Notes: Abstract In July 1985, diurnal patterns of photosynthesis and pigmentation were characterized for whole water (〉0.4 μm) and size-fractioned (〉5 μm and 0.4 to 5 μm) communities from three light depths sampled across a coastal thermal front in the Southern California Bight. Samples were collected predawn and held for 20 h in deck incubators. Variations in chlorophyll a and accessory pigment-to-chlorophyll a ratios showed no obvious diurnal trends. Timing of peak photosynthetic potential (P max) and its coincidence with variations in light-limited rates of photosynthesis (alpha), as well as diurnal amplitudes in P max and alpha, often differed between size fractions sampled within the same community. The same was true for identical size fractions collected from different depths and stations transecting the front. Primary productivity was 20-fold greater on the cold water side, where 〉5 μm diatoms dominated the mixed layer and accounted for 80% of daytime productivity. Diatoms collected from the top and bottom of the upper mixed layer displayed nearly identical diurnal patterns in P max and alpha, with midday peaks exceeding predawn values by four-fold and two-fold respectively. Above the pycnocline, the 0.4 to 5 μm fraction had lower assimilation rates than the 〉5 μm fraction and smaller diurnal amplitudes in P max and/or alpha, with daytime patterns often characterized by two peaks interspersed by a short period of photoinhibition. Within the front, the 0.4 to 5 μm fraction accounted for two-thirds of plant biomass and 〉90% of primary production. Pigment analyses by high-performance liquid chromatography revealed enrichment in 19′-hexanoyloxyfucoxanthin, indicative of enhanced numbers of prymnesiophtes. Photosynthetic activity in confined surface communities was susceptible to daytime photoinhibition, but subsurface communities exhibited midday P max peaks that were three-to seven-fold predawn values. In the warm-water mass, both algal size fractions contributed equally to photosynthesis and chlorophyll a in surface waters, with the 0.4 to 5 μm fraction becoming dominant at the base of the euphotic zone. At all depths, peak P max of the 0.4 to 5 μm fraction occurred before noon, while P max of the 〉5 μm fraction was clearly evident in the afternoon. Elevated chlorophyll b-, 19′hexanoyloxyfucoxanthin- and zeaxanthin-to-chlorophyll a ratios indicated a mixture of algal groups, including chlorophytes, cyanobacteria and prymnesiophytes.

Notes: Abstract Between July 21 and August 8, 1984, phytoplankton were collected from the surface (2 m) and/or chlorophyll maximum of a neritic front, warm-core eddy 84-E and Wilkinson's Basin in the Northwest Atlantic Ocean and incubated up to 38 h in 200-liter vats. Effects of light intensity and nutrient availability on diel patterns of cell metabolism were analyzed in a 0.6- to 1-μm fraction, where Synechococcus spp. represented 80 to 100% of the total photoautotrophs. Populations held under in situ conditions exhibited daytime peaks in photosynthetic potential (Pmax) that were an order of magnitude higher than nighttime Pmax values. Daytime phasing of Pmax peaks had no relationship to asynchronous fluctuations in cellular activities of ribulose 1,5 bisphosphate carboxylase (RUBPCase) or phosphoenol pyruvate carboxylase (PEPCase), or to variations in chlorophyll content. Daytime Pmax peaks were about 12 h out of phase with nighttime maxima in the frequency of dividing cells (FDC). The phase relationship between Pmax and FDC could be altered by manipulating environmental conditions. High light exposure of depp populations did not affect timing of the Pmax peak, but its magnitude increased and coincided with increased RUBPCase activity and chlorophyll photobleaching. In the eddy population, a major shift in the timing of peak Pmax was induced when increased light intensity was accompanied by nutrient enrichment. This change coincided with major increases in cellular chlorophyll and carboxylating enzyme activity. Lowering irradiance and/or increasing nutrient availability elicited different diel pattern in cellular metabolism in surface populations from the eddy and from Wilkinson's Basin that appeared linked to differences in the nutrient status of the cells. Rates of cell division estimated from the percentage of dividing cells in preserved samples were 0.83 divisions d-1 in surface warm-core eddy populations, supporting the view that carbon and nitrogen turnover rates in oligotrophic waters can be sufficient to promote near optimal growth of Synechococcus spp.

Notes: Abstract 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 μW 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 μW 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 μmol O2 cm-3 h-1, while photosynthetic performance ranged from 100 to 130 μmol 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 μmol O2 cm-3 h-1 and cell volumes of about 5.4×10-8 cm-3 cell-1. Below about 1250 μW 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.

Notes: Abstract Diel periodicity in parameters of photosynthesis-irradiance (P-I) curves was incorporated into calculations of integral daily phytoplankton production for the Santa Barbara Channel off southern California (USA). Model equations of the relationship between photosynthesis and light were used in combination with observed slope (α) and asymptote (P max) values presented in the preceding paper. Primary production was always 19 to 39% less than comparable estimates obtained with the assumption of constant maximum daily α and P max values. Regardless of which P-I formulation was used of 6 tested, observed production (using a temporal series of simulated in situ incubations) ranged from 13% less to 25% more than estimates from constant midday α and P max values. The amplitude and timing of diel oscillations differed somewhat among 3 field stations. Maximum to minimum ratios ranged from approximately 3 to 5 for α, and 4 to 6 for P max. The differences in amplitude and timing of oscillations in P-I curves both contributed to errors in calculating phytoplankton production. Thus, photosynthetic periodicity in the upwelling area of the Santa Barbara Channel influences phytoplankton production. There were oscillations in both α and P max, and the time-dependence of these parameters should be considered to improve the accuracy of predictive models of primary productivity.

Notes: Abstract Gonyaulax polyedra Stein was grown under bright light (330 μE m2 s-1) and then transferred to low light (80 μE m2 s-1), where the rates of photoadaptation were measured as a function of increasing cell density in increasingly nutrient-limited batch cultures. Photoadaptation required the synthesis of new pigment molecules for the photosynthetic units of the thylakoid membrane. However, as older bright-light cultures were shifted to low-light conditions, the onset of pigment synthesis was delayed, the rates of new chlorophyll synthesis slowed, and final yields of cell chlorophyll were sharply reduced. Moreover, photoadaptive capabilities of photosynthesis observed in log-phase cultures were lost in nutrient-limited stationary cultures. Addition of inorganic nutrients to stationary cultures at the time of transfer to low-light levels induced pigments synthesis at a rate and magnitude comparable to log-phase cultures and led to short-term increases in the rates of cell-and carbon-based photosynthesis. The nutrient status of the population also increased, as witnessed by increased carbon and nitrogen content and a large, but transient, increase in nitrate reductase activity. Improved photosynthetic status did not increase cell division rates over control populations. The study was made from December 1980 through March 1981.

Notes: Abstract Mechanisms of photoadaptation of photosynthesis have been studied in three strains of the symbiotic dinoflagellate Symbiodinium microadriaticum. Algal strains isolated from the clam Tridacna maxima, the sea anemone Aiptasia pulchella, and the scleractinian coral Montipora verrucosa were maintained in the defined medium ASP-8A, and were grown at irradiances ranging from 22 to 248 μE m-2 s-1 on a 14 h:10 h (light:dark) photoperiod at 26°C. All algal cultures were analysed during log-phase of growth. At all light levels, rates of cell division and photosynthesis were determined, as were cell volumes, pigmentation (including chlorophyll a, chlorophyll c 2, peridinin, β-carotene and xanthophylls), and carbon and nitrogen content. Low light-induced changes in pigmentation were evident to varying degrees in all three algal strains, although alterations in the photosynthesis-irradiance relations were distinctly different in each strain. The algae from T. maxima show the least photoadaptive capability, and seem to photoadapt by changing photosynthetic unit (PSU) size. Algae from A. pulchella appear to adapt by changing PSU number, while algae from M. verrucosa appear to photoadapt by changes in the activities of CO2-fixing enzymes or electron transport systems. These are the first observations that demonstrate functional differences in different strains of S. microadriaticum. The adaptive capabilities of the algae appear to correlate well with the ecological distribution of their respective hosts. The study was made from July 1981 through December 1982.

Notes: Abstract 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 (Pmax) 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 Pmax 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.

Notes: Abstract Short-term changes in photosynthesis were documented for 17 of 24 marine phytoplankton species, representing a range of taxonomic groups. Periodicity in phytoplankton photosynthesis on light-dark cycles (diel periodicity) was widespread but not universal for the species studied. The centric diatoms Lauderia borealis, Ditylum brightwellii, Stephanopyxis turris, Coscinodiscus rex, Chaetoceros gracile, and Biddulphia mobiliensis had strong diel periodicity in photosynthetic capacity (P max). Amplitudes of the daily variations ranged from 2.9 to 〉50, with maxima in the morning or near midday, and with minima during the dark period, and these variations were not dependent on changes in cell pigmentation. There was some evidence for sustained photosynthetic periodicity in constant conditions in several diatoms, and an endogenous rhythm may have been present. The photosynthesis-irradiance (P-I) relationship was time-dependent for representative marine diatoms, with both the initial slope (α) and the asymptote (P max) of P-I curves exhibiting significant synchronous diel oscillations. Moreover, detailed studies of the amplitude and timing of photosynthetic periodicity for the diatoms L. borealis and D. brightwellii demonstrated large temporal variations in photosynthesis with morning maxima. These P-I oscillations are discussed with reference to models of primary production which use the relationship between photosynthesis and light as a component of predictive equations for phytoplankton growth in the sea.

Notes: Abstract Gonyaulax polyedra Stein grown in increasingly nutrientlimited batch culture undergoes the following changes (collectively termed aging): there is a decline in the intracellular concentrations of carbon, nitrogen and photosynthetic pigments; nitrate reductase activity decreases; rates of respiration and photosynthesis fall; and cell division virtually ceases (accompanied in bright light by a decrease in the volume of individual cells). The effect of light intensity on these aging events was tested by growing cells in either bright or dim light. The bright light (330 μE m-2 s-1) was enough to saturate photosynthesis and the dim light (80 μE m-2 s-1) was low enough to induce significant shade adaptation of photosynthesis without lowering growth rate. At both light intensities, a decline in carbon and nitrogen content preceded or accompanied all other monitored changes, and the sequence of aging events was similar. However the onset of the decline in intracellular nutrients and photosynthetic rate in low-light cells was delayed by a least one cell division time (i.e., to twice the cell density) in comparison to cells under bright light. At both light levels, pigment-protein complexes of the photosynthetic apparatus began to break down after intracellular carbon and nitrogen had been depleted to a critically low level. The beginning of the drop in pigmentation signalled the end of log-phase growth. It is suggested that the greater pigmentation of low-light cells may represent a larger nutrient supply than found in bright-light cells and could increase the survival time of nutrient-stressed populations.

Notes: Abstract Circadian rhythms in photosynthesis were defined in field populations of phytoplankton. Measurements of carbon-dioxide fixation rates demonstrated that a diurnal periodicity of photosynthesis in samples incubated under natural light-dark (LD) cycles also were observed to continue in similar samples which had been photoadapted to constant dim light (LL) for 48 h. These changes in photosynthetic rates preceded sunset and sunrise, had daily amplitudes that ranged from 1.5 to 2.0, appeared to be independent of light-intensity, and displayed maxima about midday, while rates of dark fixation of carbon dioxide and the photosynthetic pigment content per cell were constant over the circadian cycle. Similar rhythmicity also was detected in room-temperature (22°C) chlorophyll a fluorescence yield, in both the obsence and presence of the photosynthesis inhibitor DCMU [3-(3,4-dichlorophenyl)-1, 1-dimethylurea]. However, the magnitude and timing of the fluorescence rhythm maxima seem to depend on wavelengths monitored and, in part, on the measuring technique used. Also, the circadian changes in the fluorescence intensity were abolished at low temperature (-60°C), and the shape of the emission spectra of chlorophyll fluorescence of cells in LD and LL did not change over time. The significance of the fluorescence rhythms with regard to chlorophyll a determinations and photosynthetic rates is discussed. It was concluded that there was sufficient similarity between circadian rhythms of photosynthesis in natural phytoplankton populations and in laboratory cultures of dinoflagellates to suggest that the mechanism of regulation may be the same for both of them.