolism is that, in the light
(OH) acts as hydrogen acceptor, while in the dark and
in the absence of molecular oxygen, some endogenous
substance must serve this function. Since the scheme
of Promageot has been proposed for the dehydrogenation
of thioglycollate during photosynthesis, the same scheme
may be proposed for the dark. According to this scheme
a total of eight (H) are possible from the dehydrogen
ation of one molecule of thioglycollate to sulfuric and
glycolic acids. That this amount of hydrogen is suffi
cient to account for the observed 4 : 1 ratio of carbon
dioxide to thioglycollate is indicated in the following
discussion.
Since known mechanisms for carbon dioxide fixation
in the dark do not yield the stoichiometry obtained in
our experiments and cannot explain the similarity be
tween the light and the dark metabolism, it is again
necessary to consider the role of endogenous materials
in the fixation reaction.
The reactions indicated in equations 7 and 8 on
page 48 have been found to be compatible with experi mental observations of carbonate assimilation in the
light, While photolytically-produced (OH) is reduced 53
through the dehydrogenation of thioglycollate, endoge
nous materials (R-H) serve as carbon dioxide acceptors
for non-reductiv© carboxylations.
Acetone and propionate have already been shown to
be carbon dioxide acceptors for photosynthetic bacteria
in the light. It Is unlikely that they function simi
larly in the dark for two reasons. First, so far as we
now know, both appear to require light or oxygen,
neither of which was present during our dark experi
ments. Second, the quantity of acetone or propionate
which might be present to function as carbon dioxide
acceptor within the cell would be limited and probably
could not account for the amount of carbonate assim
ilated.
Ribulose phosphate has also been suggested as a
carbon dioxide acceptor in the light. It is possible
that this substance may also function as acceptor in
the dark If the phosphorolysis of capsular mucin could make sufficient pentose phosphate available to the cell.
Assimilation of carbonate would not then seem to be
limited by the small amount of any endogenous acceptor.
Also in the dark, other acceptors than (OH) for hydrogen removed In thioglycollate oxidation would be required. Therefore, we must postulate that endogenous materials serve this additional function in the dark.
In cells containing sulfur granules, the sulfur 54
undoubtedly acts as the hydrogen acceptor since: (1)
there was no significant carbon dioxide uptake by sul
fur-free cells; (2) hydrogen sulfide was produced by
cells containing elementary sulfur; and (3) calcula
tions show (page 19) that the amount of sulfur present
within the cells was adequate to serve this function.
When intracellular sulfur granules were not present,
thiosulfate could serve as an exogenous hydrogen
acceptor. Even thioglycollate-grown cells, lacking
exogenous acceptor and visible sulfur granules, showed
a very limited carbon dioxide uptake which indicated
the presence of endogenous hydfogen acceptors.
Thus, endogenous materials may have a significant
role in both the light and the dark metabolism of
resting cells in that such materials may account for
carbon dioxide assimilation in excess of that expected
to result from the oxidation of thioglycollate alone.
Since carbon dioxide fixation occurred with exoge nous hydrogen acceptor (thiosulfate) in the dark, it is necessary that we consider why growth did not occur under these conditions. First, the reaction Indicated by equation 8 (page 48) satisfies the requirements for
carbon dioxide fixation, but not those for growth.
Growth requires an additional mechanism for the regen
eration of the carbon dioxide acceptor and the reduc tion of fixed carbon dioxide to eell substance (HCHO). + 2 H (9) R-COOH------*■ (HC H O ) ♦ R-H + H 20
Since growth did not occur in the dark, the above
reaction may be thought to be light-dependent. Further
more, the reduction, requiring hydrogen, would yield a
ratio of carbon dioxide to thioglycollate of 2 : 1
rather than the 4 : 1 ratio actually found. Thu3,
resting cells apparently do not carry out the reaction
indicated in equation 9 even when they are illuminated.
This suggests that the reaction may be growth-linked.
Second, other reactions necessary for growth may
require hydrogen acceptors. But acceptors with a suffi
ciently high potential may not be available in the dark.
For example, (OH), produced by the photolysis of water,
is not available a3 a hydrogen acceptor in the dark.
And third, it may be that both of the above limi tations prevent growth under conditions in which carbon dioxide fixation occurs.
Both growth and manometric experiments Indicated that concentrations of thioglycollate greater than about
0.025 M were inhibitory to carbon dioxide uptake and growth of the purple sulfur bacteria. In manometric experiments with both sulfide-grown and thiosulfate- grown cells, the thioglycollate could be recovered at the end of the experiments. This observation sug gested that the inhibitory effect of thioglycollate 56 was not necessarily the result of any product which may have been formed by its assimilation.
It Is possible that this Inhibitory effect could have been the result of the sulfhydryl concentration alone. For example, van Kiel (1931) has shown that some strains of purple sulflar bacteria fall to grow at concen trations of sulfide above 0.1 per cent. Other strains could tolerate sulfide concentrations up to 0.2 per cent, above which growth ceased. This Implies that In concentrations of sulfide between 0.004 and 0.008 M
N a2S, nearly all strains were completely inhibited.
Equimolar quantities of thioglycollate had no such effect. Instead, the organisms assimilated carbon di oxide and grew in concentrations of thioglycollate up to 0.025 M.
The primary evidence favoring the rejection of any scheme in which thioglycollate Inhibition may be linked with sulfhydryl concentration resulted from experiments with glycolic acid. Carbon dioxide uptake was inhibited to an almost Identical degree when equimolar concen trations of glycolic acid replaced thioglycollate. This observation precluded any possibility that the sulf hydryl group per se was responsible for the inhibition.
Furthermore, the inhibition of carbon dioxide uptake
Increased in proportion to the amount of thioglycollate added. This suggested a possible competitive inhibition 57
of carbonate assimilation by interference with the usual
sulfur oxidation mechanisms.
Eut if the inhibition were competitive, then in
creasing the concentration of normal substrate (sulfide
or thiosulfate) could be expected to reduce the inhibi
tory effect. However, neither the addition of 0.002 M
sulfide to sulfide-grown cells, nor the addition of
thiosulfate to thiosulfate-grovm cells had any effect on
the thioglycollate inhibition. Even cells oxidizing
thioglycollate which, since they had been harvested
from a thioglycollate medium, was their "normal1' sulfur
oxidation, were inhibited by increased concentrations of
this compound. This could hardly be considered a com
petitive inhibition since thioglycollate was also the
normal substrate.
The photosynthetic reduction of carbon dioxide may
be considered to occur in at least two separable reac
tions: (1) the creation of reducing power through the
photolysis of water, a light-dependent reaction; and
(2) the utilization of this reducing power for the assim
ilation of carbon dioxide, a reaction Independent of
light (Quayle et al., 1954).
If thioglycollate and glycolic acid were inhibitory
to the light reaction, then inhibition of carbon dioxide uptake would not be expected to occur in the dark.
However, this was not the case. Uptake of carbonate 58 was inhibited by thioglycollate to about the same extent
both in the light and in the dark. This implies that
the inhibition lay, not in the photodecomposition of water, but in the subsequent reactions leading to the assimilation of carbon dioxide.
In view of the previous discussion of the light and the dark metabolism of thioglycollate, and the pos sible role of pentose phosphate in carbon dioxide fixa tion, thioglycollate and glycolic acid may have been inhibitory for structural reasons.
H H H H-C-OH H-C-OH H-C-SH 9=0 c=o 9=0 OH OH
ribulose glycolic thioglycollate
Competitive inhibition could not have been demonstrated since the concentration of ribose phosphate, ribulose phosphate, or other related material to which carbon dioxide i3 ultimately attached, was not varied. SUMMARY
Several strains of purple sulfur bacteria were
studied in manometric and growth experiments. The
results of those experiments have led to the following
conclusions.
1. All strains examined grew photosynthetlcally
at the expense of sulfide, thiosulfate, sulfhydrate,
and dithionate. Sulfur was stored as intracellular
granules with 0.1 per cent sulfide and 0.3 per cent
thiosulfate, but never with 0*1 per cent thiosulfate.
Growth did not oecur on media containing sodium sulfite.
2. Of the organic sulfur compounds investigated,
only sodium thioglycollate at concentrations of 0.01 to
0.025 M sustained growth of the organisms in the light.
Sulfur was not stored within cells grown in thiogly
collate.
3. Cells having sulfur available as internal sulfur granules or external thiosulfate in solution, assimilated carbon dioxide in the light. Without an available sulfur source, there was no uptake in the light.
4. In the presence of thioglycollate carbon dioxide fixation was inhibited in proportion to the concentration of thioglycollate added and was nearly
59 60
complete at 0.05 M. Thioglycollate could be recovered
at the same concentration as that Initially added fol
lowing inhibition studies. Glycolic acid was Inhibitory
to a similar extent at equimolar concentrations. The
Inhibition was not a result of interference in the sul
fur oxidation mechanisms, but was probably an effect on
the assimilation of carbon dioxide.
5. Carbonate assimilation was observed with thio
glycollate when sulfide- or thioglycollate-grown cells were employed in manometric experiments, but not when thiosulfate-grown cells were used. This implies that
cells grown upon the more oxidized thiosulfate were not adapted to the more reduced thioglycollate.
6. It has been assumed that thioglycollate func tioned in the same manner in the light as sulfide or thiosulfate would have functioned, i.e., as a hydrogen donor through oxidation. The oxidation of thioglycol late may proceed through acetate or to glycolate.
Although acetate, but not glycolate, could be assim ilated by the organisms in the light, a pathway to glycolate would better fit the stoichiometric rela tionship observed.
7. Suspensions of cells produced a slight amount of carbon dioxide in the dark. Cells without sulfur granules and without an external supply of thiosulfate did not utilize thioglycollate and carbonate in the dark. 61
8. Suspensions of cells containing sulfur granules,
or sulfur-free cells In thiosulfate solution, assimilated
carbonate in the dark when thioglycollate was added up
to a concentration of about 0,07 M, Beyond this concen
tration an inhibition of carbonate assimilation occurred.
9, Glycolic acid produced an inhibition similar to
that of thioglycollate, but did not initiate carbon
dioxide uptake in the dark or in the light.
10. It seems most logical to assume that thiogly
collate oxidation is the same in the light as in the
dark. But thioglycollate oxidation in the dark with
endogenous sulfur or exogenous thiosulfate as hydrogen
acceptor could not yield as much energy as its oxida
tion in the light with (OH) as acceptor. Therefore,
since the same proportion of carbon dioxide is util
ized with thioglycollate in the dark and in the light,
either carbon dioxide is utilized In a different way,
or there is a significantly greater contribution made
by endogenous processes in the dark than In the light,
or both.
11, Growth of the purple sulfur baeteria In the
dark has not been demonstrated conclusively even under
circumstances comparable to those in which carbon
dioxide was assimilated in the dark in the manometric
experiments. In the absence of light, the carbonate may not be further utilized in cellular metabolism 62 and hence, not available for growth of the organisms.
Or the absence of sufficiently high potential hydrogen acceptors, such as 0H~, may prevent growth of the organisms in the dark. BIBLIOGRAPHY
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Block, R. J., .Durrum, E. L, and Zweig, G. 1955, A manual of paper chromatography and paper electro phoresis, Academic Press, Inc., N. Y., p. 161-166.
Feigl, P. 1945. Manual of spot tests, Academic Press, I n c ., N . Y.
Fromageot, C., Chatagner, M. and Bergeret, P. 1948. Formation of alanine by enzymatic desulfination of L-cysteine sulfinic acid. Biochim. et Biophys. Acta, 2, 294-307.
Fromageot, C. 1951. Oxidation of organic sulfur. In: The Enzymes, v. II, part 1, Academic Press, Inc*, N. Y., p. 609.
Gaffron, H. 1934. Uber die KohlensHure-Assimilation der roten Schwefelbakterien. I. Biochem. Z., 269. 447-453.
Gaffron, H. 1954. Mechanism of photosynthesis. In: Autotrophic Microorganisms, Cambridge University Press, London, 152-185.
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Lankester, E. R. 1873. On a peach-coloured bacterium — Bacterium rubescens n.s. Quart. J. MIcr. Sc., 13. 408-425.
Larsen, H. 1951. Photosynthesis of succinic acid by Chlorobium thiosulfatopfallum. J. Biol. Chem., 193. 167-173.
Larsen, H, 1952. On the culture and general physiology of green sulfur bacteria. J. Bact., 64, 187-196.
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Muller, F. M, 1933. On the metabolism of the purple sulfur bacteria in organic media. Arch, f . Mikro- blologie, 131-166.
Nakamura, H, 1939. Weitere Untersuchungen fiber den Wasserstoffumsatz bei den Purpurbakterien, nebst einer Bemerkung fiber die gegenseitige Beziehung zwisehen Thio- und Athiorhodaceae. Acta Phyto- chim., Japan, 11, 109-125.
Newton, J. W. and Wilson, P. W. 1953. Nitrogen fixa tion and photoproduction of molecular hydrogen by Thlorhodaceae. Antonie van Leeuwenhoek, 19. 71-77.
Pirie, N. W. 1934. The oxidation of thiosulfate to sulfate by tissue slices in vitro. Biochem. J., 28, 1063-1075.
Quayle, J. R., Fuller, R, C., Benson, A. A. and Calvin, M. 1954. Enzymatic carboxylation of ribulose diphosphate. J. Am. Chem. Soc., 76. 3610-3611,
Siegel, J. M. 1954. The photosynthetic metabolism of acetone by Bhodopseudomonas gelatinosa. J. Biol. Chem*, 208, 205-216.
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Vishniac, W. 1952, The metabolism of Thiobacillus thloparus. I, The oxidation of thiosulfate, J. Bact., 64, 363-573.
Winogradsky, S. 1887. tJber Schwefelbakterein. Botan. Ztg., 45, 489 ff, (Nos. 31-37). AUTOBIOGRAPHY
I, John Jacob Taylor, was born in Dayton, Ohio,
June 26, 1928. I received my secondary school educa tion in the public schools of Germantown and Lewisburg,
Ohio, and my undergraduate training at Heidelberg Col lege, Tiffin, Ohio, which granted me the Bachelor of
Science degree in 1950. Prom Ohio University I re ceived the degree Master of Science in 1952. While in residence there, I was an assistant to Dr. Arthur
Blickle during 1950, and an instructor of botany until
June, 1952. From 1952 to 1954, I was employed as a research specialist by the National Cash Register
Company, Dayton, Ohio. In September, 1954, I was appointed Graduate Assistant in the Department of
Bacteriology, and in July, 1956, Research Assistant with the Research Foundation a,t Ohio State University.
I held the latter position for one year while completing the requirements for the degree Doctor of Philosophy.
66