DIVERSITY OF HALOGENATED SECONDARY METABOLITES IN THE RED ALGA LAURENCIA NIPPONICA (RHODOMELACEAE,CERAMIALES)1,2

Many morphologically similar, but chemically distinct, populations have been found in the marine red alga Laurencia nipponica Yamada (Rhodomelaceae, Ceramiales) growing in Japan. Each chemical type is characterized by a specific end-product of halogenated secondaly metabolite synthesis: chamigrane-type sesquiterpenoids such as prepacifenol and halochamigrene epoxide and C₁₅ bromoethers such as laurencin, laureatin, isoprelaurefucin, epilaurallene, and kumausallene. These seven types of secondary metabolite syntheses remained the same in the wild and under various culture conditions. Because bromoethers and terpenoids are probably synthesized by different metabolic pathways, it is virtually certain that different sets of enzymes participate in their synthesis. Prepacifenol- and laureatin-producing populations were selected as representatives of terpenoid and bromoether groups, respectively. F₁ tetrasporophytes derived from crosses between reciprocal, female and male gametophytes of prepacifenol- and laureatin-producing strains bore both types of metabolites, suggesting that the genes Producing these enzyme systems are encoded by nuclear genomes. The F₁ gametophytes resulting from the reciprocal crosses produced either prepacifenol or laureatin, and the four individuals derived from spore tetrads (a set of tetraspores derived from a single tetrasporangium) produced either prepacifenol or laureatin in a 1:1 ratio, indicating that genes participating in terpenoid synthsis and those participating in bromoether synthesis are on different loci of homologous chromosomes and are segregated at meiosis (tetrasporogenesis). One individual of this interpopulational F₁ gamtophyte produced both parental types of metabolite, perhaps indicating the occurrence of a recombination type. Natural hybrid individuals, including such recombination-type gametophytes, were found in a sympatric locality at which these two chemical types occur. F₁ tetrasporophytes derived from crosses between respective prepacifenol- and laureatin-producing strains and their F₁ gametohytes produced only parental-type metabolite-producing plants. These results indicate that the diverse chemical types can be referred to as races (chemical races).     

Is the tetrasporophyte of Asparagopsis armata (Bonnemaisoniales) limited by inorganic carbon in integrated aquaculture?1

Seaweeds cultivated in traditional land‐based tank systems usually grow under carbon‐limited conditions and consequently have low production rates, if no costly artificial source of inorganic carbon is supplied. In integrated aquaculture, the fish effluents provide an extra source of dissolved inorganic carbon (DIC) to seaweeds due to fish respiration. To evaluate if the tetrasporophyte of Asparagopsis armata (Harv.) F. Schmitz (the Falkenbergia stage) is carbon limited when cultivated with effluents of a fish (Sparus aurata) farm in southern Portugal, we characterized the DIC forms in the water, assessed the species photosynthetic response to the different DIC concentrations and pH values, and inferred for the presence of a carbonic anhydrase (CA)–mediated mechanism. Results showed that A. armata relies mainly on CO₂ to meet photosynthetic needs. Nevertheless, from pH 7.5 upward, the CO₂ supply to RUBISCO seems to derive also from the external dehydration of HCO₃^– mediated by CA. The contribution of this mechanism was essential for A. armata to attain fully saturated O₂‐evolution rates at the natural seawater DIC concentration (2–2.2 mM) and pH values (～8.0). We revealed in this study that seaweeds cultivated in fish‐farm effluents benefit not only from a rich source of ammonia but also from an important and free source of DIC for their photosynthesis. If supplied at relatively high turnover rates (～100 vol · d^?1), fish‐farm effluents provide enough carbon to maximize the photosynthesis and growth even for species with low affinity for HCO₃^–, avoiding the artificial and costly supply of inorganic carbon to seaweed cultures.     

CHANGES IN NUTRIENT CONTENT OF PALMARIA PALMATA IN RESPONSE TO VARIABLE LIGHT AND UPWELLING IN NORTHERN SPAIN1

Light has been identified as one of the main factors affecting seaweed ecophysiology. We investigated the dependence of nutrient metabolism on sun and shade light conditions and whether episodes of upwelling of nutrient‐rich subsuperficial water could reduce the summer nutrient limitation driving physiological changes in Palmaria palmata (L.) Kuntze. We measured the major nutrient pools, photosynthetic pigments, and light curves, under sun and shade conditions during a summer period when one upwelling was recorded. The redundancy analysis (RDA) produced two clear groups: sun‐ and shade‐acclimated algae. Light was the major predictive factor. Sun‐acclimated algae exhibited higher carbon (C) and lower nitrogen (N) and phosphorus (P) content in association with the storage of floridoside (main C reserve) to benefit from higher irradiance (under nutrient limitation). Among N pools, N reserves (phycoerythrin, nitrate) were a lower proportion of the total N in sun‐acclimated algae, suggesting their degradation to fulfill the N demands of the cell. The orthophosphate content was also lower in sun‐acclimated algae, indicating its utilization as a nutrient reserve. In contrast, N within cell walls and membranes and chl a contributed to a similar proportion of the total N in sun‐ and shade‐acclimated algae, suggesting a response to sustain cell integrity. Transient high nutrient concentration due to the upwelling was unrelated to the nutrient content of the thallus. The storage of C as floridoside from high light exposure was shown to be the driving force for the metabolic adjustment of P. palmata at the end of summer before the onset of dormancy.     

STRUCTURE AND DISTRIBUTION OF GLUCOMANNAN AND SULFATED GLUCAN IN THE CELL WALLS OF THE RED ALGA KAPPAPHYCUS ALVAREZII (GIGARTINALES, RHODOPHYTA)

Two new polysaccharides were isolated from the cell walls of the carrageenan producing red seaweed Kappaphycus alvarezii (Doty) Doty. They were characterized by chemical analyses, enzymatic degradations, and nuclear magnetic resonance spectroscopy. One was a 4.0 M NaOH soluble β-(1,4)- d -glucomannan that mostly precipitated upon neutralization and dialysis. It was composed of about 82 residues, and 70% of its glucose and mannose were released by a commercial cellulase enzyme complex. The disaccharide β- d -Man (1→4) d -Glc was recovered from the hydrolysate during the first hours of degradation and confirmed the chemical structure of the polysaccharide. The other polysaccharide was extracted with 1.5 M NaOH and was identified as a sulfated glucan of degree of polymerization of about 180 1,4-linked β-glucose containing 10% 1,3-linkages. The sulfate was located on C-6 of 64% of the 4-linked glucose residues. A third alkali-soluble polysaccharide rich in galactose was also detected. The distribution of the glucomannan and galactose containing polysaccharides was inversely related to the algal cell size. Potential functions of these alkali-soluble polymers are discussed in the context of cell wall polysaccharide assembly.     

CARBOXYSOME CONTENT OF SYNECHOCOCCUS LEOPOLIENSIS (CYANOPHYTA) IN RESPONSE TO INORGANIC CARBON1

The carboxysome content of chemostat grown Synechococcus leopoliensis (Racib.) Komarek increases under inorganic carbon limitation. At growth rates of ca. 85%μ_max the carboxysome content (±SE) was 0.57 ± 0.09 carboxysomes·cell section^?1. Under severe carbon limitation (ca. 13%μ_max) this increased to 3.4 · 0.3 carboxysomes·cell section^?1. Corresponding to this change is a three order of magnitude decrease in the half-saturation constant of photosynthesis for dissolved inorganic carbon. Nitrogen and phosphorus limitation had no effect on carboxysome content or the kinetics of photosynthesis with respect to inorganic carbon. These results are discussed in light of the apparent lack of photorespiration in these organisms.     

EFFECT OF NUTRIENT DEPRIVATION AND RESUPPLY ON METABOLITES AND ENZYMES RELATED TO CARBON ALLOCATION IN GRACILARIA TENUISTIPITATA (RHODOPHYTA)1

The starch content of red algae normally increases during nitrogen limitation. Based on this we hypothesized that nutrient deprivation would result in an increased activity of starch‐synthesizing enzymes and a decrease in the activity of starch‐degrading enzymes, with the opposite scenario when nutrients were sufficient. We therefore examined the effect of the nutrient status of Gracilaria tenuistipitata Chang et Xia on the content of starch and floridoside and on the activity of enzymes involved in the allocation of carbon into starch, floridoside, and agar; floridoside phosphate synthase and α‐galactosidase involved in synthesis and degradation of floridoside; starch synthase and starch phosphorylase involved in the metabolism of starch; uridine 5′‐diphosphate (UDP)‐glucose pyrophosphorylase; adenosine 5′‐diphosphate‐glucose pyrophosphorylase; UDP‐glucose 4‐epimerase; and phosphoglucomutase. During the period of nutrient limitation the starch and floridoside content increased, as did dry weight and C/N ratio, whereas growth rate and protein content decreased. A general decrease in the enzyme activities during nutrient limitation was also observed, indicating a decrease in overall cellular metabolism. The addition of nutrients caused an increase in enzyme activities and a decrease in the contents of starch and floridoside. Of the enzymes examined, only the activity of UDP‐glucose pyrophosphorylase increased during nutrient limitation and decreased abruptly after nutrient addition. This implies a regulatory role for this enzyme in the supply of UDP‐glucose for starch synthesis. It also supports our suggestion that UDP‐glucose is the substrate for starch synthesis in red algae. This assertion is further strengthened by the observation that of the potential starch synthases only the UDP‐glucose starch synthase could support the observed rate of starch synthesis.     

THE ACCLIMATED RESPONSE OF GROWTH,PHOTOSYNTHESIS, COMPOSITION,AND CARBON BALANCE TO TEMPERATURE IN THE PSYCHROPHILIC ICE DIATOM NITZSCHIA SERIATA1

Nitzschia seriata Cleve, a common member of marine bottom ice communities in the Arctic, was grown in unialgal batch cultures to test for compensatory mechanisms for the low temperatures (?1.8° C) typical of its natural habitat. The upper lethal limit for growth was between 12° and 15°C, and the optimum was between 6° and 12° C. The Arrhenius function adequately (R²= 73%) fitted the relationship between growth rate and temperature from – 1.6° up to 10° C, with an average Q₁₀ of 1.9 over the entire range. Light-saturated and light-limited rates of photosynthesis (normalized to chlorophyll a or cell carbon) showed complete compensation from 12° to 4° C. Photosynthetic rates, especially at light saturation, declined rapidly at temperatures below 4° C. Susceptibility to photoinhibition was greatest at the lowest growth temperatures. Cellular composition (chlorophyll a, protein, polysaccharide, and lipid contents) was not systematically related to temperature in any simple way, although cell size (carbon per cell) was maximal at the lowest growth temperature. Dark respiration was unmeasurably low (<0.015 day^?1) at all growth temperatures. The strategy of adaptation in N. seriata may be characterized as optimizing efficiency and compensation, rather than maximization, of growth rate.     

PICOPHYTOPLANKTON BIOMASS IN RELATION TO LAKE TROPHIC STATE AND THE TN:TP RATIO OF LAKE WATER IN JAPAN1

Picophytoplankton biomass and its contribution to total phytoplankton biomass were investigated in relation to the nutrient concentration and total N: total P ratio of the epilimnetic waters of 42 Japanese lakes during the warm season in 1991 (April–October). Picophytoplankton biomass (as chlorophyll a) in meso-, eu-, and hypertrophic lakes was significantly higher than those observed in oligotrophic lakes. However, picophytoplankton biomass increased significantly with increased total P concentrations in all systems excluding hypertrophic lakes. Picophytoplankton contribution to total chlorophyll a content was significantly higher in oligo- and mesotrophic lakes than in eu- and hypertrophic lakes and was inversely correlated with total P concentrations in lake water. Picophytoplankton contribution to the total phytoplankton biomass was positively (r = 0.54, n = 42, P = 0.0003) correlated with the total N: total P ratio of lake waters. Each lake trophic type, with the exception of hypertrophic lakes, showed this trend, although the correlation was not significant. We suggest that picophytoplankton contribution is influenced by the total N: total P ratio rather than lake trophic state; however, picophytoplankton were of little importance in hypertrophic lakes.     

HIERARCHICAL SPATIAL STRUCTURE AND DISCRIMINANT ANALYSIS OF GENETIC DIVERSITY IN THE RED ALGA MAZZAELLA LAMINARIOIDES (GIGARTINALES, RHODOPHYTA)

Our study of the genetic structure of Mazzaella laminarioides (Bory) Fredericq (Gigartinales) in the central Chilean region documented a high level of genetic diversity based on random amplified polymorphic DNA (RAPD) markers and indicated the occurrence of significant genetic structure at different spatial scales. A total of 288 haploid gametophytes was analyzed with 17 polymorphic RAPD bands, which produced 202 distinct multilocus genotypes. Within stands, mean gene diversity ranged from 0.210 to 0.249 and no significant linkage disequilibrium could be detected among pairs of alleles, revealing that recombination (sexual reproduction) regularly shuffles the genes at that scale. Analysis of molecular variance within stands (less than 30 m) showed that the structure was very low, only marginally significant, and did not increase with increasing hierarchical levels at this lowest spatial scale. In contrast, at a larger spatial scale (among stands, from 5 to 60 km), increasing geographical distance seemed to account for increasing isolation between populations even if natural barriers, such as sandy beaches or river estuaries, may play a role in such isolation. Moreover, the strong genetic differentiation occurring between locations separated by 60 km allowed the assignment of individuals to their original population through a canonical discriminant analysis. This approach further allowed the identification of potential recent migrants from one population to the other. Thus, in species like M. laminarioides for which the dominance of RAPD markers can be avoided by selecting haploid individuals, RAPD analysis appeared to be specially appropriate for the study of genetic differentiation.     

INORGANIC CARBON TRANSPORT IN RELATION TO CULTURE AGE AND INORGANIC CARBON CONCENTRATION IN A HIGH-CALCIFYING STRAIN OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

The relationships among inorganic carbon transport, bicarbonate availability, intracellular pH, and culture age were investigated in high-calcifying cultures of Emiliania huxleyi (Lohmann) Hay & Mohler. Measurement of inorganic carbon transport by the silicone-oil centrifugation technique demonstrated that gadolinium, a potential Ca²⁺ channel inhibitor, blocked intracellular inorganic carbon uptake and photosynthetic ¹⁴CO₂₊ fixation in exponential-phase cells. In stationary-phase cells, the intracellular inorganic carbon concentration was unaffected by gadolinium. Gadolinium was also used to investigate the link between bicarbonate and Ca²⁺ transport in high-calcifying cells of E. huxleyi. Bicarbonate availability had significant and rapid effects on pH_i in exponential- but not in stationary-phase cells. 4′, 4′-Diisothiocyanostilbene-2,2′-disulfonic acid did not block bicarbonate uptake from the external medium by exponential-phase cells. Inorganic carbon utilization by exponential- and stationary-phase cells of Emiliania huxleyi was investigated using a pH drift technique in a closed system. Light-dependent alkalization of the medium by stationary-phase cells resulted in a final pH of 10.1 and was inhibited by dextran-bound sulphonamide, an inhibitor of external carbonic anhydrase. Exponential-phase cells did not generate a pH drift. Overall, the results suggest that for high-calcifying cultures of E. huxleyi the predominant pathway of inorganic carbon utilization differs in exponential and stationary phase cells of the same culture.     

BIOCHEMICAL COMPOSITION AND PHOTOSYNTHETIC CARBON METABOLISM OF NUTRIENT LIMITED CULTURES OF MERISMOPEDIA TENUISSIMA (CYANOPHYCEAE)1

Continuous cultures of Merismopedia tenuissima Lemmerman, limited by phosphorus, nitrogen, sulfur, or carbon, were compared to non limited batch cultures by two methods. The cellular content of photosynthetic pigments (chlorophyll and phycocyanin) was found to decrease in all nutrient limited cultures, except for the carbon limited culture. The ratio of carbohydrate to protein was 4- to 7-fold higher in P, N or S limited cultures than in non-limited or C limited cultures. The macromolecular products of photosynthesis were determined in samples to which NaH¹⁴CO₃ was added. Relative incorporation into protein decreased in P or N limited cultures, increased accumulation of low molecular weight compounds was found in S and P limited cultures, and little change was noted in C limited cultures as compared to non-limited cultures. Although relative incorporation into protein was significantly greater at 20μEin·m^?2·s^?1 light intensity than at 180 μEin·m^?2.s^?1 in non-limited cultures, this effect was abolished in all nutrient limited cultures. These results suggest that measurement of the cellular carbohydrate to protein ratio and the products of photosynthesis would be useful in the analysis of algal population dynamics in nature.     

PHYCOBILIPROTEIN COMPOSITION AND CHLOROPLAST STRUCTURE IN THE FRESHWATER RED ALGA COMPSOPOGON COERULEUS (RHODOPHYTA)1

Serial sections of uncorticated axial cells of Compsopogon coeruleus revealed a single interconnected parietal chloroplast. Phycobilisomes in such chloroplasts were hemidiscoidal in shape with a broad-face diameter of ca. 25–30 nm. The molar ratio of phycobiliproteins in whole cell extracts was IPE:3PC:1APC, similar to isolated phycobilisomes. Two spectrally distinct C-phycocyanin forms (A618 nm, F648 nm and A630 nm, F652 nm) were resolved in dissociated phycobilisomes along with B-phycoerythrin and allophycocyanin.     

CHANGES IN ENDOGENOUS CYTOKININ CONCENTRATIONS IN CHLORELLA (CHLOROPHYCEAE) IN RELATION TO LIGHT AND THE CELL CYCLE1

Endogenous cytokinins were quantified in synchronized Chlorella minutissima Fott et Novákova (MACC 361) and Chlorella sp. (MACC 458) grown in a 14:10 light:dark (L:D) photoperiod. In 24 h experiments, cell division occurred during the dark period, and cells increased in size during the light period. Cytokinin profiles were similar in both strains, consisting of five cis‐zeatin (cZ) and three N⁶‐(2‐isopentenyl)adenine (iP) derivatives. Cytokinin concentrations were low during the dark period and increased during the light period. In 48 h experiments using synchronized C. minutissima (MACC 361), half the cultures were maintained in continuous dark conditions for the second photoperiod. Cell division occurred during both dark periods, and cells increased in size during the light periods. Cultures kept in continuous dark did not increase in size following cell division. DNA analysis confirmed these results, with cultures grown in light having increased DNA concentrations prior to cell division, while cultures maintained in continuous dark had less DNA. Cytokinins (cZ and iP derivatives) were detected in all samples with concentrations increasing over the first 24 h. This increase was followed by a large increase, especially during the second light period where cytokinin concentrations increased 4‐fold. Cytokinin concentrations did not increase in cultures maintained in continuous dark conditions. In vivo deuterium‐labeling technology was used to measure cytokinin biosynthetic rates during the dark and light periods in C. minutissima with highest biosynthetic rates measured during the light period. These results show that there is a relationship between light, cell division, and cytokinins.     

PHOTOSYNTHESIS OF THE RED ALGA GASTROCLONIUM COULTERI (RHODOPHYTA) IN RESPONSE TO CHANGES IN TEMPERATURE,LIGHT INTENSITY,AND DESICCATION1

Previously reported transplantation experiments in the field showed that Gastroclonium coulteri (Harvey) Kylin could survive above its normal intertidal range (0.0–0.5 m above MLLW), except during periods of daytime low tides in spring. Net photosynthetic rate measurements in the laboratory were performed to determine which physical factors might determine the upper boundary for this species in the intertidal zone. Maximum net photosynthesis occurred between 15 and 20° C, but remained positive between 4 and 35° C. The air temperature extremes observed in the field were 2° C (only seen once) and 26° C. Net photosynthesis increased as expected with light intensity to the highest value obtainable in the laboratory, 1400 μEin m^?2 s^?1. Plants collected from the field under higher light intensity (up to 2000 μEin m^?2 s^?2) also showed high rates of photosynthesis. Neither the temperature nor light levels observed in the field were directly damaging to photosynthesis. Desiccation, however, resulted in a sharp decrease in both photosynthesis and respiration. G. coulteri fully recovered from successive daily treatments of about 35% desiccation, but not from successive treatments of 50% desiccation. One exposure to 70% desiccation allowed no recovery of photosynthetic capacity.     

STUDIES ON DEVELOPING AND RELEASED SPERMATIA IN THE RED ALGA,TIFFANIELLA SNYDERAE (RHODOPHYTA)1

Developing and released spermatia of the red alga, Tiffaniella snyderae (Farl.) Abb. were studied. Spermatia were observed under hydrodynamically defined conditions and found to be released from the exposed spermatangial heads in a spermatium-plus-strand unit that remained connected to the spermatangial head. Interactions of single-spermatial strands resulted in the formation of multi-spermatial strands as long as 600 μm with as many as 47 spermatia along their length; however, most were 100–200 μm with 8–21 spermatia. Strand length and number of spermatia were correlated. Spermatial strands contracted or extended and rotated as the water velocity past the plant was changed, and in still water the strands retracted into a clump on the spermatangial head surface. Each strand type exhibited a characteristic threshold water velocity at which it reached maximum length, and above which it broke and was carried away. Fluorescence microscopy showed that the strands did not contain nucleic acid (DNA) and could thus be differentiated from filamentous blue-green algal and bacterial epiphytes. Histochemical staining indicated that the strands and spermatial vesicles contained an acidic, sulfated polysaccharide. Chelation of Ca²⁺ with EGTA resulted in strand breakdown suggesting that this divalent cation may be involved in strand integrity. Scanning electron microscopy revealed that release from the spermatangia occurred through tears in the cuticle covering the spermatangial head if it was still present, or from exposed spermatangia. Individual spermatia were attached tangentially to a well-defined strand 0.64 μm in diameter in the contracted state to 0.2 μm in the extended state. Transmission electron microscopy of spermatangial heads showed that immature spermatangia were characterized by a centrally positioned nucleus and abundant ER cisternae filled with a moderately electron dense granular material. Later in development the spermatangia acquire two spermatial vesicles containing highly convoluted fibrillar contents. The cell becomes polarized with the nucleus displaced apically and the spermatial vesicles occupying the basal half of the spermatangium. At maturity one of the vesicles is released basally. Liberated spermatia contain a membrane-bound nucleus and mitochondria and are associated with an oblong accumulation of fibrous material similar in size and position to the strand observed with the SEM. These strands are discussed in relation to red algal fertilization and other phases of the red algal life-history.     

PHENOTYPIC PLASTICITY INDUCED IN TRANSPLANT EXPERIMENTS IN A MUTUALISTIC ASSOCIATION BETWEEN THE RED ALGA JANIA ADHAERENS (RHODOPHYTA,CORALLINALES) AND THE SPONGE HALICLONA CAERULEA (PORIFERA: HAPLOSCLERIDA): MORPHOLOGICAL RESPONSES OF THE ALGA1

The association between the red macroalga Jania adhaerens J. V. Lamour. and the sponge Haliclona caerulea is the most successful life‐form between 2 and 4 m depth in Mazatlán Bay (Mexican Pacific). J. adhaerens colonizes the rocky intertidal area and penetrates into deeper areas only when it lives in association with H. caerulea. The aposymbiotic form of the sponge has not been reported in the bay. To understand the ecological success of this association, we examined the capacity of J. adhaerens to acclimate in Mazatlán Bay using transplant experiments. The transplanted aposymbiotic J. adhaerens did not survive the first 2 weeks; however, J. adhaerens when living in association with H. caerulea, acclimated easily to depth, showing no sign of mortality during the 103 d of the experiment. We conclude that the ability of J. adhaerens to colonize in deeper areas in this hydrodynamic environment may in part rely on the protection provided by the sponge to the algal canopy. Both species contribute to the shape of the associated form. Nevertheless, the morphological variation in the association appears to be dominated by the variation in J. adhaerens canopy to regulate pigment self‐shading under light‐limited conditions and/or tissue resistance under high hydrodynamics. Consequently, our results are consistent with light as the abiotic controlling factor, which regulates the lower depth distribution of the association in Mazatlán Bay, through limiting the growth rate of J. adhaerens. Hydrodynamics may determine the upper limit of the association by imposing high mass losses.     

GROWTH OF THE FILAMENTOUS GREEN ALGA CTENOCLADUS CIRCINNATUS (CHAETOPHORALES,CHLOROPHYCEAE) IN RELATION TO ENVIRONMENTAL SALINITY1

Clones of the filamentous green alga Ctenocladus circinnatus Borzi were isolated from algae collected at Abert Lake (Oregon) and Mono Lake (California). Stock cultures were exposed to varied salinities of natural lake water to examine the effects on growth rate, cell form, chlorophyll a, and water content. Growth rates were reduced in both clones with increased salinity over the range 25–100 g·L^?1 and were almost completely inhibited at 150 g·L^?1. Chlorophyll a increased between salinities of 25 and 100 g·L^?1, reflecting slower growth, higher proportions of akinetes, and smaller cell sizes as salinity increased. Tissue water content remained essentially constant from 25 to 100 g·L^?1 salinity. Shorter cell dimensions with increased salinity suggest that a lower surface-to-volume ratio may reduce the potential for passive loss of cell water. Prior acclimation of stock cultures to elevated salinity provided no enhancement of growth response at any salinity. The results indicate that environmental salinity can limit the productivity and distribution of Ctenocladus in nature.     

FLORIDOSIDE AS A CARBON PRECURSOR FOR THE SYNTHESIS OF CELL‐WALL POLYSACCHARIDE IN THE RED MICROALGA PORPHYRIDIUM SP. (RHODOPHYTA)1

Although red algae are known to be obligatory photoautotrophs, the red microalga Porphyridium sp. was shown to assimilate and metabolize floridoside. A pulse‐chase experiment with [¹⁴C]floridoside showed that at the end of a 240‐min pulse, 70% of total ¹⁴C‐uptake by the cells remained in the floridoside fraction. To evaluate the assimilation of floridoside by Porphyridium sp. cells, we exposed Porphyridium sp. not only to [¹⁴C]floridoside but also to its constituents, [¹⁴C]glycerol and [¹⁴C]galactose, as compared with [¹⁴C]bicarbonate. The extent of incorporation of [¹⁴C] galactose by the Porphyridium sp. cells was insignificant (50–80 dpm·mL^?1), whereas uptake of ¹⁴C from [¹⁴C]glycerol into the algal cells was evident (2.4 × 10³ dpm·mL^?1) after 60 min of the pulse. The pattern of ¹⁴C distribution among the major constituent sugars, xylose, glucose and galactose, of the labeled soluble polysaccharide was dependent on the ¹⁴C source. The relative content of [¹⁴C]galactose in the soluble polysaccharide was highest (28.8%) for [¹⁴C]floridoside‐labeled culture and lowest (19.8%) for the [¹⁴C]glycerol‐labeled culture. Upon incubation of [¹⁴C]floridoside with a crude extract of a cell‐free system prepared from nonlabeled cells of Porphyridium sp., the label was indeed found to be incorporated into the sulfated polysaccharide. Our results suggested that the carbon metabolic pathway in Porphyridium sp. passes through the low molecular weight photoassimilatory product—floridoside—toward sulfated cell‐wall polysaccharide production.     

SPATIAL AND TEMPORAL VARIABILITY IN BIOMASS AND PRODUCTION OF PSAMMOPHYTIC HALIMEDA INCRASSATA (BRYOPSIDALES,CHLOROPHYTA) IN A CARIBBEAN REEF LAGOON1

Estimates for the production of calcium carbonate by Halimeda spp. have been based on limited measures in small areas or over short periods, subsequently extrapolated over larger temporal and spatial scales. The accuracy of these extrapolations depends on the variations in time and space of the parameters used for the derivations of the production, which were evaluated in the present study for Halimeda incrassata (J. Ellis) J. V. Lamour. in the Puerto Morelos reef lagoon, Mexican Caribbean. Growth, biomass, and CaCO₃ content of the branches of the thalli were determined at 1–3 monthly intervals from November 1997 until June 1999, using the marking technique with the coloring agent Alizarin Red‐S. Biomass exhibited great variability (V=25.4%), and fluctuations were closely related to changes in thallus density. Growth and CaCO₃‐content changes were related to the solar cycle, having coefficients of 15.4% and 2.5%, respectively. Additionally, calcified biomasses of the thalli were determined over a large spatial (31 stations in ～7 km² area) and time (6 y) scale. Spatial variability in calcified biomass was 59%, and the coefficient of variation attained its highest value (69%) for the samples collected over a 6 y period, from 1990 to 1996 (except 1995). Based on overall average values, branches of H. incrassata in the study area had a mean turnover of 30 d, with an annual production of 815 g CaCO₃ per square meter.