Changes In Photon Flux Density Research Articles

Light and brominating activity in two species of marine diatom

Marine organisms mediate the formation of volatile inorganic (e.g. HOBr) and organic halogens (e.g. CHBr3) and contribute to the sea-to-air emission of bromine and iodine. This air-sea halogen exchange has implications for atmospheric chemistry. It is important to establish the physiological function of halogen metabolism in key groups of marine organisms to permit predictive model development. In this study a series of laboratory experiments was performed to investigate the link between the availability of photosynthetically active radiation (PAR) and brominating activity, as measured by the bromination of phenol red, in two cold-water marine diatoms (Thalassiosira antarctica, CCAP 1085/25; Porosira glacialis, CCMP 668). Brominating activity in T. antarctica was found to change in response to short term changes in photon flux density and to have a strong positive linear relationship with gross photosynthetic rate up to 260μmol O2 (mg chla)−1h−1. Experiments performed across multiple diel cycles showed that light-phase brominating activities in T. antarctica were a factor of 2.8 (±1.0) higher than those measured in the dark. Whilst P. glacialis showed no response to short term changes in PFD, measurements across a number of diel cycles revealed that light-phase brominating activities in this diatom were significantly higher than those in the dark by a factor of 1.3 (±0.3). The addition of 0.1μM H2O2 to the medium of T. antarctica cultures led to a significant increase in brominating activity by a factor of 2.4 (±0.3) relative to no-addition controls but no such response was seen in P. glacialis. These results suggest that there is a link between PAR light availability and brominating activity in marine diatoms but that the nature of this relationship differs between species. By establishing a potential link with common ecosystem model state variables (light and photosynthesis) this work provides the first step towards developing a predictive capability for brominating activity in the marine environment. More work is needed to assess the potential for developing generalised parameterisations between PAR light availability and brominating activity in diatom species representative of a wider range of ocean regions.

Annual Fluctuations in Local Photon Counts Reflect Differential Distances from the Galaxy’s Singularity: Astronomical, Chemical and Biology Implications

A conspicuous annual variation in the photon flux density of ~10-12 W·m-2 by a photomultiplier tube housed in a hyperdark ground level setting was within the range expected for the small changes in the earth’s distance per orbit from the energetic entropy (Joules) of the singularity at the center of the galaxy. For the Bekenstein-Hawking relation to be congruent the singularity’s power must reflect the galaxy’s age. The power (W) per volume at the distance of the earth when divided into the peak-to-trough change in photon flux density for minimum-maximum distances from the galactic center converged with the Compton wavelength for an electron. Subsequent calculations indicated that the discrepancy between the magnetic moments of the orbit/spin of the electron and of the proton when the energy associated with the neutral hydrogen line is involved with the Bohr atom also solve for the Compton electron wavelength. The most likely local mechanism to satisfy the hyperfine currents that are in the order of 10-15 A within the width of plasma membrane phenomena would be proton channels when pH ranges from about 5.8 to 7.4 which can occur transiently near neurons. The measured photon flux densities multiplied by known diffusion velocities of protons in water could produce energy levels of ~10-20 J per square unit of time. The results suggest the potential for collapsing de Broglie particle-wave properties of an electron involves energies associated with neuronal action potentials coupled to cognition and that this potential may be spread through the galactic volume as pervasive photon flux densities originating from the singularity at the center.

Annual Fluctuations in Local Photon Counts Reflect Differential Distances from the Galaxy’s Singularity: Astronomical, Chemical and Biology Implications

A conspicuous annual variation in the photon flux density of ~10-12 W·m-2 by a photomultiplier tube housed in a hyperdark ground level setting was within the range expected for the small changes in the earth’s distance per orbit from the energetic entropy (Joules) of the singularity at the center of the galaxy. For the Bekenstein-Hawking relation to be congruent the singularity’s power must reflect the galaxy’s age. The power (W) per volume at the distance of the earth when divided into the peak-to-trough change in photon flux density for minimum-maximum distances from the galactic center converged with the Compton wavelength for an electron. Subsequent calculations indicated that the discrepancy between the magnetic moments of the orbit/spin of the electron and of the proton when the energy associated with the neutral hydrogen line is involved with the Bohr atom also solve for the Compton electron wavelength. The most likely local mechanism to satisfy the hyperfine currents that are in the order of 10-15 A within the width of plasma membrane phenomena would be proton channels when pH ranges from about 5.8 to 7.4 which can occur transiently near neurons. The measured photon flux densities multiplied by known diffusion velocities of protons in water could produce energy levels of ~10-20 J per square unit of time. The results suggest the potential for collapsing de Broglie particle-wave properties of an electron involves energies associated with neuronal action potentials coupled to cognition and that this potential may be spread through the galactic volume as pervasive photon flux densities originating from the singularity at the center.

Annual Fluctuations (~10<sup>-12</sup> W•m<sup>-2</sup>) in Ground Level Photon Power Densities: Quantitative Evidence for Possible Modulation From the Galactic Center

The numbers of photons per second for more than a year (2013-2014) recorded every ~2.5 s by a digital photomultiplier unit was maximum during August to October and minimum during February through April. The peak-to-peak difference in quantity for the classical, smooth variation was 2.4 x 10-12 W m-2. This value is within the order of magnitude for a photon source from the galactic center with a power of 1059 J from a spherical surface with a radius defined for a singularity that could be equivalent to the galactic mass. The calculated change in photon flux density from such a source for the annual change in the earth’s distance around the sun relative to the distance to galactic center was ~10-21 W m-2 or equivalent to 10 Solar Flux Units (SFU). Data obtained for daily SFU for the years 2009 through 2013 indicated that an average difference was 8 to 10 SFU higher for September-October when the earth would be closer to the galactic center than March-April. The most likely source of the amplitude of the annual variation is consistent with modification by the earth’s position in the solar mass field with respect to the galactic center. These data suggest that at least a subset set of background photon emissions on the earth’s surface display a clear annual variation whose source could originate from a singularity at the distance of the center of the Milky Way.

Phytotoxicity Evaluation of Type B Trichothecenes Using a Chlamydomonas reinhardtii Model System

Type B trichothecenes, which consist of deoxynivalenol (DON) and nivalenol (NIV) as the major end products, are produced by phytotoxic fungi, such as the Fusarium species, and pollute arable fields across the world. The DON toxicity has been investigated using various types of cell systems or animal bioassays. The evaluation of NIV toxicity, however, has been relatively restricted because of its lower level compared with DON. In this study, the Chlamydomonas reinhardtii testing system, which has been reported to have adequate NIV sensitivity, was reinvestigated under different mycotoxin concentrations and light conditions. The best concentration of DON and NIV, and their derivatives, for test conditions was found to be 25 ppm (2.5 × 10−2 mg/mL). In all light test conditions, DON, NIV, and fusarenon-X (FusX) indicated significant growth inhibition regardless of whether a light source existed, or under differential wavelength conditions. FusX growth was also influenced by changes in photon flux density. These results suggest that C. reinhardtii is an appropriate evaluation system for type B trichothecenes.

Sunflecks in trees and forests: from photosynthetic physiology to global change biology

Sunflecks are brief, intermittent periods of high photon flux density (PFD) that can significantly improve carbon gain in shaded forest understories and lower canopies of trees. In this review, we discuss the physiological basis of leaf-level responses to sunflecks and the mechanisms plants use to tolerate sudden changes in PFD and leaf temperature induced by sunflecks. We also examine the potential effects of climate change stresses (including elevated temperatures, rising CO(2) concentrations and drought) on the ability of tree species to use sunflecks, and advocate more research to improve our predictions of seedling and tree carbon gain in future climates. Lastly, while we have the ability to model realistic responses of photosynthesis to fluctuating PFD, dynamic responses of photosynthesis to sunflecks are not accounted for in current models of canopy carbon uptake, which can lead to substantial overestimates of forest carbon fixation. Since sunflecks are a critical component of seasonal carbon gain for shaded leaves, sunfleck regimes and physiological responses to sunflecks should be incorporated into models to more accurately capture forest carbon dynamics.

Responses of Emiliania huxleyi (Prymnesiophyceae) to step changes in photon flux density

A non-calcifying strain (Ply 92) of Emiliania huxleyi (Lohman) Hay et Mohler, was subjected to reciprocal step changes of photon flux density (PFD) between 50 and 800 µmol photons m−2 s−1 to establish the timescales of photoacclimation. Photoacclimation in E. huxleyi involved adjustment of cellular light harvesting pigment contents, but not cellular ribulose bisphosphate carboxylase-oxygenase (RuBisCO) or cellular RNA contents. There was considerable variability in both the magnitude and the rate of the response of physiological and biophysical variables to the step changes in PFD. The slowest response was observed in cellular chl a content, and the chl a/carbon ratio. Acclimation of these variables appeared to be due to dilution of pigment pools rather than to pigment turnover. Changes in the chl a-specific light absorption coefficient accompanied changes in cellular chl a content. Following the shifts in PFD, the efficiency of excitation energy transfer from the photosystem II antennae to reaction centres II (RCII), as assessed from , adjusted within 12 h. The efficiency of charge separation within RCII, as assessed from , adjusted on a similar timescale. The overall efficiency of photosystem II (PSII), as assessed from / , showed a step response to the change of PFD due to inverse changes in , and . Changes in non-photochemical excitation energy quenching were correlated with the relative and absolute abundance of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin, which appeared to be reversibly interconverted with fucoxanthin. Thus, acclimation to PFD involved co-ordinated adjustment of the quantum efficiency of RCII, the efficiency of excitation energy transfer from the light-harvesting pigment bed to RCII, and cellular light absorption on overlapping time scales.

Transcriptional regulation of genes encoding subunits of photosystem I during acclimation to high-light conditions in Synechocystis sp. PCC 6803.

Cyanobacteria, algae and plants reduce photosystem contents and modulate photosystem stoichiometry upon acclimation to high-light (HL) conditions to avoid the damage due to excess light energy. In order to elucidate the molecular mechanism of HL acclimation, mRNA levels of genes encoding subunits of photosystems were examined in response to change of photon flux density in the wild-type strain of Synechocystis sp. PCC 6803. Transcript levels and promoter activities of photosystem I (PSI) genes rapidly decreased upon the shift to HL to less than 10% of the initial level within 1 h, whereas responses of photosystem II (PSII) transcript levels were not coordinated. The prompt change in promoter activities of PSI genes, but not PSII genes, seems important for Synechocystis cells to regulate their photosystem contents in response to changes in photon flux density. The mRNA stabilities of PSI genes also decreased during HL incubation. The down-regulation of PSI transcripts under HL did not depend on de novo protein synthesis, contrasting with the requirement of newly synthesized protein factor(s) for the accumulation of PSI transcripts under low light. When gene expression profiles of wild-type cells and a pmgA-disrupted mutant that cannot modulate photosystem stoichiometry under HL were compared, a large increase in the psaAB and psaA transcript levels was observed in the mutant under prolonged exposure to HL for 6 h. Repression of the promoter activity of the psaA gene by a pmgA-dependent mechanism was shown to be essential for the adjustment of photosystem stoichiometry under HL conditions.

Characterization of the photosynthetic induction response in a Populus species with stomata barely responding to light changes

The photosynthetic induction response is constrained by stomatal and biochemical limitations. However, leaves in some plants like Populus koreana x trichocarpa cv. Peace (a hybrid clone) may have little stomatal limitation because their stomata barely respond to changes in photon flux density (PFD). We examined the induction responses of leaves of well-watered and dehydrated P. koreana x trichocarpa plants grown in a high-light or a low-light regime. With an increase in PFD from 50 to 500 micromol m(-2) s(-1), steady-state stomatal conductance (g(s)) increased by only 0.25-8.2%, regardless of the initial g(s), but steady-state assimilation rate (A) increased by 550-1810%. Photosynthetic induction times required to reach 50% (IT50) and 90% (IT90) of A at high PFD were 60-90 s and 210-360 s, respectively. Examination of the dynamic relationships between A and g(s), and between A and intercellular CO2 concentration, indicated that the induction limitation was imposed completely by the biochemical components within 30-40 s after the PFD increase. Values of IT50 and IT90 were significantly higher in low-light leaves than in high-light leaves, whereas the induction state at 60 s and the induction efficiency at 60 and 120 s after the increase in PFD were lower in low-light leaves than in high-light leaves. Dehydration reduced leaf water potential (psi) significantly, resulting in a significantly decreased initial g(s). Leaf water potential had no significant effects on induction time in high-light leaves, but a low psi significantly reduced the induction time in low-light leaves. We conclude that the photosynthetic induction response was limited almost completely by biochemical components because the stomata barely responded to light changes. The biochemical limitation appeared to be higher in low-light leaves than in high-light leaves. Mild water stress may have reduced steady-state A and g(s), but it had little effect on the photosynthetic induction response in high-light leaves.

Stomatal responses to non‐local changes in PFD: evidence for long‐distance hydraulic interactions

ABSTRACTInteractions among stomata within a single areole have recently been reported, and evidence suggests that hydraulic mechanisms may be responsible for these interactions. Such interactions may play a role in patchy stomatal behaviour by coordinating stomatal behaviour within areoles. However, models suggest that longer‐distance interactions may be required to produce the large‐scale discoordination that is characteristic of stomatal patchiness. This study was undertaken to characterize long‐distance interactions between ‘artificial patches’ of stomata under varying conditions of evaporative demand and soil water stress. Gas‐exchange was monitored in two adjacent regions (‘patches’) of a wheat leaf by two independent gas mixing and analysis systems. When photon flux density (PFD) was changed in only one of these patches, stomatal conductance responded in both patches in a manner consistent with hydraulic interactions propagated by changes in xylem water potential. These data are discussed in the context of mechanisms for patchy stomatal conductance and implications for the design and analysis of gas‐exchange experiments.

Gas-exchange analysis of chloroplastic fructose-1,6-bisphosphatase antisense potatoes at different air humidities and at elevated CO 2

Gas-exchange measurements were performed to analyze the leaf conductances and assimilation rates of potato (Solanum tuberosum L. cv. Desireé) plants expressing an antisense construct against chloroplastic fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) in response to increasing photon flux densities, different relative air humidities and elevated CO(2) concentrations. Assimilation rates (A) and transpiration rates (E) were observed during a stepwise increase of photon flux density. These experiments were carried out under atmospheric conditions and in air containing 500 micromol mol(-1) CO(2). In both gas atmospheres, two levels of relative air humidity (60-70% and 70-80%) were applied in different sets of measurements. Intercellular CO(2) concentration, leaf conductance, air-to-leaf vapour pressure deficit, and instantaneous water-use efficiency (A/E) were determined. As expected, assimilation rates of the FBPase antisense plants were significantly reduced as compared to the wild type. Saturation of assimilation rates in transgenic plants occurred at a photon flux density of 200 micromol m(-2) s(-1), whereas saturation in wild type plants was observed at 600 micromol m(-2) s(-1). Elevated ambient CO(2) levels did not effect assimilation rates of transgenic plants. At 70-80% relative humidity and atmospheric CO(2) concentration the FBPase antisense plants had significantly higher leaf conductances than wild-type plants while no difference emerged at 60-70%. These differences in leaf conductance vanished at elevated levels of ambient CO(2). Stomatal response to different relative air humidities was not affected by mesophyll photosynthetic activity. It is suggested that the regulation of stomatal opening upon changes in photon flux density is merely mediated by a signal transmitted from mesophyll cells, whereas the intercellular CO(2) concentration plays a minor role in this kind of stomatal response. The results are discussed with respect to stomatal control by environmental parameters and mesophyll photosynthesis.

Rapid changes in xanthophyll cycle‐dependent energy dissipation and photosystem II efficiency in two vines, Stephania japonica and Smilax australis, growing in the understory of an open Eucalyptus forest

Leaves of Stephania japonica and Smilax australis were characterized in situ on the coast of north‐eastern New South Wales, Australia, where they were growing naturally in three different light environments: deep shade, in the understory of an open Eucalyptus forest where they received frequent sunflecks of high intensity, and in an exposed site receiving full sunlight. In deep shade the xanthophyll cycle remained epoxidized during the day and the vast majority of absorbed light was utilized for photosynthesis. In the exposed site both deepoxidation and epoxidation of the xanthophyll cycle and changes in the level of xanthophyll‐dependent thermal energy dissipation largely tracked the diurnal changes in photon flux density (PFD). In the understory the xanthophyll cycle became largely deepoxidized to zeaxanthin and antheraxanthin upon exposure of the leaves to the first high intensity sunfleck and this high level of deepoxidation was maintained throughout the day both during and between subsequent sunflecks. In contrast, thermal energy dissipation activity, and the efficiency of photosystem II, fluctuated rapidly in response to the changes in incident PFD. These findings suggest a fine level of control over the engagement of zeaxanthin and antheraxanthin in energy dissipation activity, presumably through rapid changes in thylakoid acidification, such that they became rapidly engaged for photoprotection during the sunflecks and rapidly disengaged upon return to low light when continued engagement might limit carbon gain.

Localization of photosynthetic metabolism in the parasitic angiosperm Cuscuta reflexa

Cells capable of photosynthesis in the parasitic angiosperm Cuscuta reflexa Roxb. (dodder) are highly localized. Immunolocalization of ribulose-1,5 bisphosphate carboxylase-oxygenase (Rubisco) and autofluorescence of chlorophyll in transverse sections of stems showed that they were largely restricted to a band of cells adjacent to the vascular bundles, consequently, the concentrations of Rubisco and chlorophyll were low per unit area or fresh weight. When 14CO2 was supplied to stem segments of C. reflexa it preferentially accumulated in these cells adjacent to the vasculature. Although the conductance for CO2 movement to the cells containing chlorophyll and Rubisco was very low, both the light reactions and dark reactions of photosynthesis appeared to be functional. De-epoxidation of the xanthophyll-cycle pigments after exposure to high light, and the chlorophyll fluorescence parameters, photochemical quenching (qP), non-photochemical quenching (NPQ) and the quantum efficiency of photosystem II (φPSII) responded normally to changes in photon flux density, indicating functional light-driven electron transport. The response of CO2 exchange to photon flux density followed a typical hyperbolic curve, and positive rates of CO2 fixation occurred when external CO2 was increased to 5%. We propose that CO2 for carbon assimilation is derived from internally respired CO2 and that this layer of photosynthetic cells makes a positive contribution to the carbon budget of C. reflexa.

An improved dynamic model of photosynthesis for estimation of carbon gain in sunfleck light regimes

ABSTRACTA dynamic model of leaf photosynthesis for C3 plants has been developed for examination of the role of the dynamic properties of the photosynthetic apparatus in regulating CO2 assimilation in variable light regimes. The model is modified from the Farquhar‐von Caemmerer‐Berry model by explicitly including metabolite pools and the effects of light activation and deactivation of Calvin cycle enzymes. It is coupled to a dynamic stomatal conductance model, with the assimilation rate at any time being determined by the joint effects of the dynamic biochemical model and the stomatal conductance model on the intercellular CO2 pressure. When parametrized for each species, the model was shown to exhibit responses to step changes in photon flux density that agreed closely with the observed responses for both the understory plant Alocasia macrorrhiza and the crop plant Glycine max. Comparisons of measured and simulated photosynthesis under simulated light regimes having natural patterns of lightfleck frequencies and durations showed that the simulated total for Alocasia was within ±4% of the measured total assimilation, but that both were 12–50% less than the predictions from a steady–state solution of the model. Agreement was within ±10% for Glycine max, and only small differences were apparent between the dynamic and steady–state predictions. The model may therefore be parametrized for quite different species, and is shown to reflect more accurately the dynamics of photosynthesis than earlier dynamic models.

Water Relations and CO2 Exchange of Tropical Bryophytes in a Lower Montane Rain Forest in Panama

Abstract:For 6 tropical bryophytes, measurements of the diel courses in water status and net CO2 exchange were made in a submontane tropical rain forest in Panama. In addition, the response of gas exchange to changes in photon flux density (PPFD) and thallus water content (WC) was studied under controlled conditions. Diel variation of WC was pronounced, and both low and high WC limited carbon gain considerably. Low PPFD, e.g. during rain storms, was less important in limiting CO2 exchange. More than half of the mean diurnal carbon gain of 2.9 mg C per g thallus carbon was lost during the night as respiration. Assuming that the average 24‐h carbon gain was representative for the entire year, we estimated the net annual primary productivity of the mosses and liverworts to be 45% of the initial plant carbon content.

Operation of the Xanthophyll Cycle in Non-Stressed and Stressed Cells of Dunaliella salina Teod. in Response to Diurnal Changes in Incident Irradiation: A Correlation with Intracellular β-Carotene Content

Summary Changes in carotenoid composition were monitored on a diurnal basis in cells of Dunaliella salina Teod. grown outdoors, under natural irradiance in mass culture. Stressed cells contained approximately four times more, β-carotene than non-stressed cells. The net change in β-carotene content however, was similar for both treatments and correlated closely with changes in photon flux density during the diurnal cycle. Xanthophyll levels were higher in stressed cells suggesting that excess light was not the only factor involved in stimulating carotenoid accumulation, although xanthophyll content increased in nonstressed cells to a level similar to that of stressed cells, with increasing photon flux density. Light-response curves of zeaxanthin and β-carotene formation displayed a biphasic nature in stressed cells. In nonstressed cells however, zeaxanthin formation was rapid and the descending light-response curve showed little change in net zeaxanthin levels. High β-carotene-containing cells had substantial amounts of zeaxanthin and levels of this carotenoid remained higher in stressed cells throughout the diurnal cycle. Changes in intracellular β-carotene occurred coincident with operation of the xanthophyll cycle during the diurnal period. Non-stressed and stressed cells showed similar de-epoxidation of the xanthophyll cycle at peak photon flux denisty. Although non-stressed cells formed more β-carotene and zeaxanthin in response to increasing irradiance, stressed cells formed β-carotene and zeaxanthin more rapidly i.e. at lower photon flux density. Zeaxanthin formation in stressed cells occurred as a consequence of violaxanthin de-epoxidation. By comparasion, zeaxanthin production in non-stressed cells occurred by two probable mechanisms: 1, by rapid de-epoxidation of violaxanthin and 2, a slow but continuous de novo synthesis which increased the total zeaxanthin pool.

Leaf Nitrogen Content as a Predictor of Photosynthetic Capacity in Ambient and Global Change Conditions

Leaf assimilation capacity in Lolium perenne, grown in elevated CO2 level (700 ,umol mol-1) and/or in- creased air temperature (ambient + 4?C) could be predicted from leaf N content expressed on an area basis, although the linear relationships between maximum carboxylation rate (Vcmax) or maximum electron transport rate (Jmax) and leaf N depended on treatment. The model, based on Farquhar, Von Caemmerer & Berry (1980) showed negative long-term effects of increased air temperature on Vcmax and Jmax, while long- term exposure to increased CO2 level affected only Vcmax. Acclimation responses to these global changes therefore could not be explained by changes in N-content alone, but also in terms of changes in photosynthetic nitrogen use efficiency. Stimulation of photosynthesis by elevated CO2 was not affec- ted by reduction of leaf N in leaves developed in ambient air temperature, while part of the CO2 benefit was lost in leaves developed in increased air temperature. This suggests that N-deficient ecosystems maintain the potential to respond to elevated CO2 concentration, unless other processes than the primary carbon metabolism become limiting at low N supply. Similar to nitrogen content, changes in photon flux density did not change the CO2 benefit either, unless a transition occurred from one limiting process to another (electron transport to carboxylation or vice versa). Hypotheses on interaction be- tween CO2 level, nitrogen status of the leaf and light intensity are formulated to support these findings.

The relationship between photoacclimation and phagotrophy with respect to chlorophyll a, carbon and nitrogen content, and cell size of Chrysochromulina brevifilum (Prymnesiophyceae)

Abstract The chlorophyll content of photosynthetic algae varies as a response to changes in photon flux density. Mixotrophic algae that are predominantly heterotrophic have also been shown to alter their chlorophyll a content as a response to changes in prey density. The marine flagellate Chrysochromulina brevifilum Parke et Manton (Prymnesiophyceae) was subjected to a range of photon flux densities, both with and without Marsupiomonas pelliculata Jones (Pedinophyceae) as a prey organism, and the ingestion of prey, chlorophyll a content, carbon and nitrogen content and cell size were monitored. Results demonstrated that the chlorophyll a content per cell of C. brevifilum changed with photon flux density, but prey density had no effect. Under a relatively low photon flux density (25 μmol photons m−2 s−1) both the ingestion of prey and the cellular concentration of chlorophyll a were greater than under a higher photon flux density (100 μmol photons m−2 s−1). However, the recent history of cultures of C. bre...

Acclimation and adaptation to irradiance in symbiotic dinoflagellates. I. Responses of the photosynthetic unit to changes in photon flux density

The photosynthetic responses to variations in photon flux density were determined for Symbiodinium microadriaticum, the symbiont of the Caribbean jellyfish Cassiopeia xamachana, S. kawagutii, the symbiont of the Indo-Pacific stony coral Montipora verrucosa, and S. pilosum, the symbiont of the Caribbean zoanthld Zoanthus sociatus. Photosynthetic responses were characterized in terms of cellular pigment content, photosynthesis versus irradiance (P-I) relationships, and number and size of the photosynthetic unit (PSU). Analyses of the responses under 2 Light regimes of 40 and 250 p 0 1 quanta m-' S-' indicate that: (1) the 3 different species cultured under identical conditions possess different photosynthetic characteristics; (2) the 3 species acclimate to low photon flux density by simultaneously increasing the number and size of their PSU; (3) PSU characteristics were not correlated with the parameters of their respechve P-lcurves; (4) the 3 species analyzed have different photoacclimatory capabilities which can be correlated with their respective ecological distribution as endosymbionts. As different species demonstrate characteristic responses, photo-acclunatory adjustment in symbiotic dinoflagellates may be under genetic constraints, and thus may represent photo-adaptation (sensu 0. Bjorkman). As a whole, the results suggest that symbiont photo-adaptation may constitute an important axis of niche diversification for the intact associations.

STRATEGIES AND KINETICS OF PHOTOACCLIMATION IN THREE ANTARCTIC NANOPHYTOFLAGELLATES1

ABSTRACTThree Antarctic nanophytoflagellates (two cryptophyte species and a Pyramimonas sp.) were compared for their capacity to phiotoacclimate and for their kinetic responses in changing photic environments. Division rate, cell size cellular fluorescence, and chlorophyll a content were measured steady and transient states of semi‐continuous cultures maintain at 1.0° C. Of all parameters tested, cell size was most affected by irradiance. Acclimation kinetics were modeled using a first‐order equation. Rates of change in cell size following shifts in irradiance were comparable with rates of change in chemical composition reported for temperate algae. Response rates of cellular in vivo red and orange fluorescence were lower. In many cases, however, responses could not be described by the first‐order kinetic model. Division rates remained high for approximately 3 days following a shift down in irradiance, after which new division rates were established. The nanoflagellates studied here appear to respond to small irradiance perturbations at low rates. However, they may fail to adapt and abrupt changes in photon flux density (PFD). When shade‐adapted (25 μmol, m−2, m−2, s−1) cells were exposed to high PFD (400 μmol, m−2, s−1) for 1–3 days, cell were incapable of readapting division rate and pigment content to the initial irradiance condition (25 μmol, m−2, s−1) for about 1 month following the shift‐down step. The ecological role of the kinetics of photoacclimation in nanophytoflagellate growth performance in Antarctic ecosystems is discussed.