270. E. B.: Two Kinds of Lithotrophs Missing in Nature, Zeitschrift Für Allgemeine Mikrobiologie 17 (1977), 491-493

270. E. B.: Two Kinds of Lithotrophs Missing in Nature, Zeitschrift für Allgemeine Mikrobiologie 17 (1977), 491-493.

Zeitschrift für Allg. Mikrobiologie 1977 491-493

(Institut für Physikalische Chemie, Universität Wien) Two kinds of lithotrophs missing in nature

E. BRODA

(Eingegangen am 14. 9.1976) Two groups of lithotrophic bacteria, the existence of which may be expected on evolutionary and thermodynamical grounds, have not yet been detected: (A) photosynthetic, anaerobic, ammonia bacteria, analogous to coloured sulphur bacteria, and (B) chemosynthetic bacteria that oxidize ammonia to nitrogen with O2 or nitrate as oxidant.

The versatility of the prokaryotes in their energy metabolism has long astonished microbiologists. The bacteria have developed processes, i.e., enzymes, for the utili- zation of a wide range indeed of exergonic reactions. Attention is now drawn to further processes in energy metabolism which on the basis of considerations on the evolution of the bioenergetic processes (BRODA I975a) may be expected to have existed, or to exist, but which have not yet been found. Two kinds of "lithotrophic" bacteria with such mechanisms will now be predicted. Lithotrophs are bacteria that use in- organic reductants in their energy metabolism (FROl\fAGEOT and SENEZ 1960); all autotrophs must be lithotrophs, though the reverse need not be true. The two bacteria here predicted would generate dinitrogen (N2). The nitrifying bacteria make adenosine triphosphate, ATP, through oxidative phosphorylation coupled to the aerobic oxidation of ammonia, a highly exergonic process. Thus, in nitrification Nitrosomonas produces nitrite, and Nitrobacter makes nitrate. The redox reactions are:

NHt + 1.5 O2 = H 20 + NO;- + 2 H+; = - 65 kcal (1) NO;- + 0.5 O2 = NO;-; = - 18 kcal (2) The negativity of the free enthalpy change, is the precondition for the production of ATP and, consequently, for the endergonic reduction of CO 2 to biomass. The reduction occurs, as in plants, through the CALVIN cycle; the reductant, NADH, is obtained by reverse electron flow forced, by ATP. Clearly, the nitrificants, one main group of the "chemolithotrophic" bacteria, could evolve only after the biosphere began to contain, as a consequence of the photosynthetic activity of the blue-green algae, free oxygen (BRODA I975a, b). The tran- sition to the oxidizing biosphere took place about 2 giga-years (Gy) ago (RUTTEN 1971) Similarly the free oxygen made possible the rise of the second important class of chemolithotrophs, the colourless (white) sulphur bacteria. These "thiobacilli" make ATP on the basis of reactions of the overall types:

HS- + 0.5 O2 + H+ = H 20 + S; = - 51 kcal (3) 2 S + 1.5 O2 + H 20 = S04 + 2 H+; = - 139 kcal (4) The thiobacilli presumably descended from coloured, photosynthetic, sulphur bacteria, i.e., the "photolithotrophs" gave, after the advent of 02' rise to the chemolithotrophs. In other words, oxidative phosphorylation evolved from photosynthetic 492 E. BRODA phosphorylation. This is indicated by the "conversion hypothesis" for the origin of respiration from photosynthesis (BRODA 1975a). The basic processes in the energy metabolism of the photosynthetic sulphur bacteria are

2 HS- + 2 H+ + CO2 = (CH20) + H 20 + 2 S; = 11 kcal (5)

0.5 HS- + CO2 + H 20 = (CH20) + 0.5 H+ 0.5 = 18 kcal (6)

(CH 20) indicates unit quantity of biomass, not formaldehyde. Reactions (5) and (6), which are endergonic, are energized by light, i.c., electrons are promoted photochemi- cally. Aseparation into exergonic and endergonic partial reactions would, in contrast to the position with the chemolithotrophs, not be meaningful with the photolithotrophs because CO2 is indispensable as terminal (extracellular) electron acceptor. In reactions (1) to (4) this role is played by 02' Incidentally, for the processes (4) and (6) the term "sulphurication" might be introduced, in analogy to nitrification (processes 1 + 2). vVho, then, were the anccstors of the nitrificants? Can they, in parallel to the evolution of the sulphur bacteria, have descended frOln photosynthetic ammonia bacteria? Such (coloured) bacteria are not known. But apparently no search has ever been made for them. They may exist or else they may have existed, but died out.

The photochemical promotion of electrons from NH; to reduce CO2, the fundamental feature of such hypothetical bacteria, would from the point of view of energetics not be too difficult:

1.3 NH; + CO2 = (CH20) + 0.65 N2 + H 20 + 1.3 H+; = 12 kcal (7)

This would involve a direct biotic oxidation of NH;, i.c., of NHa, to N2 • Such a reaction is unknown. In contrast to the anaerobic and endergonic reaction (7), an aerobic and exergonic oxidation of NHa to N2 could, like that to NO;- or NO;;-, occur only after the appe- arance of O2 in the biosphere:

NH; + 0.75 O2 = 0.5 N2 + 1.5 H 20 + H+; = - 75 kcal (8)

(The exergonicity oi NHa oxidation by O2 is, of course, also evident from the fact that NHa is considered as a commercial fuel). Chemolithotrophs capable of reaction (8) would compete with the nitrificants, responsible for reactions (1) and (2). They would likewise be colourless, i.c., white. But, like reaction (7), reaction (8) has never been observed.

In reaction (8), O2 could be replaced as an oxidant by NO;- or NO;;-:

NH; + NÜ;- = N2 + 2 H 20; = - 86 kcal (9) The resulting reaction, here written dovm only for the stoichiometrically simpler case of NO;-, could also be considered as a variant of denitrification, i.c., of nitrate or nitrite dissimilation, or, in the terms of EGA:MI (TAKAIIAsm ct al. 1963), of "nitrate or nitrite respiration". Thus the lnissing photolithotrophs and chemolithotrophs would both produce N2 frOln NHa. So far only NO;- or NO;- are known as important biotic sources of N2• This is set free in denitrification:

NO;- + 0.75 (CH20) + H+ = 0.5 N2 + 0.75 CO2 + 1.25 H 20; = -95 kcal

The only exception is the production, of uncertain quantitative importance, of N20 from NHa by some aerobic chemoorganotrophs (YOSIIIDA and ALEXA.NDER 1970); the N20 further yields, abiotically, N2 (JOIINSTON 1972). Apart from this "N20 by- Two missing lithotrophs 493 pass", the biotic pathway from NH3 to N2, reversing the fixation of atmospheric N 2, must take the detour via nitrification. This is, or was, not true if the "missing lithotrophs" here put forward exist, or existed.

A cknowledgement I lilie to thank Dr. G. A. PESOHEK for discussions.

Addition in proo!

An extensive survey of the role of N 2ü in the atmosphere has now been given by HAHN and JUNGE (1977).

References BRODA, E., 1975a. Thc Evolution of the Bioenergetie Processes. Pergamon Press Oxford. BRODA, E., 1975b. The history of inorganie nitrogen in the biosphere. J. klol. Evol.,i, 87 -100. FRO:i\IAGEOT, C. and SEXEZ, J. C., 1960. Aerobic and anaerobie reaetions of inorganie substanees. In: Comparative Bioehemistry, Vol. 1, 347 -409 (:i\1. FLoRKm and H. S. 1\!Asox, Editors). Aea- demic Press New York. HAHN, J. and JUNGE, C., 1977. Atmospherous nitrous oxide: a critical review. Z. Naturforsch., 32a, 190-214. JOHNSTON, H., 1972. Newly recognized vital nitrogen cycle. Proc. nat. Acad. Sei. Wash., 69, 2369-2372. RUTTEN,:M. G., 1971. The Origin of Life by Natural Causes. Elsevier Amsterdam. TAKAHAsm, H., TAXIGUCm, S. and EGA:ilII, F., 1963. Inorganie nitrogen cOlllpounds: Distribution and llletabolislll. In: COlllparative Biochclllistry, Vol. 5, 92-202 (kL FLoRKm and H. S. kIASON, Editors). Acadelllic Press New York. YosmDA, T. and ALEXAXDER, M., 1970. Nitrous oxide formation by Nit1'Osomonas europea and heterotrophie organisllls. Soil Science Alller. Proe., 34,880-882.

Mailing address: Prof. Dr. E. BRODA Institute of Physical Chemistry, University Währinger Straße 42 A-I090 Wien, Austria