Adaptation to low photon flux densities in Protogonyaulax tamarensis var. excavata,with reference to chloroplast photomorphogenesis

Changes in cellular chlorophyll content, cell volume, and light scatter of a New England red tide dinoflagellate, Protogonyaulax tamarensis var. excavata (clone GT-429), cultured in various light regimes are reported. Individual cells were analyzed, using flow cytometry and compared to traditional bulk measurements. Compared to high photon flux densities (182 Ein m^-2 s^-1), changes were measured that reflected increased chlorophyll fluorescence and increased cell volume at reducec photon flux densities when cell division was sustained, and increased flourescence and decreased cell volume when cell division ceased. These optical changes were accompanied by conformational changes in the chloroplasts. We found no change in photosynthetic carboxylating enzyme activities. We suggest that this photomorphogenesis of the chloroplasts at low photon flux densities may be an indication of stress and survival vs adaptive value to these persistent cells.     

Contribution ofSynechococcus spp. to size-fractioned primary productivity in three water masses in the Northwest Atlantic Ocean

The distribution of phycoerythrin-richSynechococcus spp. relative to eukaryotic algae and the contribution ofSynechococcus spp. toin situ primary production were compared at a neritic front, in warm-core eddy 84-E, and at Wilkinson's Basin, during a cruise to the Northwest Atlantic Ocean in July/August 1984. Immunofluorescence analyses ofSynechococcus strains demonstrated the restricted distribution of the tropical oceanic serogroup to the warm-core eddy, while strains of the neritic serogroup and those labelled by antiserum directed against a motile strain, were abundant in all three water masses. Although the majority ofSynechococcus spp. cells were observed in the 0.6 to 1 m fraction, an increasing proportion of the totalSynechococcus spp. cells were found in the 1 to 5 m fraction as nitrate concentrations increased near the base of the thermocline. From immunofluorescence analyses, we determined that the increasing proportion of largerSynechococcus spp. cells at depth was not the result of a change in strain composition, and may therefore be associated with increasing cell volume due to the enhanced nutrient supply. The contribution of the different size fractions to the total standing crop of chlorophyll and thein situ rate of photosynthesis was distincty different for the three water masses. At the neritic front, the larger photoautotrophs of the 1 to 5 m and >5 m fractions were the major contributors to chlorophyll concentrations and primary production.Synechococcus spp. appeared to provide only 6% of the dawn-to-duskin situ primary production at the neritic front. In modified Sargasso water in the warm-core eddy,Synechococcus spp. contributed 25% to thein situ rate of integrated primary production. In this warm-core eddy, the 0.2 to 0.6 m fraction made a major contribution to the standing crop of chlorophyll and primary production that equalled or exceeded that of the larger sze categories. Furthermore, at the bottom of the euphotic layer, eukaryotes numerically dominated the 0.2 to 0.6 m fraction, which contributed 61% of the primary productivity. At Wilkinson's Basin, theSynechococcus spp.-dominated 0.6 to 1.0 m fraction made the greatest contribution to the standing crop of chlorophyll an primary production, while smaller photoautotrophs (0.2 to 0.6 m) accounted for little of the chlorophyll or photosynthetic rates measured over the euphotic layer. Largest numbers ofSynechococcus spp. (2.9x10⁸ cells l^-1) occurred at the 18% isolume, coincident with a shoulder in the chlorophyll fluorescence profile and the site of maximumin situ primary productivity. At Wilkinson's Basin,Synechococcus spp. contributed 46% to thein situ photosynthesis integrated over the water-column.     

Diurnal patterns in photosynthetic capacity and depth-dependent photosynthesis-irradiance relationships inSynechococcus spp. and larger phytoplankton in three water masses in the Northwest Atlantic Ocean

The photosynthetic characteristics of prokaryotic phycoerythrin-rich populations of cyanobacteriaSynechococcus spp. and larger eukaryotic algae were compared at a neritic frontal station (Pl), in a warm-core eddy (P2), and at Wilkinson's Basin (P3) during a cruise in the Northwest Atlantic Ocean in the summer of 1984.Synechococcus spp. numerically dominated the 0.6 to 1 m fraction, and to a lesser extent the 1 to 5 m size fractions, at most depths at all stations. At P2 and P3, all three size categories of phytoplankton (0.6 to 1 m, 1 to 5 m, and >5 m) exhibited similar depth-dependent chages in both the timing and amplitude of diurnal periodicities of chlorophyllbased and cell-based photosynthetic capacity. Midday maxima in photosynthesis were observed in the upper watercolumn which damped-out in all size fractions sampled just below the thermocline. For all size fractions sampled near the bottom of the euphotic zone, the highest photosynthetic capacity was observed at dawn. At all depths, theSynechococcus spp.-dominated size fractions had lower assimilation rates than larger phytoplankton size fractions. This observation takes exception with the view that there is an inverse size-dependency in algal photosynthesis. Results also indicated that the size-specific contribution to potential primary production in surface waters did not vary appreciably over the day. However, estimates of the percent contribution ofSynechococcus spp. to total primary productivity in surface waters at the neritic front were significantly higher when derived from short-term incubator measurements of photosynthetic capacity rather than from dawn-to-duskin situ measurements of carbon fixation. The discrepancy was not due to photoinhibitory effects on photosynthesis, but appeared to reflect increased selective grazing pressure onSynechococcus spp. in dawn-to-dusk samples. Low-light photoadaptation was evident in analyses of the depth-dependency ofP-I parameters (photosynthetic capacity,P _max; light-limited slope, alpha;P _max alpha,I _k ; light-intensity beyond which photoinhibition occurs,I _b ) of the > 0.6 m communities at all three stations and was attributable to stratification of the water column. There was a decrease in assimilation rates andI _k with depth that was associated with increases in light-limited rates of photosynthesis. No midday photoinhibition ofP _max orI _b was observed in any surface station. Marked photoinhibition was detected only in the chlorophyll maximum at the neritic front and below the surface mixed-layer at Wilkinson's Basin, where susceptibility to photoinhibition increased with the depth of the collected sample. The 0.6 to 1 m fraction always had lower light requirements for light-saturated photosynthesis than the > 5 m size fraction within the same sample. Saturation intensities for the 1 to 5 m and 0.6 to 1 m size fractions were more similar whenSynechococcus spp. abundances were high in the 1 to 5 m fraction. The > 5 m fraction appeared to be the prime contributor to photoinhibitory features displayed in mixed samples (> 0.6 m) taken from the chlorophyll maxima. InSynechococcus spp.-dominated 0.6 to 1 and 1 to 5 m size fractions, cellular chlorophylla content increased 50- to 100-fold with depth and could be related to increases in maximum daytime rates of cellularP _max at the base of the euphotic zone. Furthermore, the 0.6 to 1 m and > 5 m fractions sampled at the chlorophyll maximum in the warm-core eddy had lower light requirements for photosynthesis than comparable surface samples from the same station. Results suggest that photoadaptation in natural populations ofSynechococcus spp. is accomplished primarily by changing photosynthetic unit number, occuring in conjuction with other accommodations in the efficiency of photosynthetic light reactions.     

In situ characterization of phytoplankton from vertical profiles of fluorescence emission spectra

Vertical profiling of the upper ocean with a laser/fiber optic fluorometer enabled the determination of fluorescence emission spectra of photosynthetic pigments over small vertical scales. Simultaneous acquisition of phycoerythrin (PE) and chlorophyll (chl) emission spectra allowed in situ differentiation between PE-containing cells (cryptomonads and cyanobacteria) and other chl-containing autotrophs. Further, fluorescence spectral peak shifts associated with different species of PE-containing cells resulted in even finer scale in situ taxonomic differentiation. We found that the phycoerythrin fluorescence emission maxima shifted from 578 nm near the surface, to 585 m at the base of the shallow thermocline (30% light level), and to 590 nm below the thermocline at the base of the euphotic zone (1% light level). These shifts in peak emission coincided with a taxonomic change in the PE-containing cells (as determined from analysis of discrete bottle samples) from a greater proportion of Synechococcus spp. in the upper water column to a greater proportion of cryptomonads at the base of the euphotic zone. These results indicate that the composition of the phytoplankton assemblage may be assessed in situ without sample collection.     

Photosynthesis and respiration in the alga Ahnfeltia plicata in a flow-through system

Photosynthesis and respiration in Ahnfeltia plicata (Huds.) Fries (Gigartinales) was measured in a seawater flowthrough system at different temperatures, salinities and photon flux densities (PFD). The exchanges of dissolved oxygen and inorganic carbon were continuously recorded with an oxygen probe and a pH electrode measuring variation in CO₂–HCO ₃ ^- equilibrium as pH changes. Highest apparent photosynthesis at moderate photon flux density (PFD 50 E m^-2 s^-1) was found at 15°C and 33 S. Photosynthesis was measured up to PFD 500 E m^-2 s^-1 and no light saturation was documented. In the present experimental set-up, with continuous supply of fresh seawater, the number of limiting factors during photosynthesis measurements is reduced.     

Different patterns of growth and nitrogen uptake in two clones of marineSynechococcus spp.

We grew marineSynechococcus Clones WH7803 and WH8018 at 150µE m^–2 s^–1 in dilute batch cultures with NH ₄ ⁺ as the limiting nutrient. The maximal uptake capacities for NH ₄ ⁺ and NO ₃ ^- were measured in frequent experiments during log and stationary phases of growth. Clone WH7803, originally isolated from oceanic waters, had a specific uptake rate of NH ₄ ⁺ that approximated the maximum (log phase) specific growth rate (ca ~ 0.025 h^–1). NO ₃ ^- uptake was observed only after nitrogen in the media was depleted; the NO ₃ ^– uptake capacity was ca 12% the capacity for NH ₄ ⁺ uptake throughout the nitrogen depleted period. Growth was arrested upon nitrogen depletion, but resumed soon after reinoculation into fresh media, even after 5 d of starvation. Clone WH8018, originally isolated from coastal waters, revealed a five-fold enhancement in the NH ₄ ⁺ uptake rate relative to growth rate at the time of nitrogen depletion. As nitrogen starvation proceeded, this enhancement was reduced. This clone, too, was able to take up NO ₃ ^- once nitrogen in the media was depleted, but only after ca 20 h. Growth continued for a limited period during nitrogen depletion, but nitrogen-starved cells were slow to recover upon reinoculation into fresh media. We speculate that clonal differences may reflect differences in the molecular regulation of nitrogen assimilation.     

Photosynthetic characteristics of Prochloron sp./ascidian symbioses

The prokaryotic green alga Prochloron sp. (Prochlorophyta) is found in symbiotic association with colonial didemnid ascidians that inhabit warm tropical waters in a broad range of light environments. We sought to determine the light-adaptation features of this alga in relation to the natural light environments in which the symbioses are found, and to characterize the temperature sensitivity of photosynthesis and respiration of Prochloron sp. in order to assess its physiological role in the productivity and distribution of the symbiosis. Colonies of the host ascidian Lissoclinum patella were collected from exposed and shaded habitats in a shallow lagoon in Palau, West Caroline Islands, during February and March, 1983. Some colonies from the two light habitats were maintained under conditions of high light (2 200 E m^–2 s^–1) and low light (400 E m^–2 s^–1) in running seawater tanks. The environments were characterized in terms of daily light quantum fluxes, daily periods of light-saturated photosynthesis (H_sat), and photon flux density levels. Prochloron sp. cells were isolated from the hosts and examined for their photosynthesis vs irradiance relationships, respiration, pigment content and photosynthetic unit features. In addition, daily P:R ratios, photosynthetic quotients, carbon balances and photosynthetic carbon release were also characterized. It was found that Prochloron sp. cells from low-light colonies possessed lower chlorophyll a/b ratios, larger photosynthetic units sizes based on both reaction I and reaction II, similar numbers of reaction center I and reaction center II per cell, lower respiration levels, and lower P_max values than cells from high-light colonies. Cells isolated from low-light colonies showed photoinhibition of P_max at photon flux densities above 800 E m^–2 s^–1. However, because the host tissue attenuates about 60 to 80% of the incident irradiance, it is unlikely that these cells are normally photoinhibited in hospite. Collectively, the light-adaptation features of Prochloron sp. were more similar to those of eukaryotic algae and vascular plant chloroplasts than to those of cyanobacteria, and the responses were more sensitive to the daily flux of photosynthetic quantum than to photon flux density per se. Calculation of daily minimum carbon balances indicated that, though high-light cells had daily P:R ratios of 1.0 compared to 4.6 for low-light cells, the cells from the two different light environments showed nearly identical daily carbon gains. Cells isolated from high-light colonies released between 15 and 20% of their photosynthetically-fixed carbon, levels sufficient to be important in the nutrition of the host. Q₁₀ responses of photosynthesis and respiration in Prochloron sp. cells exposed briefly (15–45 min) to temperatures between 15° and 45°C revealed a discontinuity in the photosynthetic response at the ambient growth temperatures. The photosynthetic rates were found to be more than twice as sensitive to temperatures below ambient (Q₁₀=3.47) than to temperatures above ambient (Q₁₀=1.47). The Q₁₀ for respiration was constant (Q₁₀=1.66) over the temperature range examined. It appears that the photosynthetic temperature sensitivity of Prochloron sp. may restrict its distribution to warmer tropical waters. The ecological implications of these findings are discussed in relation to published data on other symbiotic systems and free-living algae.     

Diel patterns of carbon incorporation into biochemical constituents of Synechococcus spp. and larger algae in the Northwest Atlantic Ocean

Diel patterns of ¹⁴C-bicarbonate incorporation in>5 m algal communities were compared with those in cyanobacterial populations of Synechococcus spp. (0.6 to 1.0 m), collected from the surface and/or chlorophyll maximum at three stations in the Northwest Atlantic Ocean (a neritic front; in Warm-Core Eddy 84-E; and Wilkinson's Basin) from 21 July to 8 August, 1984. Cell constituents were chemically separated into four fractions: lipids, low molecular weight (LMW) metabolites, polysaccharides/nucleic acids, and proteins. The in situ diel pattern of ¹⁴C assimilation was virtually the same for >5 m algal communities adapted to different environments. Protein synthesis appeared to continue at a reduced rate at night using energy derived from the catabolism of polysaccharides and the mobilization of LMW compounds. Synechococcus spp. populations exhibited inherent physiological differences in their in situ diel pattern of carbon fixation from that in>5 m algal communities taken from the same water mass. There was no nighttime protein-synthesis in Synechococcus spp. The relative proportion of ¹⁴C-protein remained constant over night, while that of ¹⁴C-polysaccharides/nucleic acids declined and that of labelled LMW metabolites increased. Daytime light-intensity manipulations did not alter the diel pattern of carbon fixation in any of the>5 m algal assemblages, while changes in the carbon metabolism of surface and shadeadapted Synechococcus spp. populations could be rapidly induced by altering the light intensity.Bigelow Laboratory Contribution No. 86004     

Vertical distributions and photosynthetic action spectre of two oceanic picophytopianlcters,Prochlorococcus marinus andSynechococcus sp.

Two picophytoplankters,Prochlorococcus marinus andSynechococcus sp., were isolated from the bottom of the euphotic zone (150 m depth) in the western Pacifie Ocean. The concentration ofP. marinus at this depth was more than 10⁴ cells ml^–1 while that ofSynechococcus sp. was less than 10² cells ml^–1. TheP. marinus isolate has a high divinyl-chlorophylla:b ratio similar to that of the Mediterranean strain, while theSynechococcus sp. isolate is of the phycourobilinrich type. The growth rate ofP. marinus was higher thanSynechococcus sp. when both were cultured under weak blue-green to blue-violet light (ca. 2 E m^–2 s^–1). While the chlorophyll-specific absorption spectra showed higher values inSynechococcus sp., the photosynthetic action spectre revealed thatP. marinus was able to use blue-violet light, whereasSynechococcus sp. was able to use blue-green light, more efficiently for photosynthesis. The photosynthetic quantum yield ofP. marinus was higher than that ofSynechococcus sp. at any wavelength between 400 and 700 nm. The calculated in situ photosynthesis rates per Gell volume forP. marinus were estimated to be higher than forSynechococcus sp. at 50 and 150 m depth. These results indicate thatP. marinus photosynthetically surpassesSynechococcus sp. in the blue-light-rieh environment of the oceanic euphotic zone. This may be why the former predominates at depths in temperate to tropical open ocean waters.     

Photoadaptation of photosynthesis in Gonyaulax polyedra

Gonyaulax polyedra Stein exhibited a combination of photoadaptive strategies of photosynthesis when only a single environmental variable, the light intensity during growth, was altered. Which of several biochemical/physiological adjustments to the light environment were employed depended on the level of growth irradiance. The photoadaptive strategies employed over any small range of light levels appeared to be those best suited for optimizing photosynthetic performance and not photosynthetic capacity. (Photosynthetic performance, P _i, is defined as the rate of photosynthesis occurring at the level of growth irradiance.) Among all photosynthetic parameters examined, only photosynthetic performance showed a consistent correspondence to growth rates of G. polyedra. Above 3500 to 4000 W cm^-2, where photosynthetic performance was equal to photosynthetic capacity, cells were not considered light-limited in either photosynthesis or growth. At these higher light levels, photosynthetic perfomance, cell volume, growth rates and respiration rates remained maximal; photosynthetic pigment content varied only slightly, while the photosynthetic capacity of the cells declined. At intermediate light levels (3000 to 1500 W cm^-2), photosynthesis, not growth, was light-limited, and photoadaptive strategies were induced which enhance absorption capabilities and energy transfer efficiencies of chlorophyll a to the reaction centers of G. polyedra. Photosynthetic capacity remained constant at about 280 mol O₂ cm^-3 h^-1, while photosynthetic performance ranged from 100 to 130 mol O₂ cm^-3 h^-1. Major increases in photosynthetic pigments, especially peridinin-chlorophyll a-proteins and an unidentified chlorophyll c component, accompanied photoadaptation to low irradiances. Maximal growth rates of 0.3 divisions day^-1 were maintained, as were respiration rates of about-80 mol O₂ cm^-3 h^-1 and cell volumes of about 5.4×10^-8 cm^-3 cell^-1. Below about 1250 W cm^-2, photosynthesis in G. polyedra was so light-limited that photosynthetic performance was unable to support maximal growth rates. Under these conditions, G. polyedra displayed photostress responses rather than photoadaptive strategies. Photostress was manifested as reduced cell volumes, slower growth, and drastic reductions in pigmentation, photosynthetic capacity, and rates of dark respiration.     

Fractionated phytoplankton primary production,exudate release and bacterial production in a Baltic eutrophication gradient

The distribution of phytoplankton primary production into four size fractions (>10 m, 10-3 m, 3-0.2 m and <0.2 m), the utilization of algal exudates by bacteria and the bacterial production were studied in a eutrophication gradient in the northern Baltic proper. The polluted area exhibits substantially increased nutrient, especially nitrogen, levels while only minor differences occur in salinity and temperature regimes. Total primary production was 160 g C · m^-2 · yr^-1 at the control station and about 275 g C · m^-2 · yr^-1 at the eutrophicated stations. The estimated total exudate release was 16% of the totally fixed ¹⁴CO₂ in the control area and 12% in the eutrophicated area (including the estimated bacterial uptake of exudates). The difference in¹⁴CO₂ uptake rates between incubation of previously filtered water (<3, <2, <1 m) and unfiltered water was used to estimate bacterial uptake of phytoplankton exudates which were found to contribute about half of the estimated bacterial carbon requirement in both areas. Bacterial production was estimated by the frequency of dividing cells (FDC) method as being 38 g C · m^-2 · yr^-1 at the control station and 50 g C · m^-2 · yr^-1 at the eutrophicated stations. To estimate the mean in situ bacterial cell volume a correlation between FDC and cell volume was used. The increased annual primary production in the eutrophicated area was due mainly to higher production during spring and autumn, largely by phytoplankton cells (mainly diatoms) retained by a 10 m filter. Primary production duringsummer was similarin the two areas, as was the distribution on different size fractions. This could possibly explain the similar bacterial production in the trophic layers at all stations since the bulk of bacterial production occurs during summer. It was demonstrated that selective filtration does not quantitatively separate photoautotrophs and bacteria. A substantial fraction of the primary production occurs in the size fraction <3 m. The primary production encountered in the 3-0.2 m fraction was due to abundant picoplankton (0.5 to 8 · 10⁷ ind · l^-1), easily passing a 3 m filter. The picoplankton was estimated to constitute up to 25% of the total phytoplankton biomass in the control area and up to 10% in the eutrophicated area.     

Ecotypic differentiation in the marine diatom Skeletonema costatum: influence of light intensity on the photosynthetic apparatus

Three genetically distinct clones of Skeletonema costatum (Grev.) Cleve were grown at 20°C under high (274 E m^-2 s^-1) and low (27 E m^-2 s^-1) light conditions and their photoadaptive photosynthetic responses compared. When all three clones were grown under low light, pigment analyses and fluorescence excitation spectra demonstrated that the accessory pigments, chlorophyll c and fucoxanthin, became more important in light-harvesting compared to chlorophyll a. Photosynthetic unit sizes increased for Photosystems I and II in low light, but photosynthesis vs irradiance characteristics were not reliable predictors of photosynthetic unit features. Fluorescence excitation spectra and photosynthesis vs irradiance (P-I) relationships indicated that changes in energy transfer occurred independent of changes in pigment content. Large increases in accessory pigment content were not accompanied by large increases in excitation from these pigments. Changes in energy transfer properties were as important as changes in PSU size in governing the photoadaptive responses of S. costatum. When the three clones were grown under identical conditions, each had a separate and distinct pattern of photoadaptation. Significant differences among clones were found for pigment ratios, photosynthetic unit sizes for Photosystems I and II and efficiency of energy transfer between pigments. These strikingly different photoadaptive strategies among clones may partially account for the great ecological success of the diatom species. This is the first quantitative investigation of the importance of both chlorophyll c and fucoxanthin to the adaptive responses of diatoms to light intensity, and represents the most complete characterization of the photoadaptive responses of a single species of marine phytoplankter to differences in light environment.     

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.     

Photosystem II photoinhibition and altered kinetics of photosynthesis during nutrient-dependent high-light photoadaptation in Gonyaulax polyedra

Gonyaulax poledra Stein was transferred at different cell densities from increasingly nutrient-limited low-light (LL, 80 E m^-2 s^-1) batch-cultures to high-light (HL, 330 E m^-2 s^-1) growth conditions. Several age-dependent differences in HL-adaptation strategies were apparent. Short-term (3h) susceptibility to photosynthetic photoinhibition increased with culture age, with light-limited rates of photosynthesis exhibiting greater photosuppression than light-saturated rates at all stages of growth. These shortterm changes were not accompanied by photobleaching of chlorophyll but were directly related to age-dependent photoinactivation of Photosystem II electron-transport rates. The capacity of electron transport by Photosystem I was only slightly affected. Prolonged exposure of LL log-phase cells to HL conditions did induce photobleaching of chlorophyll associated with increased cell volume, a transient decrease of organic carbon and nitrogen content, enhanced cellular-, carbon-and chlorophyll-based rates of light-saturated photosynthesis (P _max) and suppressed cellular rates of light-limited photosynthesis. As a result, the density of LL log-phase cells doubled and their cellular photosynthetic performance nearly tripled within 1 d of HL exposure while cellular respiratory demands remained unchanged. By contrast, prolonged HL incubation of LL stationary populations induced a transitory burst in cell division and a large reduction in cell volume, leading to a short-term increase in volume-based organic carbon and nitrogen content. Despite reduced cell volume and lowered carbon demand, the cellular-, carbon-and chlorophyll-based rates of P _max in nondividing populations fell by 64, 48 and 27%, respectively, over a 4 d exposure to HL, while light-limited rates were almost fully suppressed within 1 d and chlorophyll a content was reduced by 56%. As a result, the photosynthetic performance of LL-aged cells declined immediately under HL conditions. Addition of inorganic nutrients to LL stationary cultures at the time of HL transfer led to immediate and complete suppression of photosynthesis and cell lysis within 1 d. Addition of nutrients following transfer to HL induced cell responses intermediate to those described for LL log and aged cells exposed to HL. Results support the view that declining nutrient-status impairs HL photoadaptive responses in phytoplankton populations and that the rate and pattern of photoadaptive responses may be used as physiological growth indicators in field studies. The study was conducted from March 1981 to May 1983.     

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.     

Light absorption,photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 mol m^-2 s^-1. Growth was lightsaturated at PFDs in excess of 100 mol m^-2 s^-1, with a mean light-saturated growth rate at 23 °C of 1.5×10^-5s^-1 (1.2 d^-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 mol m^-2s^-1. Dark-respiration rates were independent of PFD, averaging 1.7×10^-6 mol O₂ mol^-1 C s^-1 (0.14 mol O₂ mol^-1 C d^-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O₂ mol^-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10^-3 m² mg^-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg^-1 chl a between PFDs of 14 and 200 mol m^-2 s^-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.     

Seasonal photosynthetic characteristics of Ascoseira mirabilis (Ascoseirales,Phaeophyceae) from King George Island,Antarctica

Monthly variation in photosynthesis, dark respiration, chlorophyll a content and carbon: nitrogen (C:N) ratios in different lamina sections of adult plants of Ascoseira mirabilis Skottsberg from King George Island, Antarctica, was investigated between September 1993 and February 1994. Light saturated net photosynthesis (P _max) showed maximum values in September (12 to 25 mol O₂ g^-1 fr wt h^-1), and decreased towards the summer to values ranging between 2.0 and 5.0 mol O₂ g^-1. In the distal section, however, a second optimum occurred in December (25 mol O₂ g^-1 fr wt h^-1). Dark respiration rates were also highest in October and November and decreased strongly in December to February (6.0 and 1.0 mol O₂ g^-1 fr wt h^-1, respectively). Gross photosynthesis exhibited high values between September and December. Concomitant with the seasonal decrease of photosynthetic efficiency () from mean values of 1.2 mol O₂ g^-1 fr wt h^-1 (mol photons cm^-2 s^-1)^-1 in September to 0.3 mol O₂ g^-1 fr wt h^-1 (mol photons cm^-2 s^-1)^-1 in January, the initial light saturating point (I _k) gradually increased from 19 to 60 mol photons m^-2 s^-1. Likewise C:N ratios were low in spring (12 to 13) and increased in summer (20). In general, the photosynthetic parameters P _max, gross photosynthesis, and Chl a concentrations were significantly higher in the distal section of the thallus. In contrast, C:N ratios were lower in the distal section of the lamina. The results show that photosynthesis obviously strongly supports growth of the alga in late winter to spring, as it does in some morphologically related brown algae from temperate and polar regions. The question whether growth is additionally powered     

Photosynthetic characteristics of marine Synechococcus spp. with special reference to light environments near the bottom of the euphotic zone of the open ocean

The present study aimed to resolve the question why marine Synechococcus spp. abundantly occur even at the bottom of the euphotic zone in the Kuroshio are. Photosynthesis under such conditions was examined using simulated blue-green model light (BGL). Results indicated that photosynthesis of marine Synechococcus spp. under BGL is as active enough to support growth of these organisms. Examination of light-harvesting under BGL indicated that active photosynthesis is permitted by an unusually high abundance of phycoerythrin (PE), which is the main light-harvesting pigment for photosystem II (PSII), due to a phycobilisome (PBS) structure which is different from ordinary hemidiscoidals. Although the absorption maximum of PE is located at longer wavelengths than the energy maximum of BGL, PE was found to absorb BGL significantly. Thus, BGL cannot be a typical photosystem I (PSI) light. PSII is also significantly excited by BGL. Carotenoids, which largely absorb BGL, were found to be effective in light-harvesting for PSI. Based on the results obtained, possible reasons why marine Synechococcus spp. commonly occur in warm waters were discussed. Two strains of Synechococcus spp. isolated from the Gulf Stream in 1981 and from Kuroshio, Japan in 1983 were used in the present study.     

High turnover of inorganic carbon in kelp habitats as a cause of δ13C variability in marine food webs

In a study to assess qualitatively the importance of organic matter derived from kelp production in the Aleutian Islands of subarctic Alaka, replicated samples of autotrophic sources and primary and secondary consumer organisms were sampled for ¹³C among sources, sites, (treatment) islands, and years. Unanticipated variation in the ¹³C of kelps occurred among overtly similar sites at different islands. Variation in the ¹³C of the surface canopy-forming kelp Alaria fistulosa was particularly extreme, ranging from-15.5 to-28.0 compared to the understory kelps, Laminaria spp. A. fistulosa ¹³C varied by as much as 6 to 7 among similar sites at a given island within years, and by as much as 3 to 4 between years at the same sampling site. In serveral cases, ¹³C variation was weakly tracked by some consumer organisms, suggesting that even detritus pathways through the food web can be localized and tightly coupled. Dynamic cycles in the concentration and ¹³C of dissolved inorganic carbon (DIC) and aqueous CO₂ concentration ([CO₂]_aq) were measured at three sites on one island. The ¹³C or organic carbon fixed by A. fistulosa, calculated from diurnal DIC concentration and ¹³C measurements, varied by 15 and varied inversely with [CO₂]_aq concentrations. Local DIC variability, probably resulting from high productivity and decreased turbulence in dense kelp habitats, provides a possible mechanism of variation in kelp ¹³C. The short-term variability in the ¹³C of organic carbon fixed by kelps indicates that sampling methodology and design must assess this potential variation in marine macrophyte ¹³C before making assumptions about the transfer of ¹³C-invariate organic matter to higher trophic levels. On the positive side, a predictable relationship between [CO₂]_aq concentration and kelp ¹³C offers a potentially robust means to assess productivity effects on CO₂ limination in kelps and other complex aquatic macrophyte habitats.     

Photoinhibition at low quantum flux densities in a marine dinoflagellate (Amphidinium carterae)

Growth and photosynthetic properties of the marine dinoflagellate Amphidinium carterae Hulbert were examined under continuous illumination in batch cultures at four different irradiances between 2 and 150 E m^-2 s^-1. The slope of both cell- and Chl a-based photosynthesis versus the irradiance curves was greatest for cells grown at 15 E m^-2 s^-1. The relative Chl a values cell^-1 were 1, 1.5 and 2 for cultures grown at 150, 80 and 15 E m^-2 s^-1, respectively. A low-temperature (-196°C) fluorescence technique was used to examine cells for photoinhibiton. Photoinhibition was greatest for cells grown at 150 E m^-2 s^-1. However, significant photoinhibition of this species was noted even at 80 E m^-2 s^-1. No significant difference in the fluorescence pattern was found between cells grown at 2 and 15 E m^-2 s^-1. Time course studies indicate that photoinhibition may occur within 2 h following exposure to 350 E m^-2 s^-1 in cells grown at 15 E m^-2 s^-1 and is reversible when light levels are lowered within 4 h. The ecological significance of phytoplankton unable to cope with excess photosynthetic excitation energy is discussed.