Acclimation and adaptation to irradiance in symbiotic dinoflagellates. II. Response of chlorophyll–protein complexes to different photon-flux densities

The response of chlorophyll–protein complexes to super- and sub-saturating photon-flux densities, PFD (250?μmol quanta m^?2?s^?1 and 40?μmol quanta m^?2?s^?1, respectively) were analyzed for Symbiodinium microadriaticum Freudenthal, the symbiont of the Caribbean jellyfish Cassiopeia xamachana; S. kawagutii Trench and Blank, the symbiont of the Indo-Pacific scleractinian Montipora verrucosa; and S. pilosum Trench and Blank, the symbiont of the Carribbean zoanthid Zoanthus sociatus. The results indicate that each species exhibits a quantitatively distinct chlorophyll (chl) a distribution among its chl–protein complexes when cultured under standardized high and low light conditions. In response to sub-saturating PFD, the three species differentially increased the cellular concentrations of most of the chl–protein complexes. Increases in P₇₀₀ (reaction center of Photosystem I) under sub-saturating PFD correlate with an increase in the cellular concentrations of the Photosystem I-enriched complexes. Similarly, increases in photosynthetic unit (PSU) size correlate with an increase in the cellular concentrations of the water-soluble peridinin–chl-a–protein (PCP) complexes and the membrane-bound chl a–chl c ₂–peridinin–protein (acpPC) complexes, which together represent the light-harvesting components of this group. In S. microadriaticum, acclimation to sub-saturating PFD uniquely includes the preferential enrichment of the dimeric form of PCP. Under super-saturating PFD, an enrichment in photo-protective xanthophylls was detected in acpPC from S. microadriaticum and S. pilosum, but not from S. kawagutii. Each species demonstrated a characteristic photo-acclimatory response which correlates with its distribution as endosymbiont in nature, supporting the concept that different species of symbiotic dinoflagellates are adapted (sensu Björkman 1981) to different photic environments. The study was conducted between May 1992 and November 1994.     

Effect of feeding regime and irradiance on the photophysiology of the symbiotic sea anemone Aiptasia pulchella

To determine how the animal and algal components of the symbiotic sea anemone Aiptasia pulchella respond to changes in food availability and culture irradiance, sea anemones from a single clone were maintained at four irradiance levels (320, 185, 115, and 45 E m^-2 s^-1) and either starved or fed for 5 wk. Changes in protein biomass of sea anemones maintained under these conditions were not related to the productivity of zooxanthellae, since the protein biomass of fed A. pulchella decreased with increase in irradiance and there was no difference in protein biomass among starved sea anemones at the four irradiance levels. Except for the starved high-light sea anemones, the density of symbiotic zooxanthellae was independent of culture irradiance within both starved and fed. A. pulchella. Starved sea anemones contained over twice the density of zooxanthellae as fed sea anemones. Within both starved and fed individuals, chlorophyll per zooxanthella increased with decreasing culture irradiance while algal size remained constant (in fed sea anemones) at about 8.80 m diameter. Chlorophyll a: c ₂ ratios of zooxanthellae increased with decreasing culture irradiance in zooxanthellae from starved sea anemones but remained constant in zooxanthellae from fed sea anemones. As estimated from mitotic index data, the in situ growth rates of zooxanthellae averaged 0.007 d^-1 and did not vary with irradiance or feeding regime. Photosynthesis-irradiance (P-I) responses of fed A. pulchella indicated an increase in photosynthetic efficiency with decreasing culture irradiance. But there was no consistent pattern in photosynthetic capacity with culture irradiance. Respiration rates of fed sea anemones also did not vary in relation to culture irradiance. The parameter I _k , defined as the irradiance at which light-saturated rates of photosynthesis are first attained, was the only parameter from the P-I curves which increased linearly with increasing culture irradiance. The daily ratio of net photosynthesis to respiration for A. pulchella ranged from 1.6 to 2.8 for sea anemones maintained at the three higher irradiances, but was negative for those maintained at 45 E m^-2 s^-1. Since the final protein biomass was greatest for sea anemones maintained at the lowest irradiance, these results indicate that sea anemone growth cannot be directly related to productivity of zooxanthellae in this symbiotic association.     

Indications from photosynthetic components that iron is a limiting nutrient in primary producers on coral reefs

Because iron is not available generally in oxygenated sea water, it may be a limiting factor in marine primary production. This hypothesis was tested in the context of Davies Reef, Latitude 18°50′S (one of the coral reefs in the central region of the Great Barrier Reef system). Samples were collected for study in the period August, 1980 to March, 1981. Sea water around the reef contained ≦2x10^-6 M Fe, surface sediments from the reef contained 66±26 (1 SD) ppm total Fe, and interstitial water near the surface contained ≧5x10^-7 M Fe. Thus, Fe constituted a trace component of the reef environment, but limited Fe should be available to algae associated with the sediments. Specific biochemical analyses to test the Fe status of benthic photosynthetic organisms were carried out with a common blue-green alga, Phormidium sp., and a ubiquitous symbiotic dinoflagellate, Gymnodinium microadriaticum (zooxanthellae). The blue-green alga contained the electron transport protein, flavodoxin, which is found only in Fe-deficient organisms. Supporting evidence for Fe stress in this organism included chlorosis in the presence of plentiful biliprotein, and very low extractable photosynthetic cytochrome, c-553. The latter observations were shown to be the result of Fe deficiency in laboratory cultures of a blue-green alga, Synechococcus sp. These cultures showed that production of flavodoxin is not a universal response of algae to Fe stress, but that lowered cellular concentrations of Fe-containing proteins involved in photosynthesis probably is universal. The zooxanthellae from a soft coral, Sinularia sp., had three-fold lower total Fe and ferredoxin (an electron transport protein), than the same alga from a clam, Tridacna maxima. Thus, some algae in symbiotic associations may also suffer Fe-deficiency. It was concluded that the degree and extent of Fe-stress in primary producers on a coral reef may influence growth rates, biomass, and distribution of species.     

Capability of dynamic photoinhibition in Arctic macroalgae is related to their depth distribution

The capability of several macroalgal species to protect photosynthesis from excessive irradiance by dynamic photoinhibition was investigated relative to their depth distribution in summer 1995 in the Kongsfjord (79°N; 12°E, Ny Ålesund, Spitsbergen, Norway). Photoinhibition of photosynthesis was induced by exposure of algae from different water depths to a high photon fluence rate of 500?μmol?m^?2?s^?1 for 2?h. Changes in optimal quantum yield (F _v/F _m) were measured during the inhibition phase. Recovery of photosynthesis was subsequently induced by dim white light (10?μmol?m^?2?s^?1) and observed as changes in the variable fluorescence. With a newly developed mathematical model different parameters of the response kinetics of inhibition and recovery were calculated and related to the depth distribution of each algal species. It is shown that two components with slow and fast reaction kinetics, respectively, are involved in photoinhibition and recovery of photosynthesis. Their possible molecular bases are discussed. The half-life time (τ) of the inhibition and recovery phases, i.e. the time necessary to reach half maximal response, is clearly related to the depth distribution of the investigated species. Algae collected close to the water surface show a fast reaction of both photoinhibition and recovery and, hence, have a low τ. With increasing depth the reactions become slower and τ increases. τ was highest in deep water algae. Further analysis of the reaction kinetics in Laminaria saccharina shows that the relative proportion of the two kinetics involved change with the collection depth. In contrast, a significant difference in the reaction rates of both kinetics was not observed.     

Photosynthesis,photorespiration, and dark respiration in eight species of algae

The rates of photosynthesis and dark respiration for 7 marine algae and 1 fresh-water alga were measured and compared. The dinoflagellates Glenodinium sp. and zooxanthellae have high dark respiration rates relative to photosynthetic rates, which may decrease their net growth rates. Photorespiration in the 8 algal species was studied by examining the effects of the concentration of oxygen on the rates of photosynthesis, on the incorporation of ¹⁴CO₂ into the photorespiratory pathway intermediates glycine and serine, and on the postillumination burst of carbon dioxide production and oxygen consumption. A combination of these results indicates that all the algae tested can photorespire, but that Glenodinium sp., Thalassiosira pseudonana, and zooxanthellae either have a photorespiratory pathway different from that proposed for freshwater algae (Tolbert, 1974), or an additional pathway for glycolate metabolism.     

Photosynthetic performance of giant clams, Tridacna maxima and T. squamosa, Red Sea

Two species of giant clams, Tridacna maxima and T. squamosa, coexist in the Red Sea, but exhibit distinctly different depth distributions: T. maxima mostly occurs in shallow waters (reef flat and edge), while T. squamosa may occur down to the lower fore-reef slope. Giant clams have been described as mixotrophic, capable of both filter-feeding and photosynthesis due to algal symbionts (zooxanthellae), therefore, observed depth preferences were investigated in relation to possible differences in autotrophy vs. heterotrophy. This study was conducted from April to June 2004, at the reef near the Marine Science Station, Aqaba, Gulf of Aqaba, Red Sea, and in May 2007, at a reef near Dahab, Sinai Peninsula, Egypt. In situ measurements using a submersible pulse amplitude modulated fluorometer (Diving PAM), revealed no significant differences in effective PSII quantum yield (ΔF/Fm′) and relative electron transport rates (ETR) between the two species; but rapid light curves (ETR vs. light, photosynthetically active irradiance, PAR) showed significant differences in maximum photosynthetic rates (ETR_max), with 20% higher values in T. maxima. Chamber incubations displayed higher net and gross oxygen production by T. maxima (88.0 and 120.3 μmol O₂ cm⁻² mantle area day⁻¹) than T. squamosa (56.7 and 84.8 μmol O₂ cm⁻² mantle area day⁻¹); even under shading conditions (simulated depth of 20 m) T. maxima still achieved 93% of the surface gross O₂ production, whereas T. squamosa reached only 44%. A correlation was found between ETR and net photosynthesis measured as oxygen production (T. maxima: R ² = 0.53; T. squamosa: R ² = 0.61). Calculated compensation depth (CD) (gross photosynthesis equals respiration) in T. maxima (16 m) matches the maximum depth of occurrence in this study (17 m). By contrast, the CD of T. squamosa (9 m) was much shallower than the maximum vertical range (42 m). Findings suggest T. maxima is a strict functional photoautotroph limited by light, whereas T. squamosa is a mixotroph whose photoautotrophic range is extended by heterotrophy. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effect of phosphate and ammonium levels on photosynthetic and respiratory responses of the red alga Gracilaria verrucosa

Gracilaria verrucosa (Hudson) Papenfuss exposed to nutrient enriched media (0.1 mM PO₄; 1.0 mM NH ₄ ⁺ ) by pulse feeding 2 h every third day for a period of 5 wk at 20°C and 25–30 salinity showed significantly higher rates of photosynthesis regardless of photon flux density correlated with increased pigment levels. Algae in nonenriched media showed significantly higher levels of soluble carbohydrates and decreased levels of phycoerythrin and chlorophyll a. Photosynthetic and respiratory responses to temperature 15°, 25°, 30°C and salinity (15, 25, 30 S) combinations indicate broad tolerances by both nutrient enriched and non-nutrient enriched algae. Photosynthetic and respiratory rates were highest at the high temperatures. Pulse-fed algae had significantly higher photosynthetic rates than non-nutrient enriched plants at all temperature and salinity combinations. Non-nutrient enriched algae had significantly higher respiratory rates than nutrient enriched algae at only 30°C and 15. The respiratory rates of both nutrient enriched and non-nutrient algae decreased under combinations of higher temperatures and salinities. G. verrucosa, grown without nutrients, has lower tolerances to environmental stresses.     

Photosynthesis-irradiance responses and photosynthetic periodicity in the sea anemone Aiptasia pulchella and its zooxanthellae

Sea anemones (Aiptasia pulchella) containing zooxanthellae (Symbiodinium microadriaticum) were maintained in a long-term laboratory culture on a 12 h light (100 E m^-2 s^-1):12 h dark cycle. Photosynthetic oxygen production was measured for the symbiotic association and for freshlyisolated zooxanthellae. Light utilization efficiencies () were similar for both sets of zooxanthellae, suggesting negligible shading of zooxanthellae by animal tissue in this association. Whereas freshly-isolated zooxanthellae were photoinhibited at high irradiances (800 to 1 800 E m^-2 s^-1), zooxanthellae in the host continued to function at photosynthetic capacity. Time of day may influence photosynthetic measurements in symbiotic organisms, as it was found that photosynthesis in A. pulchella followed a diel periodicity at both light-saturating (1 200 E m^-2 s^-1) and subsaturating (150 E m^-2 s^-1) irradiances. There was a peak period of photosynthesis between 12.00 and 14.00 hrs. Light stimulated dark respiration rates of A. pulchella. Dark respiration of sea anemones increased somewhat towards the end of the light cycle and was always greater after exposure to high irradiances.     

Depth-dependent photoadaption by zooxanthellae of the reef coral Montastrea annularis

Zooxanthellae living in colonies of the Caribbean reef coral Montastrea annularis photoadapt to depth-dependent attenuation of submarine light. Studies carried out at Discovery Bay, Jamaica, show that in shallow-living coral colonies, the zooxanthellae appear photoadapted to function at high light intensities, and do poorly if transplanted to low light intensities; in contrast, zooxanthellae in deeper-living coral colonies can be damaged by high light intensities. The adaptation to decreasing light intensity and changing spectral quality appears to be accomplished by increasing the size of the photosynthetic unit (PSU), as opposed to increasing the number of PSU's per cell. Whole cell absorption increases with depth, partially offsetting the loss of light energy due to depth-dependent attenuation. Calculations of photosynthetically usable radiation, the light an alga is capable of absorbing in its own submarine habitat, suggest that the algae at different depths are optimizing rather than maximizing their ability to harvest submarine light energy.     

Obligate mixotrophy inLaboea strobila,a ciliate which retains chloroplasts

The planktonic ciliateLaboea strobila Lohmann sequesters photosynthetically functional chloroplasts derived from ingested algae. The chloroplasts lie free in the cytoplasm and are most abundant just under the pellicle of the ciliate. The maximum rate of photosynthesis (P_max) was 925 pg C ciliate^-1h^-1 (3.7 pg C pg chl.a ^-1h^-1). At saturating irradiance, the amount of carbon fixed h^-1 equaled 12.6% of the body carbon of the ciliate. To grow,L. strobila requires both light and algal food. In the absence of food, survival ofL. strobila is significantly longer in the light than in the dark. Based on ingestion rate and photosynthetic rate, we calculate that photosynthesis can make an important contribution to this ciliate's carbon budget even when algal food is plentiful.     

Essential amino acid synthesis and nitrogen recycling in an alga–invertebrate symbiosis

When aseptically-cultured sea anemones, Aiptasia pulchella, were incubated with ¹⁴C-labelled glucose, aspartate and glutamate, radioactivity was incorporated into animal protein. Radioactivity was recovered from all amino acids in the protein hydrolysates of A. pulchella bearing the symbiotic alga Symbiodinium sp., and from all but seven of the amino acids in A. pulchella experimentally deprived of their algae. These data suggest that these seven amino acids (histidine, isoleucine, leucine, lysine, phenylalanine, tyrosine and valine) may be synthesized by the symbiotic algae and translocated to the sea anemone's tissues; and that methionine and threonine, two amino acids traditionally considered as dietary essentials for animals, are synthesized by A. pulchella. Essential amino acid translocation from the symbiotic algae to the animal host is a core element in symbiotic nitrogen-recycling. Its nutritional value to the animal host is considered in the context of the amino acid biosynthetic capacity of the host. Received: 26 October 1998 / Accepted: 28 June 1999     

Effects of light of altered spectral composition on coral zooxanthellae associations and on zooxanthellae in vitro

Pocillopora damicornis (Linnaeus) and Montipora verrucosa (Lamarck) were collected from Hawaiian reefs. In two experiments (September 1979-January 1980: ca. 4 mo; August-October 1980; ca. 2 mo), these reef corals were grown under sunlight passed through filters producing light fields of similar quantum flux but different spectral composition. In vitro cultures of symbiotic zooxanthellae (Symbiodinium microadriaticum Freudenthal) from M. verrucosa were cultured under similar conditions for 15 d. Blue or white light promoted more coral skeletal growth than green or red light. In both coral species, blue light increased the total amount of chlorophyll a of the coral-zooxanthellae association. In the perforate species, M. verrucosa, the pigment concentration was elevated by an increase in the density of zooxanthellae, but the pigment concentrations per algal cell remained unchanged; in the non-perforate species, P. damicornis, it appears that pigment concentration was elevated by an increase in pigment per algal cell, and not by an increase in density of zooxanthellae. The sunloving reef-flat coral P. damicornis did not grow as rapidly as the shade-species M. verrucosa at the low quantum flux (about 10% sunlight) provided by the experimental treatments. The in vitro cultures of zooxanthellae from M. verrucosa exhibited growth rates in light of altered spectral quality that correlated with the responses of the host coral species: blue and white light supported significantly greater growth than green light, and red light resulted in the lowest growth rate.Contribution No. 678 of the Hawaii Institute of Marine Biology     

The ammonium/methylammonium uptake system of Symbiodinium microadriaticum

The substrate analogue [¹⁴C]-methylammonium was used to study ammonium/methylammonium uptake by Symbiodinium microadriaticum (zooxanthellae). The value of the Michaelis constant (K _m) for the uptake system was approximately 35 M with methylammonium as substrate; ammonium was a competitive inhibitor of methylammonium uptake, and the K _m for ammonium uptake (determined as the inhibition constant, K _i, for methylammonium) was 6.6 M. Methylammonium uptake by zooxanthellae was light-dependent. Methylammonium uptake rates of zooxanthellae which had been freshly isolated from the hermatypic coral Acropora formosa (0.85±0.05x10^-10 mol min^-1 cell^-1) were lower than those of axenic cultures of the zooxanthellae from Montipora verrucosa (Acroporidae) grown under various nitrogen regimes (1.6 to 12x10^-10 mol min^-1 cell^-1). Maximum uptake rates were found for ammonium-starved cultured M. verrucosa zooxanthellae (10.2 to 12x10^-10 mol min^-1 cell^-1); M. verrucosa zooxanthellae growing with ammonium as nitrogen source and zooxanthellae which had been freshly isolated from A. formosa gave similar and considerably lower uptake rates (0.85 to 1.6x10^-1 mol min^-1 cell^-1). These results suggest that either coral tissue contains sufficient ammonium to repress synthesis of the uptake system of the algal symbionts or, alternatively, there are additional barriers to ammonium transport for zooxanthellae in vivo.     

Carbon acquisition strategies for marine macroalgae

A model system was developed to analyse differences in carbon acquisition strategies among macroalgae. During photosynthesis in a limited volume of seawater the capability of the algae to assimilate inorganic carbon as well as to change the alkalinity of the seawater was analysed. These properties were then related to the status of the carbonate equilibrium system of the seawater. The experimental system was assumed to simulate the conditions in the boundary layer during periods of low water exchange or high intensity irradiations. Fundamental differences were found between different algal classes, suggesting that capabilities to adapt to specific environmental conditions may be connected with dissimilarities in carbon acquisition strategies. In general, green algae were able to reach the highest pH (10.8 at 5°C), and thus to achieve the highest reduction in the level of inorganic carbon via a simple HCO₃ ^–/OH^– ion exchange process. For brown algae, pH increases due to carbon uptake never exceeded pH 9.7 (9.5 in a saltwater scale). In spite of this, members of the Fucaceae (littoral brown algae) were able to extract almost all of the dissolved inorganic carbon (DIC). This was achieved through a gradual decrease in the alkalinity of the enclosed water, so that the carbon assimilation could continue without any concomitant increase in pH. For red algae, the specific response was an increase in the level of inorganic carbon. Thus, for this algal class, no specific strategy for handling a shortage of inorganic carbon was documented. Within each algal class, differences in pH and DIC compensation points could be related to differences in the depths at which the algal species occurred. This paper also introduces a low cost and convenient method of analysing DIC in seawater.     

Action spectra and spectral quantum yield of photosynthesis in marine macroalgae with thin and thick thalli

Net photosynthesis at 10mol photons m^-2 s^-1 in each of 24 wavelengths was measured in absolute units by an O₂-electrode and corrected for dark respiration to construct action spectra for gross photosynthesis in nine species of algae, which included plants with thin and thick thalli from each of four major pigment groups. The photosynthesis of green and brown algae with thin thalli decreased in green light, but species with thick thalli from these two groups had action spectra which were almost flat, and matched the optical blackness of the thalli but did not reflect the pigment differences between the species. Among the red algae, on the other hand, there was little difference between the action spectra for thin and thick algae. Only wavelengths absorbed by the phycobilin pigments were effective in photosynthesis, even in species (e.g. Chondrus, Phyllophora) which absorbed all visible wavelengths strongly. Maximal quantum yields of 0.10 to 0.12 O₂ molecules per absorbed photon were recorded for thin green and brown algae, but thicker algae in these two groups had lower values. Red algae exhibited maximal values close to 0.10 O₂ molecules per absorbed photon, irrespective of thallus thickness or phycocyanin content, but the quantum yields of phycoerythrin-rich species in the 600 to 650 nm waveband were lower than those of phycocyanin-rich species.     

Diurnal changes in photosynthesis of Antarctic fast ice algal communities determined by pulse amplitude modulation fluorometry

The aim of this project was to determine both the diurnal changes in photosynthetic activity of Antarctic sea ice algae and also the protective mechanisms they use to mitigate the effects of in situ UV radiation. Changes in the diurnal photosynthetic parameters of fast ice algal communities at McMurdo Sound were measured in situ, using a custom designed monitoring pulse amplitude modulation fluorometer. The sea ice microalgae were able to adapt rapidly to either increasing or decreasing ambient irradiances. ΔF/F_m' values were between 0.2 and 0.51, while E_k varied between 2.1 and 18 μmol photons m^-2 s^-1. ΔF/F_m', E_k, and relative electron transfer rate (rETR) all varied sequentially over the course of a day. rETR and E_k were highest at midday at the highest irradiances, when there was apparent midday down regulation of photosynthesis, while ΔF/F_m' was highest at midnight. The effects of natural UV radiation on sea ice were examined, but it was not possible to detect the effect of either UVB or UVA and UVB on photosynthesis. This was considered to be largely because of the large spatial and temporal heterogeneity of the under ice community, changing irradiances throughout the day and the relatively small change caused by UV.     

Pathway and products of CO2-fixation by green prokaryotic algae in the cloacal cavity of Diplosoma virens

Light-dependent ¹⁴CO₂ fixation by the algae of Diplosoma virens (Hartmeyer) ranged between about 3 and 27 moles mg^-1 chlorophyll h^-1. The principal first products of ¹⁴C fixation were 3-phosphoglyceric acid and phosphorylated sugars, indicating that ribulose bisphosphate carboxylase was the primary carboxylation enzyme. The activity of this enzyme in crude extracts of the algae was 4 to 6 moles CO₂ mg^-1 chlorophyll h^-1. The principal end product of ¹⁴C fixation by these algae in the ascidian host was a water-soluble oligosaccharide which was an -1,4-glucan. A maximum of 7% of the ¹⁴C fixed was found in insoluble materials of the algae or its host after 60 min ¹⁴CO₂ fixation. Whether the -1,4-glucan is a product of algal or animal metabolism remains to be determined.     

Amino acid synthesis in the symbiotic sea anemone Aiptasia pulchella

Symbiotic Aiptasia pulchella and freshly isolated zooxanthellae were incubated in NaH¹⁴CO₃ and NH₄Cl for 1 to 240 min, and samples were analysed by reverse-phase high-performance liquid chromatography (HPLC) and an online radiochemical detector. NH₄ ⁺ was first assimilated into ¹⁴C-glutamate and ¹⁴C-glutamine in the zooxanthellae residing in A. pulchella. The specific activities (dpm nmol⁻¹) of ¹⁴C-glutamate and ¹⁴C-glutamine in vivo, were far greater in the zooxanthellae than in the host tissue, indicating that NH₄ ⁺ was principally incorporated into the glutamate and glutamine pools of the zooxanthellae. ¹⁴C-α-ketoglutarate was taken up from the medium by intact A. pulchella and assimilated into a small amount of ¹⁴C-glutamate in the host tissue, but no ¹⁴C-glutamine was detected in the host fraction. The ¹⁴C-glutamate that was synthesized was most likely produced from transamination reactions as opposed to the direct assimilation of NH₄ ⁺. The free amino acid composition of the host tissue and zooxanthellae of A. pulchella was also measured. The results presented here demonstrate that NH₄ ⁺ was initially assimilated by the zooxanthellae of A. pulchella. Received: 3 February 1997 / Accepted: 24 October 1997     

Photoadaption in marine phytoplankton: Response of the photosynthetic unit

Some species of phytoplankton adapt to low light intensities by increasing the size of the photosynthetic unit (PSU), which is the ratio of light-harvesting pigments to P700 (reaction-center chlorophyll of Photosystem I). PSU size was determined for 7 species of marine phytoplankton grown at 2 light intensities: high (300 E m^-2 s^-1) and low (4 E m^-2 s^-1); PSU size was also determined for 3 species grown at only high light intensity. PSU size varied among species grown at high light from 380 for Dunaliella euchlora to 915 for Chaetoceros danicus. For most species grown at low light intensity, PSU size increased, while the percentage increase varied among species from 13 to 130%. No change in PSU size was observed for D. euchlora. Photosynthetic efficiency per chlorophyll a (determined from the initial slope of a curve relating photosynthetic rate to light intensity) varied inversely with PSU size. In contrast, photosynthetic efficiency per P700 was enhanced at larger PSU sizes. Therefore, phytoplankton species with intrinsically large PSU sizes probably respond more readily to the rapid fluctuations in light intensity that such organisms experience in the mixed layer.Contribution No. 1180 from the Department of Oceanography, University of Washington, Seattle, Washington, USA     

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.