Nutritional effect of five species of marine algae on the growth,development, and survival of the brine shrimp Artemia salina

Five species of unicellular algae of the same age, cultured bacteria-free under standard growth conditions, were analyzed for chemical composition and fed to different size classes of Artemia salina. The green algae Chlamydomonas sphagnicolo, Dunaliella viridis, Platymonas elliptica and Chlorella conductrix had significantly higher percentages of protein and lipid than did the diatom Nitzschia closterium. Total ash value was highest in populations of N. closterium. Shrimp fed Chlamydomonas sphagnicolo cells assimilated highest percentages of organic matter, while those fed Chlorella conductrix had lowest assimilation rate. Respiration rates were inversely proportional to animal size (weight) and algal cell volume. Growth, survival, rate of sexual maturtion, and sex ratio were dependent on the growth and assimilation efficiencies obtained from each respective algal food. Shrimp fed Chlamydomonas sphagnicolo, D. viridis, or P. elliptica cells displayed highest growth and assimilation efficiencies.     

Growth and competition of the marine diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. II. Light limitation

The marine diatoms Phaeodactylum tricornutum (Bohlin) and Thalassiosira pseudonana (Hasle and Heimdal) were grown under both continous illumination and a 14 h light: 10 h dark cycle at light intensities ranging from 1.53×10^-4 to 2.95×10^-1 ly min^-1. Under both photoperiods, T. pseudonana exhibited higher division rates than P. tricornutum at high light intensities, but the reverse was true at all light intensities <3×10^-3 ly min^-1. Comparison of these results with available data on light-limited growth of other planktonic algae suggests that P. tricornutum may be unusually efficient at maintaining its cell division rate at low light intensity. This efficiency may contribute substantially to its success in turbid, nutrient-enriched mass algal culture systems, the only environments in which it is known to attain great numbers.Contribution No. 4086 from the Woods Hole Oceanographic Institution.     

Some observations on the influence of salinity on growth and photosynthesis inPorphyra umbilicalis

Notwithstanding the great importance of the salinity factor in the marine environment, the knowledge of influence of salinity on growth of marine benthic algae is very limited. Rate of growth (mg, cm²) and O₂ output of the intertidal red algaPorphyra umbilicalis from Helgoland, North Sea, were measured during a 3 week culture in 3 different salinities (1/2-, 1- and 2-concentrated artificial sea water; Table 1). Under hypertonic conditions (2-concentrated sea water) growth rate and photosynthesis rate were depressed, compared to values obtained in normal concentrated sea water. Under hypotonic conditions (1/2-concentrated sea water), growth expressed in mg was the same as in normal concentrated sea water, or higher when expressed in cm³. Rate of O₂ output was almost unaltered in one of the two experiments, lowered in the other. Cell size increased at higher salinity, while swelling of cell walls and intercellular substances as well as the intensity of colouring decreased with salinity. The discrepancies between growth and photosynthesis under hypotonic conditions cannot be completely explained by the observed influences of salinity on morphological structures (cell size, swelling of cell substances). Detailed studies on the time course of photosynthesis and respiration rates, and preparation of a metabolic balance for the algae are necessary.     

Phytoplankton photosynthetic adaptation to high frequency light fluctuations simulating those induced by sea surface waves

In order to test the ability of phytoplankton to adapt to the high frequency light fluctuations induced by sea surface waves, the green alga Dunaliella tertiolecta was grown under both steady and fluctuating (0.1, 1.0 and 10 Hz) illuminations. The latter conditions reproduced those fluctuations experienced by phytoplankton in the upper photic layer. For each culture, photosynthesis versus irradiance were measured under four incubation frequencies (steady, 0.1, 1.0 and 10 Hz fluctuating illuminations). Results indicated that growth rates were similar for algae grown under steady light and 10 Hz fluctuating light (0.26–0.33 d^–1). Cells grown at 0.1 and 1.0 Hz showed lower growth rates (0.17–0.26 d^–1). Chlorophyll a and b were significantly higher under 0.1 and 10 Hz frequencies than under steady illumination; at 1.0 Hz, there were no significant differences with steady light. No changes in carotenoids were evidenced at any frequency tested. Photosynthetic measurements showed that algae grown under steady illumination had higher photosynthetic efficiency and capacity when incubated under steady and 0.1 Hz fluctuating light. Photosynthetic characteristics of algae grown under 0.1 Hz illumination did not show any clear responses to fluctuating light. Algae grown under 1.0 or 10 Hz had higher photosynthetic efficiency and capacity than those grown under steady illumination, when incubated under 1.0 and 10 Hz light. This suggests that microalgae grown under high frequency illumination (1.0 and 10 Hz) can adapt their photosynthetic characteristics to the rapidly fluctuating light regime experienced during growth, and that algae grown under steady conditions respond better to steady or slowly fluctuating (0.1 Hz) light. Such an adaptation provides a means of probing the photosynthetic responses of phytoplankton to vertical mixing.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Growth pattern andβ-dimethylsulphoniopropionate (DMSP) content of green macroalgae at different irradiances

Growth rates and intracellular-dimethylsulphoniopropionate (DMSP) concentrations of five green algal species collected from different geographic regions in 1986 and 1989 were determined under four photon flux rates. InUlothrix implexa, U. subflaccida andAcrosiphonia arcta from Antarctica, growth was light-saturated at lower irradiances than in temperateUlva rigida from Southern Chile andBlidingia minima from Germany. The DMSP content ofUlothrix implexa, A. arcta andUlva rigida was directly correlated with the light factor: with increasing irradiance, algal DMSP level increased. In contrast, inUlothrix subflaccida andB. minima DMSP concentrations gradually decreased up to a photon flux rate of 30µmol m^–2 s^–1, then increased markedly under the highest photon flux rate tested. In non-growing, dark-incubatedA. arcta DMSP content was reduced by 35%, while the DMSP pool of all other species remained unchanged, at the level of pre-culture conditions. Under full darkness all plants exhibited a significantly higher DMSP concentration compared with algae grown at low photon flux rates of 2 to 30µmol m^–2 s^–1. These data show a correlation between growth pattern and DMSP biosynthesis, and may point to a species-specific minimum amount of light energy necessary for DMSP accumulation.Contribution no. 302 of the Alfred Wegener Institute of Polar and Marine Research     

Otolith ring deposition in relation to growth rate in herring (Clupea harengus) and turbot (Scophthalmus maximus) larvae

Larvae of Clyde spring-spawning Clupea harengus L. and hatchery-produced Scophthalmus maximus (L.) were reared from hatching through metamorphosis in 1980 and 1981 in laboratory tanks and in large enclosures under various light, temperature, and feeding regimes in order to study otolith ring deposition and growth under different conditions. Ring deposition and growth rates were significantly affected by rearing conditions in both species. The ring deposition rates observed under the conditions tested ranged from 0.34 to 0.92 rings d^-1 in herring larvae, and from 0.07 to 1.0 rings d^-1 in turbot larvae. Growth rates ranged from 0.11 to 0.42 mm d^-1 in herring and from 0.05 to 0.27 mm d^-1 in turbot. The number of otolith rings was dependent on the growth rate of the individual larva. At the population level, higher ring deposition rates were observed in faster growing populations. In herring larvae, the relationship between average growth rate and average ring deposition rate was logarthmic, reaching an asymptote at 1 ring d^-1 for growth rates approaching 0.40 mm d^-1. The relationship was linear for turbot larvae for the range of growth rates observed.     

Relative growth of different species of marine algae in wasterwater-seawater mixtures

In recent studies, we developed a combined nutrient removal-marine aquaculture process for the tertiary treatment of wastewater and the production of commercially important shellfish. Part of this process consists of an outdoor mass cultivation system for marine algae. During our previous experiments we observed that marine diatoms almost exclusively are the dominant algal species in our outdoor cultures. To better understand this phenomenon of diatom dominance we grew 16 species of marine algae in continuous monoculture under laboratory conditions simulating to some degree the conditions prevailing in our outdoor experiments. Species such as Skeletonema costatum, Monochrysis lutheri and Tetraselmis sp., which were never dominant in our outdoor cultures, grew as well in monoculture, as Phaeodactylum tricornutum, frequently, the prevalent species outdoors. However, when monocultures of Dunaliella tertiolecta and Thalassiosira pseudonana (3H) were purposely contaminated with P. tricornutum, the latter species quickly became dominant. It is suggested that a complex interaction of environmental factors is usually responsible for the dominance of a particular species; one such factor may be the nitrogen source in the growth media. Under conditions of virtually, complete nitrogen assimilation, the carbon: nitrogen ratio in the algae was high (7 to 8) when NO ₃ ^- –N was the source of nitrogen, and low (4 to 6) when NH ₄ ⁺ –N was the prime form of nitrogen. When algal growth was low, resulting in a large inorganic nitrogen residue, the carbon:nitrogen ratio was low regardless of whether NO ₃ ^- –N or NH ₄ ⁺ –N was the main nitrogen source.Contribution No. 3297 from the Woods Hole Oceanographic Institution.     

Photosynthetic acclimation and biochemical responses ofGelidium sesquipedale cultured in chemostats under different qualities of light

The red algaGelidium sesquipedale (Clem.) Born. et Thur. has been cultured in chemostats to assess the effects of light quality and photon-fluence rate (PFR) on growth, photosynthesis and biochemical composition. Plants under blue and red light (BL and RL) showed higher growth rates than under white light (WL) of the same PFR (40 mol m^–2 s^–1). The light-saturated rate of photosynthesis was higher for algae grown under BL and RL than for algae grown under WL. When algae were transferred to WL of moderate PFR (100 mol m^–2 s^–1), the light-saturated rate of photosynthesis decreased, being higher in previously RL-grown algae than in previously BL- and WL-grown algae. The initial slope of photosynthesis-irradiance (PI) curves () was affected by PFR but not by light quality. Pigment content was little affected by light quality. Light-quality treatments also affected the biochemical composition of the alga; previous exposure to various light treatments activate or repress several metabolical pathways that are fully expressed in the subsequent phase of WL of moderate PFR. Thus, phycobiliproteins and soluble proteins increased for previously BL- and RL-grown algae, whereas insoluble carbohydrate concentration was reduced, indicating a change of the C-partitioning between carbon compounds and organic nitrogen compounds. Inorganic nitrogen metabolism was also affected by light: under WL of moderate PFR, NO₃ ^– was totally depleted from sea water, and maximal values of NO₃ ^– uptake were recorded. In addition, neither NO₂ ^– nor NH₄ ⁺ was released. However, when algae were transferred to a low PFR, there was a drastic reduction of NO₃ ^– uptake under WL, which only partially recovered over time. It was accompanied by the release of NO₂ ^–, but not NH₄ ⁺, to the culture medium. Under BL and RL, however, there was a transient enhancement of NO₃ ^– uptake that was followed by a net release of NO₂ ^– and NH₄ ^–. Growth rates were not correlated with PFR. This could be due to the the dynamics of internal carbon mobilization and accumulation in the algae. When algae were exposed to a moderate PFR of WL, carbon requirements for growth were satisfied by photosynthesis. Thus, there was a net accumulation of carbon in the tissue. In contrast, when algae were exposed to low PFRs of either WL, BL or RL, observed growth rates could not be maintained by photosynthesis and carbon was mobilized.     

Differences between cell division and carbon fixation rates associated with light intensity and oxygen concentration: Implications in the cultivation of an estuarine diatom

The effect of light intensity and oxygen concentration on the growth of an estuarine diatom was investigated. Differences between rates of cell division and net carbon fixation were found to be dependent upon light intensity and oxygen concentration. Under conditions favoring large differences between cell division and net carbon fixation cultures of Thalassiosira pseudonana clone 3H depart from exponential and enter stationary phase at low cell concentrations. It is suggested that single cell algae may not be able to balance maintenance, growth, and division outside a fairly narrow range of environmental conditions.     

Ethologie alimentaire d'invertébrés benthiques filtreurs (ascidies). I. Dispositif expérimental. Taux de filtration et de digestion chez Phallusia mammillata

In a study on ascidian feeding ethology, rates of filtration and digestion of the unicellular algae Monchrysis lutheri by Phallusia mammillata (Cuvier, 1815) have been determined experimentally. All results were obtained under constant conditions of temperature, salinity, pH, light, and food, using 5 individuals in 9 experiments of 24 h each. The method of Winter (1969) has been modified by using an automatic system which enables constant food concentrations (±5%) to be maintained. Controls are made by a fluorometer. The rate of filtration is deduced from the quantity of culture food added each hour; this rate varies; a well defined rhythm is not observed, but alternating maxima and minima. The values obtained for, specimens of medium size (10 to 12 cm) vary from 3856 to 4730 ml/h and per gramme organic dry weight, the average being 4380 ml. The quantity of food actually converted by each ascidian, once it has passed the alimentary canal, is calculated from the amount of proteins present in food and faeces. Digestion rate varies from 4.65 to 5.75 mg of albumin equivalent/24 h and per gramme organic dry weight (mean 5.30 mg). The percentage food conversion (filtrated amount=100%) is high: 86 to 93%, with a mean of 90%.     

Photosynthesis and respiration of exposed salt-marsh fucoids

Photosynthesis and respiration of the salt-marsh fucoids Ascophyllum nodosum ecad scorpioides and Fucus vesiculosus were investigated using an infrared CO₂ gas analyzer under a variety of light intensities, temperatures, and levels of desiccation while the algae were exposed to the atmosphere. Results indicated that net photosynthesis (0.5 to 2.0 mg C/g dry weight/h) saturated rapidly at light intensities (0.1 to 0.2 g cal/cm²/min) which were approximately 10 to 50% of the daily summer maximum intensities for algae found under phanerogam (Spartina alterniflora) canopies. Desiccation exhibited the most pronounced effect on photosynthesis, which increases slightly between 0 and 25% water loss, levels off, and decreases sharply at water losses greater than 50%. Dark respiration (0.1 to 0.3 mg C/g dry weight/h) is also inhibited by desiccation. Both species of algae appear to be broadly adapted to all three parameters investigated.This research was supported by research grants AG-375 and BO 38018 from the National Science Foundation and, in part, by the State University of New York Research Foundation and the Energy Research and Development Administration (ERDA).Communicated by M.R. Tripp, Newark     

Carbon fixation and coccolith detachment in the coccolithophore Emiliania huxleyi in nitrate-limited cyclostats

This study addresses carbon fixation and coccolith production and detachment in the cosmopolitan species Emiliania huxleyi (Lohmann) Hay et Mohler, under conditions of nitrate limitation and high light typical of surface water in the ocean. Cells were grown under controlled growth conditions using nitrate-limited cyclostat cultures at four growth rates between 0.2 and 0.7 d⁻¹ in 1995. Both photosynthesis and calcification rates increased with growth rate. Coccolith dimensions remained constant at all cell growth rates. Specific rates of coccolith detachment also increased linearly with cell-specific growth rate at a ratio not significantly different from 1.00. Estimates of coccolith carbon content decreased with increasing cell growth rates. Received: 18 March 1997 / Accepted: 8 October 1998     

Light quality in relation to growth,photosynthetic rates and carbon metabolism in two species of marine plankton algae

The effect of light quality on growth, photosynthesis and carbon metabolism in two species of marine algae,Cyclotella nana (Hustedt) andDunaliella tertiolecta (Butcher), was examined. Relative growth constants forC. nana were 0.37, 0.29 and 0.25 in blue, white and green light, respectively. Corresponding constants were 0.41, 0.31 and 0.29 forD. tertiolecta. Photosynthetic rates in both species were higher in blue light and lower in green light compared with white light of the same intensity. More than 60% of¹⁴C assimilated byC. nana orD. tertiolecta grown in blue or green light was incorporated into the ethanol-insoluble fraction, compared with 10 to 30% in this fraction in white light. The relative importance of the various components within this fraction was independent of light quality. Although less¹⁴C was assimilated into the ethanol-soluble fraction in blue or green light, there was a relative increase in some amino acids and organic acids in this fraction and a decrease in sugars and sugar phosphates relative to white light of the same intensity. These differences were independent of light intensity, photosynthetic rate and cell density in the cultures.     

The concept of light intensity adaptation in marine phytoplankton: Some experiments with Phaeodactylum tricornutum

The historical background on adaptation of algae to various light intensities is analysed. It is argued that there is little evidence to suggest that previous growth at low light intensities enhances the ability of an alga to utilize these low light levels. Rather, the published evidence suggests that the most general response to growth at sub-optimal light intensities is a reduced ability to utilize saturating levels. The present experiments with Phaeodactylum tricornutum Bohlin have tested this concept of light intensity adaptation. Changing photosynthetic abilities during batch growth depended on the light intensity used for growth and these changes affected interpretations of the data. When measurements were made intensities appeared to photosynthesize (at all intensities) better than did those grown at higher light levels. When the changes during batch growth were taken into account, or when the alga was grown in turbidostat cultures, a different picture was obtained. Growth at reduced light intensities was accompanied by (a) increased chlorophyll content, (b) decreased rate of light-saturated photosynthesis expressed on a chlorophyll, cell number or cell protein basis, and (c) decreased activity of RuDP carboxylase. One result suggested that growth at a suboptimal light intensity did enhance the ability to utilize lower light levels. The light-saturation curve of cells grown in batch culture at 0.7 klux showed higher slopes at the low light intensities than did those grown at 12 klux. This was most marked when photosynthesis was expressed per cell, but was also apparent when it was put on a per chlorophyll basis.     

Time-course of photoadaptation in the photosynthesis-irradiance relationship of a dinoflagellate exhibiting photosynthetic periodicity

Cultures of the marine dinoflagellate Glenodinium sp. were light-shifted and rates of photoadaptation determined by monitoring changes in cell volume, growth rate, pigmentation, parameters of the photosynthesisirradiance (P-I) curves and respiration. To approximate physiological conditions of field populations, cells were cultured on an alternating light-dark cycle of 12hL:12hD, which introduced a daily periodicity of photosynthesis. One result of the present study was to demonstrate how specific parameters of the P-I relationship influenced by periodicity of the light: dark cycle are distinguished from photosynthetic parameters influenced by changes in light level. Under steady-state conditions, rates of both light-saturated (P_max) and light-limited photosynthesis changed in unison over the day; these changes were not related to pigmentation, and displayed their maxima midday. This close relationship between P_max and the slope (a) of the cellular P-I curves in steadystate conditions was quickly adjusted when growth illumination was altered. Rates of light-limited photosynthesis were increased under low light conditions and the periodicity of cellular photosynthesis was maintained. The short-term responses of the P-I relationship to changing light level was different, depending on (1) whether the light shift was from high to low light or vice versa, and (2) whether the high light levels were sufficient to promote maximal photosynthesis rates. Major increases in the photosynthetic carotenoid peridinin, associated with a single type of light-harvesting chromo protein in the chloroplast, was observed immediately upon shifting high light cultures to low light conditions. Following pigment synthesis, significant increases in rates of light-limited photosynthesis were observed in about one-tenth the generation time, while cellular photosynthetic potential was unaffected. it is suggested that general results were consistent with suggested that general results were consistent with earlier reports that the major photoadaptive strategy of Glenodinium sp. is to alter photosynthetic unit (PSU) size. Photoadaptive response times to high light were light-dependent, but appeared to be shower than photoadaptive responses to low light. If light intensities were bright enough to maximize growth rates, photosynthetic response times were on the order of a generation period and pigmentation fell quickly as cells divided at a faster rate. If light-intensities were not sufficient to maximize growth rates, then pigment content did not decline, while rates of light-limited photosynthesis declined quickly. In all cases, photoadaptation was followed best by monitoring fast changes in half saturation constants for photosynthesis, rather than fluctuating changes in pigmentation. Results compared well with time-course phenomena reported for other groups of phytoplankton. Overall, results suggest phytoplankton can bring about photo-induced changes in photosynthesis very quickly and thus accommodate widely fluctuating light regimes over short periods of time.     

Studies on the digestive proteases of the milkfish Chanos chanos

The protease activity of crude extracts from various organs of the digestive tract of two groups of milkfish was determined. One group (Sample A) derived their food from ponds that had predominantly unicellular algae while the other group (Sample B) were reared on ponds dominated by the filamentous green algae Chaetomorpha brachygona. In general, crude extracts from Sample A fish had a higher protease activity than Sample B fish. In both samples, high protease activity was observed in crude extracts from the pyloric caeca, intestines and pancreas. Chymotryptic activity was observed in crude extracts of pancreas, intestines and pyloric caeca of both fish samples. Tryptic activity was, however, observed only in fish grown on unicellular algae. Experimental evidence suggests that a powerful trypsin inhibitor in Chaetomorpha brachygona may account for the absence of tryptic activity in all crude extracts of Sample B fish. The presence of this inhibitor may also explain the widely observed poor growth rate of milkfish reared on this natural food.SEAFDEC contribution no. 112     

Ethologie alimentaire d'invertébrés benthiques filtreurs (ascidies). II. Variations des taux de filtration et de digestion en fonction de l'espèce

Rates of filtration and digestion of 4 species of ascidians (Clavelina lepadiformis), Müller, Ciona intestinalis (Linné, 1767), Halocynthia papillosa (Linné, 1767) and Microcosmus sabatieri (Roule, 1885) from a rocky shore at banyuls-sur-Mer, France have been studied using the unicellular algae Monochrysis lutheri as food and the methods previously applied to Phallusia mammillata (Fiala-Médioni, 1973). Eleven experiments of 24 h each were performed under constant conditions of temperature, pH, salinity, oxygen and food concentration. No recognizable feeding rhythm emerged; filtration is irregular, varying around a mean value. Observed filtration rates averaged 2489 ml/h/g organ dry weight in Clavelina lepadiformis, 3515 in Ciona intestinalis, 6349 in Halocynthia papillosa and 6909 in Microcosmus sabatieri. The higher rates are related to larger size and higher complexity of the gills. No pseudo-faeces are formed; a very small part of the faecal material is discharged within 24 h. The mean rates of digestion, in mg albumin equivalent/24 h/g organ dry weight, are: Clavelina lepadiformis, 3.05 mg; Ciona intestinalis, 4.74 mg; Halocynthia papillosa, 9.25 mg; Microcosmus sabatieri, 10.41 mg. The amount of digested algae corresponds to 83–92% (mean=85%) of algae filtered. This high percentage indicates good assimilation of Monochrysis lutheri by ascidians.

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Ethologie alimentaire d'invertébrés benthiques filtreurs (ascidies). II. Variations des taux de filtration et de digestion en fonction de l'espèce

     

Different responses of photosynthesis and flow cytometric signals to iron limitation and nitrogen source in coastal and oceanic Synechococcus strains (Cyanophyceae)

Iron plays an important role in marine primary productivity, and Synechococcus species as major contributors to the total photosynthetic biomass in the world’s oceans might be limited by iron supply in some regions. The present study aimed to compare the photosynthesis and flow cytometric signals of four Synechococcus strains grown under different iron concentrations with either nitrate or ammonium as the sole nitrogen source. Two oceanic strains were much more sensitive to iron limitation than two coastal strains. The inhibition of iron limitation on the growth, maximal PSII photochemical yield, maximal rate of relative electron transport and photochemical quenching of the two oceanic strains was higher than for their coastal counterparts. Under iron limitation condition, the connectivity factor between individual photosynthetic units (ρ) increased for the two coastal strains, while decreased for the two oceanic strains. Furthermore, iron limitation accelerated the Q _A re-oxidation of the two oceanic strains and the PQ pool re-oxidation of the two coastal strains. Under iron limitation condition, the cell size of the two coastal strains and intracellular pigment concentrations of the two oceanic strains decreased, while the side light scatter/front light scatter (SS/FS) ratio of the two coastal strains increased. In contrast to iron limitation, nitrogen source only marginally affected the photosynthesis of the four Synechococcus strains. Ammonium enhanced the growth of the two coastal strains under iron-replete condition. For the two oceanic strains, ammonium increased their cell size and decreased their SS/FS ratio and intracellular pigment concentrations under iron-deplete and iron-replete conditions.     

Nitrogen- versus phosphorus-limited growth and sources of nutrients for coral reef macroalgae

Recent investigations of nutrient-limited productivity in coral reef macroalgae have led to the conclusion that phosphorus, rather than nitrogen, is the primary limiting nutrient. In this study, comparison of the dissolved inorganic nitrogen:phosphorus ratio in the water column of Kaneohe Bay, Hawaii, with tissue nitrogen:phosphorus ratios in macroalgae from Kaneohe Bay suggested that nitrogen, rather than phosphorus, generally limits productivity in this system. Results of nutrient-enrichment experiments in a flow-through culture system indicated that inorganic nitrogen limited the growth rates of 8 out of 9 macroalgae species tested. In 6 of the species tested, specific growth rates of thalli cultured in unenriched seawater from the Kaneohe Bay water column were zero or negative after 12 d. These results suggest that, in order to persist in low-nutrient coral reef systems, some macroalgae require high rates of nutrient advection or access to benthic nutrient sources in addition to nutrients in the overlying water column. Nutrient concentrations in water samples collected from the microenvironments inhabited or created by macroalgae were compared to nutrient concentrations in the overlying water column. On protected reef flats, inorganic nitrogen concentrations within dense mats of Gracilaria salicornia and Kappaphycus alvarezii, and inorganic nitrogen and phosphate concentrations in sediment porewater near the rhizophytic algae Caulerpa racemosa and C. sertularioides were significantly higher than in the water column. The sediments associated with these mat-forming and rhizophytic species appear to function as localized nutrient sources, making sustained growth possible despite the oligotrophic water column. In wave-exposed habitats such as the Kaneohe Bay Barrier Reef flat, water motion is higher than at protected sites, sediment nutrient concentrations are low, and zones of high nutrient concentrations do not develop near or beneath macroalgae, including dense Sargassum echinocarpum canopies. Under these conditions, macroalgae evidently depend on rapid advection of low-nutrient water from the water column, rather than benthic nutrient sources, to sustain growth. Received: 1 December 1997 / Accepted: 9 July 1998     

Do correlations exist between chromatophore arrangement and photosynthetic activity in seaweeds?

In Dictyota dichotoma, as in many other plants, the chromatophores which at low intensities occupy the cell walls perpendicular to the light beam move to the side walls parallel to the light beam if exposed to high light intensities. The aim of this investigation was to find out whether or not the changes from low- to high-intensity arrangement and vice versa function as an active control mechanism to regulate photosynthetic activity in D. dichotoma under the respective light condition. Four different experimental approaches were made: (a) In white and blue light experiments the changes of the transmittance and of the rate of photosynthetic oxygen production in high- and low-intensity arrangement were compared. (b) The kinetics of the depression and recovery of the PS-rates, as well as of the transmittance changes, were determined during high- and low-intensity movement, respectively. (c) Transmittance and PS-rates of thalli under illumination with polarized and unpolarized light of the same intensity (1,000 1x) were compared. (d) PS-rates of thalli after darkening as well as after preirradiation with weak and strong red light, conditions under which the chromatophores occupy the same position in the cells, were measured. In all these experiments the photosynthetic activity was strongly influenced by pre-illumination, but was independent of the respective chromatophore arrangement. This finding was confirmed by experiments with two other algae: (1) In the brown alga Alaria esculenta which does not display light-induced chromatophore displacements and concomitant transmittance changes, pre-irradiation with high light intensities decreases the PS-rates. (2) In the green alga Ulva lactuca, which shows circadian chloroplast movements, the PS-rates depend on the pre-irradiation only, irrespective of the chloroplast position. Thus we may conclude that in these organisms the function of chromatophore displacements is not the regulation of photosynthetic activity. Other ecological functions are discussed.