Diel periodicity of photosynthesis in marine phytoplankton

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 () 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.     

Diel oscillations of the photosynthesis-irradiance (P-I) relationship in natural assemblages of phytoplankton

Diel oscillations in the photosynthesis-irradiance (P-I) relationship are described for marine phytoplankton assemblages at 6 stations in an upwelling area off the southern California coast (USA) between May and August 1980. The initial slope () and asymptote (P _max) of P-I curves changed significantly over the day; both parameters were in phase and had similar changes in amplitude. The diel oscillations in photosynthesis appeared unrelated to changes in chlorophyll a concentrations. Amplitudes of daily variations in photosynthesis ranged from approximately 3 to 9, as measured by the maximum to minimum ratio for photosynthetic capacity (P _max). Diatom-rich samples collected during an upwelling event and those dominated by dinoflagellates both had midday to early afternoon maxima in and P _max. Samples from other locations had peak photosynthetic activity later in the afternoon. The relationship between and P _max was consistent in all phytoplankton samples analyzed, with a surprisingly high correlation considering the spatial and temporal scales encompassed in this study. These results indicate that the photosynthesis-irradiance (P-I) relationship is time-dependent and, moreover, that changes in and P _max are closely coupled for a variety of natural phytoplankton assemblages.     

Effects of nitrate on the diurnal vertical migration,carbon to nitrogen ratio,and the photosynthetic capacity of the dinoflagellate Gymnodinium splendens

A non-thecate dinoflagellate, Gymnodinium splendens, was studied in a 12 d laboratory experiment in 2.0x0.25 m containers in which light, temperature, and nutrients could be manipulated. Under a 12 h light: 12 h dark cycle, the dinoflagellates exhibited diurnal vertical migrations, swimming downward before the dark period began and upward before the end of the dark period. This vertical migration probably involved geotaxis and a diel rhythm, as well as light-mediated behavior. The vertical distribution of nitrate affected the behavior and physiology of the dinoflagellate. When nitrate was present throughout the container, the organisms resembled those in exponential batch culture both in C:N ratios and photosynthetic capacity (P_max); moreover, they migrated to the surface during the day. In contrast, when nitrate was depleted, C:N ratios increased, P_max decreased, and the organisms formed a subsurface layer at a depth corresponding to the light level at which photosynthesis saturated. When nitrate was present only at the bottom of the tank, C:N ratios of the population decreased until similar to those of nutrient-saturated cells and P_max increased; however, the dinoflagellates behaved the same as nutrient-depleted cells, forming a subsurface layer during the light period. Field measurements revealed a migratory subsurface chlorophyll maximum layer dominated by G. splendens. It was just above the nitracline during the day, and in the nitracline during the night, which concurs with our laboratory observations.     

Diurnal patterns of size-fractioned primary productivity across a coastal front

In July 1985, diurnal patterns of photosynthesis and pigmentation were characterized for whole water (>0.4 m) and size-fractioned (>5 m and 0.4 to 5 m) 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 m 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 m fraction had lower assimilation rates than the >5 m 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 m 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 m fraction becoming dominant at the base of the euphotic zone. At all depths, peak P _max of the 0.4 to 5 m fraction occurred before noon, while P _max of the >5 m fraction was clearly evident in the afternoon. Elevated chlorophyll b-, 19hexanoyloxyfucoxanthin- and zeaxanthin-to-chlorophyll a ratios indicated a mixture of algal groups, including chlorophytes, cyanobacteria and prymnesiophytes.     

Influence of light and temperature on the growth rate of estuarine benthic diatoms in culture

Four species of estuarine benthic diatoms: Amphiprora c. f. paludosa W. Smith, Nitzschia c. f. dissipata (Kützing) Grunow, Navicula arenaria Donkin, and Nitzschia sigma (Kützing) W. Smith were grown in unialgal cultures. The growth rates of the diatoms were determined as the rate of increase of the chlorophyll a content of the cultures. The diatoms were cultured at different combinations of temperture, daylength, and quantum irradiance. The highest growth rates of Navicula arenaria occurred at 16° to 20°C; the other 3 species had their optimum at 25°C or higher. The small-celled species had higher growth rates at their optimum temperature, but at lower temperatures the growth rates of all 4 species became very similar. The minimum daily quantum irradiance that could effect light-saturated growth at 12° and 20°C ranged from 2.5 to 5.0 E.m^-2.day^-1. At 12°C, two species had their highest growth rates under an 8 h daily photoperiod. At 20°C, the three species tested all had highest growth rates under 16 h daily photoperiod. The growth response of the benthic diatoms is comparable to that of several cultures of planktonic diatoms, as described in the literature. The influence of temperature and quantum irradiance on the diatoms in the present investigation was comparable to the influence of temperature and light intensity on the ¹⁴C-fixation of marine benthic diatoms (Colijn and van Buurt, 1975).     

Effects of light intensity and nutrient availability on diel patterns of cell metabolism and growth in populations of Synechococcus spp.

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-m fraction, where Synechococcus spp. represented 80 to 100% of the total photoautotrophs. Populations held under in situ conditions exhibited daytime peaks in photosynthetic potential (P_max) that were an order of magnitude higher than nighttime P_max values. Daytime phasing of P_max 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 P_max peaks were about 12 h out of phase with nighttime maxima in the frequency of dividing cells (FDC). The phase relationship between P_max and FDC could be altered by manipulating environmental conditions. High light exposure of depp populations did not affect timing of the P_max 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 P_max 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.     

Light-dependence of carbon and sulfur production by polar clones of the genus Phaeocystis

Blooms of the marine prymnesiophyte genus Phaeocystis link the oceanic and atmospheric compartments of the carbon and sulfur cycles. Modeling the fluxes of dimethylsulfide from the ocean to the atmosphere has been limited due to a lack of information on functional responses to environmental variables. In this study, the light-dependence of extracellular carbon production and dimethyl sulfide (DMS) production by non-axenic polar clones of Phaeocystis spp. was examined at different growth stages. Comparative experiments were run with non-axenic arctic clones of the diatoms Thalassiossira nordenskioeldii and Skeletonema costatum. A large portion of carbon incorporated by the colonial stage of Phaeocystis spp. is released extracellularly, in particular in stationary colonies. This extracellular production can be modeled as a function of irradiance, as for carbon incorporation. In Phaeocystis spp., cellular and extracellular carbon incorporation represent different uptake rates, indicating the formation of two distinct carbon pools. The release of extracellular carbon by polar Phaeocystis spp. was not a constant fraction of total production over the irradiance range used. We observed little extracellular carbon production by cells at high irradiance, and maximal rates were observed at intermediate irradiance. Newly incorporated carbon that accumulates in the mucilage of the colonial stage of antarctic Phaeocystis sp. during photosynthesis was not reutilized for cellular growth during the dark period, as observed for temperate clones. In contrast, only a minor fraction of the radiocarbon incorporated by the diatoms was released extracellularly for all growth stages. The production of DMS was an order of magnitude higher for Phaeocystis spp. than for diatoms. The chlorophyll-specific production of DMS and DMSP (dimethylsulphoniopropionate, the precursor to DMS) by Phaeocystis spp. showed a hyperbolic response to irradiance, while arctic diatoms (weak or non-producers of DMS), on the other hand, did not show any light-dependency of DMS production. An inverse relationship between DMS and DMSP production in stationary clones of arctic P. pouchetii was observed, but not for the exponentially growing antarctic clone. Stationary colonies also had higher DMS and dissolved DMSP production rates than exponentially growing ones. These relationships can be extrapolated to the field in areas where Phaeocystis spp. dominates.     

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.     

Light-shade adaptation by the oceanic dinoflagellates Pyrocystis noctiluca and P. fusiformis

Species-specific rates of photosynthetic carbon uptake (P), chlorophyll a content and P versus irradiance (P-I), have been measured for cells of Pyrocystis noctiluca and P. fusiformis isolated from natural populations collected in the euphotic zone within and below the surface mixed layer in the Sargasso Sea. These same measurements and the assay for ribulose bis-phosphate carboxylase (RuBP-Case), have been made for cultures of P. noctiluca in a 12 h L: 12 h D photoperiod at 9 different constant or at changing light intensities. In nature chl a cell^-1 was constant throughout the euphotic zone. The photosynthetic capacity (P_max), of cells captured below the surface mixed layer was lower by a factor of 10 compared with cells collected from the surface mixed layer. The P_max for P. noctiluca collected and incubated within the surface mixed layer was the same as for cell cultures grown under high light, non nutrient-limiting conditions, suggesting that photosynthesis in the natural system was not nutrient limited. In laboratory cultures under constant low light intensities, chl a cell^-1 increased by a factor of 5 while both P_max and RuBPCase activity decreased by a factor of ca 4 compared with high light intensities. In changing light intensities both P_max and RuBPCase activities were decreased by factors of 4 during low light intervals while chl a cell^-1 approached a constant intermediate value. The change in chl a cell^-1 in response to prolonged exposure to constant low light intensities was first order with a rate constant of 0.33 d^-1. For all irradiance conditions in culture, the P-I dependence could be described by the simple Michaelis-Menten formula. The ratio of P_max to K_I, (the light intensity where P=P_max/2) was a constant with a Coefficient of Variation of 12%: The constancy of this ratio, the parallel changes in RuBPCase activity with P_max and the constant chl a cell^-1 in the Sargasso Sea imply that for P. noctiluca and presumably P. fusiformis in nature, a dark enzymatic step rather than changes in photosynthetic pigment concentrations may regulate the photosynthetic capacity in the changing photic environment.Contribution no. 1141 from McCollum-Pratt Institute and Department of Biology, The Johns Hopkins University. Supported by DOE contract no. EY 76S20 3278, NSF no. OCE 76-02571 and ONR no. N300014-81-C-0062     

Primary production as influenced by diel periodicity of phytoplankton photosynthesis

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 () 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 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 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 , 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 and P _max, and the time-dependence of these parameters should be considered to improve the accuracy of predictive models of primary productivity.     

Growth,photosynthesis and carbon metabolism in the temperate marine diatom Skeletonema costatum adapted to low temperature and low photon-flux density

The temperate diatom Skeletonema costatum (Grev.) Cleve was grown in low temperature and/or low light conditions. The cultures were acclimatized for at least three months before experiments were begun. Our data indicate that the initial slope of the photosynthesis vs irradiance curve () is controlled predominantly by light history and the light-saturated photosynthesis (P _max) by temperature. The number of divisions per day decreased with decreasing light intensity, but was identical for cultures grown at 3° or 18°C. The metabolic pathways of inorganic carbon fixation were not fundamentally affected by low temperature or low light intensity, but both these factors increased labelling of C₃ compounds, synthesized by the Calvin-Benson cycle, and decreased that of phosphoenolpyruvate (PEP) and other metabolites. This indicates an enhancement of ribulose-1,5-bisphosphate (RuBP) carboxylase activity, which is the first step in the C₃ pathway (3-phosphoglycerate and sugar phosphate synthesis); this may optimize cell functions. At low temperatures, a seven-fold increase in RuBP carboxylase activity per cell was observed. S. costatum is able to adapt to low irradiance by increasing and decreasing I _k (the ratio of P _max:, light intensity at onset of light saturation), and to low temperature by increasing its cellular chlorophyll a and RuBP carboxylase content. However, in the latter case, adaptation is not optimal. This study revealed two main features: (1) there is evidence that RuBP carboxylase has a key function in adjustment to high rates of photosynthesis at suboptimal temperatures or irradiances; (2) adaptive mechanisms are dynamic processes and the role of the time scale in physiological adaptation should be considered.     

Diel changes in phytoplankton photosynthetic efficiency in Brackish waters

From 18 to 23 September 1974, investigations on the diel changes in phytoplankton were carried out in the Baltic Sea. Every 4 h, water samples were collected from 2 and 15 m, and PO₄, chlorophyll a, temperature, salinity, pH, phytoplankton composition and phytoplankton light photosynthesis relationship were determined. Continuous measurements of surface irradiance and some estimations of zooplankton were also made. P ^B (photosynthesis per unit chlorophyll a at low light levels of 2·10^-2 cal cm^-2 min^-1) revealed only random variation during the sampling period, i.e., 1.0 to 1.6 mg C (mg chlorophyll a)^-1 h^-1. P _m ^B (Light-saturated photosynthesis per unit of chlorophyll a) displayed pronounced diel fluctuations with the highest value of about 6 mg C (mg chlorophyll a)^-1 h^-1 around noon, and the lowest value of about 2.5 mg C (mg chlorophyll a)^-1 h^-1 during the night, during which latter period the value of P _m ^B was more or less constant. Reasons for the diel fluctuations are discussed, and an equation which describes these fluctuations is proposed. Using this equation, the daily phytoplankton production estimated in incubators by a previously described method can be corrected for the time of day at which samples are collected.     

Light response curve methodology and possible implications in the application of chlorophyll fluorescence to benthic diatoms

Chlorophyll a fluorescence has been increasingly applied to benthic microalgae, especially diatoms, for measurements of electron transport rate (ETR) and construction of rapid light response curves (RLCs) for the determination of photophysiological parameters [mainly the maximum relative ETR (rETR_max), the light saturation coefficient (E _k) and the maximum light use coefficient (α)]. Various problems with the estimation of ETR from the microphytobenthos have been identified, especially in situ. This study further examined the effects of light history of the cells and light dose accumulation during RLCs on the fluorescence measurements of ETR using the benthic diatom Navicula phyllepta. RLCs failed to saturate when using incremental increases in irradiance, however, curves with decreasing irradiance did saturate. Patterns indicating photoacclimation in response to light histories were observed, with higher rETR_max and E _k, and lower α, at high light compared to low light. However, these differences could be negated by increasing the RLC irradiance duration from 30 to 60 s. It is suggested that problems arose as a result of rapid fluorescence variations due to ubiquinone (Q_A) oxidation and non-photochemical chlorophyll fluorescence quenching (NPQ) which depended upon the light history of the cells and the RLCs accumulated light dose. Also, RLCs with irradiance duration of 10 s were shown to have a high level of error possibly specific to the fluorimeter programming. It is suggested that RLCs, using a Diving-PAM fluorimeter on benthic diatoms, should be run using decreasing irradiance steps of 30 s duration.     

Effects of four environmental variables on photosynthesis-irradiance relationships in Antarctic sea-ice microalgae

The effects of temperature, salinity, growth irradiance and diel periodicity of incident irradiance on photosynthesis-irradiance (P-I) relationships were examined in natural populations of sea-ice microalgae from McMurdo Sound in the austral spring of late 1984. Both P _m ^b (photosynthetic rate at optimum irradiance) and ^b (initial slope or P-I curve) were temperature-dependent reaching optimal rates at approximately +6° and +2°C, respectively. P-I relationships showed little difference at 20 and 33 S; however, no measurable photosynthesis by sea-ice microalgae was detected in a 60 S solution of brine collected from the upper layers of congelation ice. Although diel periodicity characteristic of the under-ice light field appeared to have little effect on P-I relationships, changes in growth irradiance had a profound effect. An increase in growth irradiance from 7 E m^-2 s^-1 (ambient) to 35 or 160 E m^-2 s^-1 resulted in a transient three-fold increase in P _m ^b and I _k (index of photoadaptation) during the first four days, followed by a sharp decline. The effects of these environmental factors on ice algal photosynthesis may influence the distribution of microalgae in sea-ice environments.     

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.     

Effects of salinity and possible interactions with temperature and pH on growth and photosynthesis of <Emphasis Type="Italic">Halophila johnsonii</Emphasis> Eiseman

The effects of salinity, temperature, and pH variations on growth, survival, and photosynthetic rates of the seagrass Halophila johnsonii Eiseman were examined. Growth and survival responses to salinity were characterized by aquarium experiments in which plants were exposed to seven different salinity treatments (0, 10, 20, 30, 40, 50, and 60 psu) during 15 days. Photosynthetic behavior was assessed for short-term salinity exposures (1 or 20 h) by incubation experiments in biological oxygen demand (BOD) bottles and by measuring photosynthesis versus irradiance (PI) responses in an oxygen electrode chamber. In the bottle experiments the possible effects of interactions between salinity and temperature (15, 25, and 35°C) or pH (5, 6, 7, and 8.2) were also examined. Growth and survival of H. johnsonii were significantly affected by salinity, with maximum rates obtained at 30 psu. Salinity also altered the parameters of the PI curves. Light-saturated photosynthesis (P _max) and the photosynthetic efficiency at subsaturating light (α) increased significantly up to an optimum of 40 psu, decreasing again at the highest salinities. Dark respiration rates and compensating irradiance (I _c) showed minimum values at 40 and 50 psu, while light-saturation point (I _k) was maximum at 30–50 psu. An interaction between salinity and temperature was not found although an increase of temperature alone produced an increase in α, P _max, respiration rates, and I _k. An interaction between salinity and pH was only found in the P _max response: P _max increased with pH=5 at 30 psu. In addition, reducing the pH increased α significantly. In the BOD bottles experiment a significant reduction in the dark respiration with decreasing pH was observed, but the opposite trend was observed in the photosynthetic rate. These results suggest that the endemic seagrass H. johnsonii could be negatively affected by hypo- or hypersalinity conditions, although salinity changes did not seem to alter the tolerance of this species to other environmental factors, such as temperature or pH.     

Diurnal photosynthetic performance of seaweeds measured under natural conditions

Statements comparing photosynthetic performance characteristics of species rely upon empirical data, usually light-saturation curves (photosynthesis, P, versus incidentlight flux-density, I _o, relationships) derived from instantaneous measurements. The specific comparative parameters are initial slope and maximum photosynthesis, P _max. For phytoplankton, diurnal variation in specific productivity at maximum incident light, I _max is typically asymmetrical, i.e., there is a morning maximum followed by an afternoon depression. Five seaweed species, numerical dominants from the Outer Banks of North Carolina, were examined for patterns of diurnal photosynthetic performance in sunlight of habitat equivalence. These were Codium decorticatum (Woodw.) Howe and Ulva curvata (Kütz.) De Toni in the Chlorophyceae, Dictyota dichotoma (Huds.) Lamour. and Petalonia fascia (O.F. Müll.) Küntze in the Phaeophyceae, and Gracilaria foliifera (Forssk.) Børg. in the Rhodophyceae. Diurnal patterns of oxygen exchange were varied, some symmetrical about the midday axis, others asymmetrical, and were specific for (1) species, (2) derived habitat, (3) thallus absorptance (1-I/I _o, where I is the transmitted light), (4) developmental stage, and (5) diurnal photosynthetically active radiation (PAR) history. All species show a depression in oxygen exchange rates at less than 0.1 I _max, and show varying degrees of recovery when I _o decreases from that value. Diurnal photosynthetic performance of some species at 0.03 I _o (total diurnal maximum) exceeds several times that at 0.70 I _o (total diurnal maximum), an observation not predicted by instantaneous measurements. Specific day-rate integrals of I _o vary, producing transient initial slope and P _max values. Thus, initial slope and P _max values derived from instantaneous measurements in the laboratory bear little relationship to actual diurnal production. At this time there appears to be no substitute for direct measurement of diurnal photosynthesis.     

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.     

Effects of irradiance and ultraviolet radiation on photoadaptation in the zooxanthellae of Aiptasia pallida: primary production,photoinhibition, and enzymic defenses against oxygen toxicity

Cnidarians which contain symbiotic algae are constantly faced with the challenges of a changing photic regime and a hyperoxic environment. Zooxanthellae (Symbiodinium sp.) from the sea anemone Aiptasia pallida (Verrill), collected and cultured at Bermuda Biological Station in 1986, exhibit a suite of compensatory responses to changes in irradiance, ultraviolet radiation (UV), and to the toxicity resulting from their interaction with photosynthetically produced oxygen. Superoxide dismutase (SOD) and catalase inactivate superoxide radicals (O₂ ^-) and hydrogen peroxide (H₂O₂), which are mediators of oxygen toxicity, show an increase in specific activity with irradiance and in response to UV, both in cultured zooxanthellae (CZ) and freshly isolated zooxanthellae (FIZ) from acclimated anemones. CZ and FIZ exposed to environmentally realistic UV levels show a 30 to 40% increase in SOD activities compared with zooxanthellae exposed to similar irradiances without UV. CZ consistently show higher activities of both SOD and catalase compared to FIZ. Both CZ and FIZ exhibit changes in chlorophyll content and in the relationship between photosynthesis and irradiance which suggest photoadaptive changes in CO₂-fixing enzymes, the photosynthetic-electron transport system, or in photosynthetic unit size (PSU). UV has a greater effect on the photosynthetic capacity (P _max) of FIZ when compared to CZ acclimated at an equivalent irradiance with or without a UV component. UV also enhances the photoinhibition observed at high irradiance in both CZ and FIZ. Differences in enzyme activity between CZ and FIZ suggest an important role for the host in the protection of zooxanthellae against the direct effects of environmentally realistic UV while the photosynthetic performance of zooxanthellae in situ may not be as well protected.     

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.