Studies on the Mineral Content of Calcareous Algae

CORAL REEF PROJECT-PAPERS IN MEMORY OF DR. THOMAS F. GOREAU. 7.

STUDIES ON THE MINERAL CONTENT OF CALCAREOUS ALGAE

E. L. B6HMl Department of Physiology and New York Zoological Society Coral Reef Project, University of the West Indies, Kingston, Jamaica

ABSTRACT The mineral content has been determined in a total of 16 species from four genera of calcareous algae all belonging to the family Udoteaceae. The most heavily calcified species are those belonging to the genus Hali- meda. Udotea is the least calcified genus. Rhipocephalus and Penicillus take intermediate positions. The maximum mineral content expressed in percentage of the algal dry weight is as follows: Halimeda, 97 per cent; Penicillus and Rhipocephalus, approximately 60 per cent; and Udotea, 37 per cent. Within the genus Halimeda, the maximum mineral content varies with species. The approximate figures are: Halimeda gracilis and H. copiosa, 97 per cent; H. opuntia and H. discoidea, 90 per cent; H. tuna, 80 per cent; H. goreauii, 50 per cent; and H. cuneata, 33 per cent. In some species of Halimeda, there is a tendency for the mineral content to increase with depth. The MgCOg concentration varies with genus and species, and within different parts of the same organism. In Halimeda, Penicillus, and Rhipo- cephalus, MgC03 is in the region of 0.2-3.0 mol %. In Udotea, MgC03 tends to be higher and in the range 3-4 mol %. In most algae investigated, the MgCOg concentration was found to be higher in the younger, scarcely calcified segments in the apical region than in the fully developed, more heavily calcified segments farther below. Juvenile individuals of Udotea and Penicillus also contain more MgCOg than adult individuals. Halimeda cuneata is unusual, in that the apical segments are more heavily calcified than the fully developed segments near the branching points. A comprehensive analysis of all major cations and anions is given for the alga Halimeda opuntia.

INTRODUCTION The elementary chemical composition of skeletal carbonates deposited by calcareous algae has been determined by several authors. Collections of these data (Vinogradov, 1953) provide many important criteria for the biology and paleontology of these plants and are of great value in the study of the geology and geochemistry of carbonate minerals. The work reported here developed while investigating the influence of ecological factors on the degree of algal calcification. Field observations had suggested that the genus Halimeda is more heavily calcified in deeper

1 Present address: Department of Biochemistry, University of Cape Town, Cape Town-Rondebosch, South Africa. 178 Bulletin of Marine Science [23(2) water (Goreau, 1963). In order to test this hypothesis, Halimedas and other algae were collected over a wide range of depth and the mineral content was determined. Results are presented which support the hypothesis. In addition, it is shown that the MgC03 concentration varies in different regions of the alga.

ACKNOWLEDGMENTS Certain sections of this project have been included in a thesis presented to the University of the West Indies in partial fulfillment of the require- ments for the degree of Ph.D. I am deeply indebted to my supervisor the late Professor T. F. Goreau for support and guidance. I also wish to thank Dr. L. S. Land for criticism and for the analysis of controls. My sincere thanks are due to Miss E. A. Graham and Dr. S. S. Seagrief for algal collections, Miss C. Spence and Me. E. S. F. Lee for expert assis- tance, Dr. M. J. Orren for AAS-measurements, and Drs. R. F. Nigrelli and E. A. Kean for research facilities. Financial support from the University of the West Indies, the New York Zoological Society, and USNSF-grant GB-350 is also gratefully acknowledged.

MATERIALS AND METHODS Algal callections.-Except for Halimeda cuneata, all algae were collected in Jamaica, West Indies, during the month of August. Specific localities are given in the tables. The surface temperature of the water in Jamaica was between 26.8° and 27.7°C. Halimeda cuneata was from Umnagazi Mouth near Port St. John's, South Africa, and Riet River, near Port Alfred, South Africa. The algae were found in shallow water approximately 3 feet in depth in the littoral region, where they occur with Caulerpa {ilifarmis and Hypnea spicifera (Seagrief, pers. commun.). The algae were collected in summer, during the months of January and February. The water temperature was 19°C at Riet River, and 23°C at Umnagazi Mouth. All contaminants, such as encrusting algae, small molluscs, and Foraminif- era, were removed with fine tweezers. The plants were washed in filtered sea water and distilled water. The air-dried algae were ground to a fine powder, and further dried in vacuo over P205 to constant weight.

Determination of Cations.-Ca and Mg were determined by differential titration with EDTA, using caIconcarboxylic acid (Merck) and Eriochrom- black T (indicator buffer tablets, Merck) as indicators (Anonymous). The titrations were carried out at pH 10 (Ca + Mg) and pH 12.5 (Ca) in the presence of hydroxylamine hydrochloride, potassium cyanide, and sodium potassium tartrate, to avoid interference with iron and other heavy metals (Bisque, 1961). In the ecological study, Ca was estimated gravimetrically as the oxalate (Goreau, 1963). In Halimeda cuneata, Mg was determined 1973] Bohm: Mineral Content of Calcareous Algae 179 by AAS on a Perkin Elmer Model 303 instrument. Lanthanum chloride was added to suppress the lines of Na and Ca. Sr was determined by flame photometry on a Hitachi-Perkin Elmer Model 139 spectrophotometer. In- terference with Ca and other ions was corrected for by the method of standard additions. Na and K were determined on an EEL model A flame photometer. The influence of interfering emitters was eliminated by the standard addition method. Determination of Anions.-In the ecological study, carbonate was estimated gravimetrically as BaC03 (Goreau, 1963). In all subsequent analy- ses, carbonate was determined as CO2 at constant pressure, by a volumetric method adapted from the technique of Umbreit, Burris, & Stauffer (1957). The mol volume of CO2 was taken as 22.246 (Glass tone, 1946). Chloride was determined as AgCl (Vogel, 1961). Sulfate was precipitated as benzi- dine sulfate and determined photometrically by the method of Letanoff & Reinhold (1936). Silicate was determined gravimetrically as Si02 after wet oxidation in HCI04 and Na2Cr04 as catalyst. From the insoluble resi- due, the Si02 content was obtained from the weight loss after vaporization of SiF4 with concentrated H2S04 and an excess of HF. Phosphate was determined after precipitation of Si02 with HCI04 and extraction of the phos- pho-heteropolymolybdic acid complex with amyl alcohol as molybdenum blue by a photometric method (Strickland & Parsons, 1965).

RESUL TS AND DISCUSSION Elementary Composition and Degree of Mineralization.-The maximum total mineral matter in the dry weight of the algae was found to be as follows: Halimeda, 97 per cent; Penicillus, 60 per cent; Rhipocephalus, 57 per cent; and Udotea, 39 per cent (Fig. 1 and Table 1). Chlorodesmis was not investigated. The exact position of Rhipocephalus is somewhat uncer- tain, because only two species were tested. It appears that algae belonging to the genus Halimeda are the most heavily calcified, and that the least calcified calcareous algae are those belonging to the genus Udotea. When the same considerations are applied to the genus Halimeda alone, Halimeda gracilis and Halimeda copiosa rank at the top, with a maximum mineral content of about 97 per cent. Next come Halimeda opuntia and Halimeda discoidea, with approximately 90 per cent, followed by Halimeda tuna, near 80 per cent, Halimeda goreauii, in the region of 50 per cent, and fi- nally Halimeda cuneata, in the vicinity of 30 per cent (Fig. 1 and Table 1).

The change of the CaC03 content in relation to depth in seven species of Halimeda is illustrated in Figure 1. Generally speaking, the findings are in agreement with earlier observations (Goreau, 1963) and suggest that these species of Halimeda tend to be more heavily calcified in deeper than 180 Bulletin of Marine Science [23(2) ,., Halimeda opuntia Halimeda discoidec 0 100 M 100 U u0 9S ~ 9' .>i! .- 90 - 90 - 8' S 8' S 80 80

7' 90 '5 ! IS 120 180 .71 lIJ

Halimeda simulans 100 ------100 Halimeda ~p':ioso - 9' .-- 9S - 90 - 90 S 8' 8S

80 80

,. 75 100 50 100

100 ~gracilis '00 ~lunQ - 95 95 - - 90 - 90

85 85

75 75 50 70 flO deplhCfeet) d.plh (feet)

FIGURE 1. CaCOa content in relation to depth in some species of Halimeda from Jamaica. The localities were as follows: H. opuntia, 2 ft, Lime Cay; 90 ft, Mangrove Point; H. simulans, 6 ft, South Cay; 100 ft, Eaton Hall; H. gracilis, 4 ft, North East Cay Pedro; 50 ft, South Cay Trench; 70 ft, Maria Buena Bay; 170 ft, Maria Buena Bay; H. discoidea, 15 ft, South Cay Trench; 120 ft, Maria Buena Bay; 180 ft, Maria Buena Bay; H. copiosa, 50 ft, South East Cay; 100 ft, Runaway Bay; H. tuna, 3 ft, Duncans, San-San; 150 ft, Duncans; H. goreauii, 4 ft, North West Cay Pedro; 6 ft, South Cay; 15 ft, Duncans; 100 ft, Runaway Bay; 150 ft, Maria Buena Cay. The number of collections analyzed is given in each column. 1973] Bohm: Mineral Content of Calcareous Algae 181 in shallower water. The effect seems to be most pronounced in Halimeda discoidea, but less so in Halimeda opuntia, Halimeda tuna, Halimeda gracilis, and Halimeda elongata. Halimeda discoidea clearly differs from thc other species, in that the degree of calcification in plants from shallow- water habitats is very low and that the CaC03 content increases sharply with depth (Fig. 1). Our finding of 47.4 per cent CaC03 (Table 1) in the shallow-water col- lection of Halimeda discoidea from Jamaica is in agreement with a recent report giving 47.8 ± 3.65 per cent CaC03 for the same species collected at the 8- to 12-foot level in Puerto Rico (Stark, Almodovar & Krauss, 1969). However, the assumption that Halimeda discoidea is less heavily calcified than Halimeda opuntia (Stark et al., 1969) seems to apply only to shallow- water populations. The results presented here suggest that Halimeda discoidea tends to be more heavily calcified in deeper water, and that the average CaCOa content of Halimeda discoidea and Halimeda opuntia in deeper water is approximately the same (Fig. 1 and Table 1). Two algae which seem to deviate from the general pattern are Halimeda goreauii and Halimeda simulans. In Halimeda goreauii, there is a distinct decrease of the mineral content with depth. In Halimeda simulans, the change of the mineral content with depth is rather small, and it is not possible to say whether this alga follows the general trend or is an excep- tion and must be grouped with Halimeda goreauii. Attempts to explain the observed trends met with difficulties. The algae were collected during midsummer in August, a time when temperature, il- lumination, and transparency of the water are optimal, and the algae are in a prolific state of growth. Variations in the average age may occur, because Halimedas grow in bursts which are often separated by more-or-Iess pro- longed quiescent periods (Goreau, 1963). Since it is not known when these bursts of growth occur, how frequent they are, and by which factors they are controlled, it may be that at certain times surface populations are quiescent, whereas at greater depth the same species are in an active state of growth. The state of growth must have an effect on the mineral content, because the number of less heavily calcified segments in the apical region of the alga is higher during active growth than during quiescence. Assum- ing that the frequency of bursts of growth is the same over the entire depth range, it may be that the rate of tissue growth and the rate of skeletogenesis differ. Recent investigations have indicated that the rate of photosynthesis in calcareous algae is greatly reduced at lower intensities of light, whereas the rate of calcification is much less affected and may be very high, even in complete darkness (Goreau, 1963). The poor response of algal calcification to changes of light intensity is illustrated by the fact that the light-to- dark calcification ratios do not deviate much from unity. Goreau (1963), using eight species of Halimeda, came to a value of 1.4. Stark et al. (1969), 182 Bulletin of Marine Science [23(2)

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turnover and Ca net accretion). In Halimeda opuntia, CO2 has been found to stimulate the incorporation of calcium (Stark et al., 1969). The effect of CO2 on photosynthesis was not determined. Since the CO2 content of sea water usually increases with depth, it is within the realm of possibility that these changes are high enough to affect the degree of mineralization. Chloride and the Alkalines.-Na, K, and CI were determined in three samples of Halimeda opuntia (Table 3). Except for Sample III, occluded sea water was not removed. The actual content of Na and Cl in samples I and II can therefore be expected to be slightly lower. The concentration of the remaining elements of sea water is so small as to have little influence on the results. As expected, there was cytoplasmic enrichment of K. This is re- flected by the Na/K weight ratios, which amount to 17.5, 16.5, and 13.5 in samples I, II, and III, respectively, and are considerably lower than in sea water (27.6). Magnesium.-Assuming that all Mg found in the algal dry matter repre- sents MgCOa, the proportion of MgCOa in the skeleton of Halimeda is in the region of 0.2-3.0 mol %, but may attain higher concentrations in young and partially calcified segments at the apex (Tables 1 and 2). In Halimeda opuntia, H. simulans, H. tridens, and H. manUe, the concentration of MgCOa has previously been given to be in the region of 1 per cent (reduced analysis: Clarke & Wheeler, 1922). This corresponds to 1.2 mol %. More recent publications indicate 5.8 mol % MgCOa for Halimeda tuna (Prat & Hamackova, 1946),0.3-0.4 mol % MgCOa for Halimeda opuntia, and 2.2 mol % MgCOa for Halimeda discoidea (Stark et al., 1969). In Penicillus and Rhipocephalus, MgC03 has here been found to be between 1.6 and 2.5 mol % and to be in the same range as in Halimeda. In Udotea, MgCOa is shown to be slightly higher and in the region of 3-4 mol % (Table 1). That the MgCOa concentration in algal carbonates varies is generally accepted for calcitic algae, where it has been shown that the Mg content changes with genus and species, with age, and within the same organism 184 Bulletin of Marine Science [23(2)

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Componentt Sample I Sample lit Sample lIlt Na 1.40 1.65 1.35 K 0.08 0.10 0.10 Mg 0.15 0.22 0.25 Ca 33.60 32.40 27.20 Sr 0.80 0.80 0.70 COs 48.40 48.30 40.20 Cl 1.45 1.35 1.00 SO. 0.60 0.60 0.70 PO. 0.01 0.01 0.01 SiOs 0.10 0.10 0.09

$ From shallow water, Rackhoms Cay, Jamaica. t When the components are expressed in mequiv/l00 mg algal dry matter it can be calculated that the three samples show a cation excess which averages to 0.07 mequiv/100 mg. :j: In another laboratory (University of Texas) the following values were found: Sample II-Ca: 32.34%; COs: 48.00% Sample III-Ca : 27.36%; CO. : 40.76%

(Clarke & Wheeler, 1922; Baas-Becking & Galliher, 1931; Johnson, 1961). The temperature of the sea water has also been shown to have an influence on the Mg content (Chave, 1954). Electronmicroprobe images of the red alga Goniolithon have indicated that the distribution of Mg in the skeletal material is nonuniform (Schmalz, 1965). In the red algae A mphiroa and Goniolithon, evidence has also accumulated which suggests that the Mg concentration varies across the more recently deposited growth layers and that the fluctuations diminish in older layers (Land, pers. commun.). The results reported here for aragonitic algae show that the Mg concentration varies with genus and species, although to a lesser extent than in the calcitic algae (Table 1). In addition, there are indications that the Mg content in aragonitic algae changes during development. Young and partially calcified segments in the apical region have often been found to contain more Mg than the more heavily calcified segments near the branching points and in the basal part of the alga (Table 2). In juvenile individuals of Udotea and Penicillus, the MgCOs content was also higher than in adult algae (Table 2). The trend of the MgCOs concentration to decrease while the degree of mineralization increases is not shown by the two species of Rhipocephalus. The deviation may be explained on the basis that the mineral content in the juvenile and adult individuals was not much different. Therefore, no great difference in the MgCOs content may be expected. Halimeda cuneata was unusual, in that the apical segments contained between 32.4 and 34.6 per cent CaCOs and more mineral mass 186 Bulletin of Marine Science [23(2) TABLE 4 MgC03 CONTENT IN SAMPLES OF SEDIMENT FROM DISCOVERY BAY, JAMAICA, W.I.

Depth (ft) mol % MgC03 75 2.5-3.6 100 2.3-3.2 125 1.3-2.0 150 1.8-3.2 175 1.6-1.7 190 0.9-1.1 200 0.8-0.9 225 1.0-1.6 250 1.2-1.6

than the segments near the branching points. Here the CaCO;j content was lower and between 20.5 and 23.5 per cent (Table 2). All algae analyzed were in excellent condition and there was no indication of waning in any parts of the algae. However, in the more fully developed segments below the apical meristem, the filamentous network was invariably fleshier, thicker, and denser. Such a pronounced proliferation of filaments was never observed in any of the West Indian species. In Halimeda ClIneata it therefore seems that the decrease of the mineral content towards the branching points is the result of a relative increase of the filamentous over the skeletal tissues. In the fully developed segments near the branching points and in the basal part of the alga, the MgC03 concentration was 5.1- 5.2 mol %, as opposed to 2.3-2.6 mol % in the apical, more heavily calcified segments. Whether depth has an effect on the MgC03 concentration is difficult to assess, because the degree of mineralization varies with depth (Fig. 1). In sediment samples, which were collected from the top layer of sediment between 75 and 225 feet below the surface of the water, and which consisted

mainly of fragments of Halimeda, it was found that the MgC03 concentration is between 0.8 and 3.6 mol %, showing a trend to decrease with depth (Table 4). However, not much significance can be attached to this finding, because the exact composition of the sediment samples with respect to genus and species was not known. Further complications arise from the fact that most of these sediments originate in shallow water and are transported into deeper regions by various mechanisms (Goreau, 1964). The variation of the MgC03 concentration in aragonitic algae and the finding that the MgC03 concentration often exceeds 1 mol %, generally accepted as the upper limit for aragonites, raises the question whether it is justified to assume that all magnesium was present as carbonate. X-ray powder diffraction patterns (Cu-Kll') of Halimeda opuntia (three samples, 1973] Bijhm: Mineral Content of Calcareous Algae 187 Table 3) and Halimeda cuneata (four samples; asterisk, Table 2) showed the typical aragonite reflections at 26.20 and 27.2°, 2 6. In two samples of Halimeda cuneata (5 mol % MgCOa) and one sample of Halimeda opuntia (1.1 mol '7"0 MgCOa), halite (NaCI) reflections at 31T and 45S, 2 6, were also seen. This indicates that contamination by residual sea water cannot be excluded in all cases. One sample of Halimeda cuneata (5.2 mol % MgCOa) and another sample of Halimeda cuneata (2.5 mol % MgCOa) also showed a weak, broad reflection between 29.4° and 29.6°, 2 6. This almost certainly is the high-intensity reflection for calcite. Since the halite reflections were not present in all cases, and since there was no correlation between the intensity of the halite reflections and the Mg content, the influence of residual sea water cannot be ascertained. Had MgCl2 from sea water contributed to a significant extent, one would have expected the Mg content to decrease along with the chloride content. Table 3 shows that the opposite was the case. Calcite, either as a distinct phase, or as a contaminant, is a possible source for MgC03. Contributions from chloro- phyll and the mucilages are too low to playa role. Since low concentrations of MgCOa in CaC03 cannot be estimated directly as MgCOa, some element of doubt must remain. In connection with algal calcification, a variation of the MgCOa concentration in the skeleton could be ascribed to changes of the remodelling activity. This interpretation is in agreement with the hypothesis that the skeleton consists of exchangeable and nonexchangeable regions (Goreau, pers. commun.). Evidence for active mineral exchange and the possibility of remodelling activity has been obtained for the calcareous alga Halimeda opuntia, in which it has been shown that algal calcification involves high turnover of calcium and low net accretion of calcium (Bohm, 1968, 1969; Bohm & Gareau, in press).

Strontium.-Strontium, when expressed as per cent SrC03 in the total skeletal matter (reduced analysis), is 1.58 per cent, 1.35 per cent, and 1.18 per cent for samples I, II, and III, respectively (Table 3). These results compare well with the figure of 1.3 per cent SrC03, which has previously been given for Halimeda (Emery, Tracey & Ladd, 1954). The SrjCa atomic ratios (atoms Sr per 1000 atoms Ca) have been found to be 10.89, 11.29, and 11.77 for samples I, II, and III, respectively (Table 3). These figures fall into the range 11-13 given for aragonitic algae (Odum, 1950, 1957). The high ratio of 11.77 found in young and poorly calcified segments (Sample III) may be an indication that the proportion of Sr in the algal skeleton also changes during development. The Anions.-Most of the algae were analyzed for carbonate only. In Halimeda opuntia, chloride, sulfate, phosphate, and silicate were also de- 188 Bulletin of Marine Science [23(2) termined (Table 3). The carbonate results are interesting, from the point of view that there seems to be a stoichiometric imbalance between carbonate and the carbonate-binding cations. The imbalance is also apparent when the sum of the cations and the sum of the anions are considered, and has been found to be 0.07 milliequivalents per 100 mg algal dry matter (Bohm, 1972b). The equivalents of chloride and sulfate largely balance with the alkalines. Sulfate has been found to be slightly higher than given in a previous report (Clarke & Wheeler, 1922). Phosphate has been found to be at at trace level and in the order of 0.01 per cent. This is in agreement with the results of Clarke & Wheeler (1922), although no lower limit has been given by these workers. In another publication, the phosphorus content in

Halimeda opuntia has been given as 0.14 ± 0.03 per cent P20ii (Stark et al., 1969). This equals 0.09 ± 0.02 per cent P04 and is somewhat higher than the value given here. The concentration of silicate has been found to be in the region of 0.1 per cent. This is less than the value communicated by Clarke & Wheeler (1922), who give 0.4 per cent Si02 for Halimeda opuntia. Their high value could be due to diatoms, which have always been found to be associated with the algae, and which are so inconspicuous that they are often not noticed unless the specimen is examined more closely under the micro- scope. Silicate has been found to be associated with two organic fractions of the algae (Bohm, 1972a).

SUMARIO

ESTUDIOS DEL CONTENIDO MINERAL EN ALGAS CALCAREAS

EI contenido mineral expresado como la suma de CaC03 Y MgCOa ha sido determinado en siete especies de Halimeda, dos especies de Rhipo- cephalus, tres especies de Penicillus y cuatro especies de Udotea, todas algas pertenecientes a la familia Udoteaceae (Caulerpales, Chlorophyta). Las especies de algas mas calcificadas son las pertenecientes al genero Halimeda. EI contenido mineral maximo en el peso seco de las algas es: Halimeda gracilis y H. copiosa, 97 por ciento; H. opuntia y H. discoidea, 90 por ciento; H. tuna, 80 por ciento; H. goreauii, 50 por ciento, y H. cuneata, 33 por ciento. En algunas especies de Halimeda, hay la tendencia a aumentar el grado de mineralizaci6n con la profundidad. La explicaci6n de esta tendencia probablemente descansa en la relaci6n entre la fotosintesis y la calcificaci6n del alga (transferencia de calcio y aumento neto de calcio).

El contenido de CO2 del agua de mar puede tambien jugar un papel en

6sto. La concentraci6n de MgC03 en Halimeda, Penicillus y Rhipocephalus

esta entre 0.2-3.0 mol %. En Udotea, MgC03 esta entre 3-4 mol %. En ejemplares j6venes y en regiones parcialmente calcificadas de Halimeda 1973] Bohm: Mineral Content of Calcareous Algae 189 opuntia y algunas otras Udoteaceae, la concentraci6n de MgC03 tiende a ser varias veces mayor que en los mas viejos, estando las regiones mas fuertemente calcificadas en la parte inferior de la planta. Halimeda cuneata es poco usual, en que los segmentos superiores estan mas fuertemente calcificados que los que estan cerca de la base de la planta. En la mayoria de las Udoteaceae investigadas, la proporci6n de MgCOa en e1 esqueleto varia con las especies y dentro de las diferentes regiones del organismo. En el alga Halimeda opuntia se han analizado los siguientes componentes adicionales: Na, K, Sr, CI, S04, P04 Y Si02•

LITERATURE CITED

ANONYMOUS 1967. Complexometric assay methods with Titriplex. Third Edition, E. Merck AG, Darmstadt, pp. 26-29. BAAS-BECKING, L. G. M. AND E. W. GALLIHER 1931. Wall structure and mineralization in coralline algae. J. phys. Chern., 35: 467-479. BISQUE, R. E. ] 961. Analysis of carbonate rocks for calcium, magnesium, iron and aluminum with EDTA. J. sedim. Petro!., 31: 113-122. BOHM, E. L. 1968. Aspects of calcification in the calcareous alga Halimeda opuntia (L) (Chlorophyta, Udoteaceae). UNESCO symposium on investigations and resources of the Caribbean Sea and adjacent regions. Wil- lemstad, Curac;:ao, November 1968. F.A.O. Fish. Rep., No. 71.1, Rome 1969, No. 3.16: 135. 1969. Calcification in the calcareous alga Halimeda opuntia (L) (Chloro- phyta, Udoteaceae). Thesis, University of the West Indies, Kingston, Jamaica, 132 pp. 1972a. Concentration and distribution of AI, Fe and Si in the calcareous alga Halimeda opuntia (L) (Chlorophyta, Udoteaceae). Int. Revue ges. Hydrobio!., 57: 631-636. 1972b. Cation-anion balance in some calcium carbonate depositing algae and the detection of organic calcium fractions in the calcareous alga Hali- meda opuntia (L) (Chlorophyta, Udoteaceae). Int. Revue ges. Hy- drobio!., 57: 685-693. BOHM, E. L. AND T. F. GaREAU In press. Rates of turnover and net accretion of calcium and the role of calcium binding polysaccharides during calcification in the calcareous alga Halimeda opuntia (L). Int. Revue ges. Hydrobiol., 58. CHAVE, K. 1954. Aspects of the biogeochemistry of magnesium. I. Calcareous marine organisms. J. Geo!., 62: 266-283. CLARKE, F. W. AND F. C. WHEELER 1922. The inorganic constituents of marine invertebrates. Prof. Pap. U.S. geol. Surv., No. 124: 1-62. EMERY, K. 0., J. I. TRACEY, AND H. S. LADD 1954. Geology of Bikini and nearby atolls. Prof. Pap. U.S. geo!. Surv., No. 260-A. 190 Bulletin of Marine Science [23(2)

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