Light-adapted Cells Research Articles

Combining Nitellopsis obtusa autofluorescence intensity and F680/F750 ratio to discriminate responses to environmental stressors

Detection of autofluorescence parameters is a useful approach to gain insight into the physiological state of plants and algae, but the effect of reabsorption hinders unambiguous interpretation of in vivo data. The exceptional morphological features of Nitellopsis obtusa made it possible to measure autofluorescence spectra along single internodal cells and estimate relative changes in autofluorescence intensity in selected spectral regions at room temperatures, avoiding the problems associated with thick or optically dense samples. The response of algal cells to controlled white light and DCMU herbicide was analyzed by monitoring changes in peak FL intensity at 680 nm and in F680/F750 ratio. Determining the association between the selected spectral FL parameters revealed an exponential relationship, which provides a quantitative description of photoinduced changes. The ability to discern the effect of DCMU not only in the autofluorescence spectra of dark-adapted cells, but also in the case of light-adapted cells, and even after certain doses of excess light, suggests that the proposed autofluorescence analysis of N. obtusa may be useful for detecting external stressors in the field.

Antarctic benthic diatoms after 10 months of dark exposure: consequences for photosynthesis and cellular integrity.

Antarctic algae are exposed to prolonged periods of extreme darkness due to polar night, and coverage by ice and snow can extend such dark conditions to up to 10 months. A major group of microalgae in benthic habitats of Antarctica are diatoms, which are key primary producers in these regions. However, the effects of extremely prolonged dark exposure on their photosynthesis, cellular ultrastructure, and cell integrity remain unknown. Here we show that five strains of Antarctic benthic diatoms exhibit an active photosynthetic apparatus despite 10 months of dark-exposure. This was shown by a steady effective quantum yield of photosystem II (Y[II]) upon light exposure for up to 2.5 months, suggesting that Antarctic diatoms do not rely on metabolically inactive resting cells to survive prolonged darkness. While limnic strains performed better than their marine counterparts, Y(II) recovery to values commonly observed in diatoms occurred after 4-5 months of light exposure in all strains, suggesting long recovering times. Dark exposure for 10 months dramatically reduced the chloroplast ultrastructure, thylakoid stacking, and led to a higher proportion of cells with compromised membranes than in light-adapted cells. However, photosynthetic oxygen production was readily measurable after darkness and strong photoinhibition only occurred at high light levels (>800 µmol photons m-2 s-1). Our data suggest that Antarctic benthic diatoms are well adapted to long dark periods. However, prolonged darkness for several months followed by only few months of light and another dark period may prevent them to regain their full photosynthetic potential due to long recovery times, which might compromise long-term population survival.

Photosynthesis supported by a chlorophyll f-dependent, entropy-driven uphill energy transfer in Halomicronema hongdechloris cells adapted to far-red light.

The phototrophic cyanobacterium Halomicronema hongdechloris shows far-red light-induced accumulation of chlorophyll (Chl) f, but the involvement of the pigment in photosynthetic energy harvesting by photosystem (PS) II is controversially discussed. While H. hongdechloris contains negligible amounts of Chl f in white-light culture conditions, the ratio of Chl f to Chl a is reversibly changed up to 1:8 under illumination with far-red light (720-730nm). We performed UV-Vis absorption spectroscopy, time-integrated and time-resolved fluorescence spectroscopy for the calculation of decay-associated spectra (DAS) to determine excitation energy transfer (EET) processes between photosynthetic pigments in intact H. hongdechloris filaments. In cells grown under white light, highly efficient EET occurs from phycobilisomes (PBSs) to Chl a with an apparent time constant of about 100ps. Charge separation occurs with a typical apparent time constant of 200-300ps from Chl a. After 3-4 days of growth under far-red light, robust Chl f content was observed in H. hongdechloris and EET from PBSs reached Chl f efficiently within 200ps. It is proposed based on mathematical modeling by rate equation systems for EET between the PBSs and PSII and subsequent electron transfer (ET) that charge separation occurs from Chl a and excitation energy is funneled from Chl f to Chl a via an energetically uphill EET driven by entropy, which is effective because the number of Chl a molecules coupled to Chl f is at least eight- to tenfold larger than the corresponding number of Chl f molecules. The long lifetime of Chl f molecules in contact to a tenfold larger pool of Chl a molecules allows Chl f to act as an intermediate energy storage level, from which the Gibbs free energy difference between Chl f and Chl a can be overcome by taking advantage from the favorable ratio of degeneracy coefficients, which formally represents a significant entropy gain in the Eyring formulation of the Arrhenius law. Direct evidence for energetically uphill EET and charge separation in PSII upon excitation of Chl f via anti-Stokes fluorescence in far-red light-adapted H. hongdechloris cells was obtained: Excitation by 720nm laser light resulted in robust Chl a fluorescence at 680nm that was distinctly temperature-dependent and, notably, increased upon DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) treatment in far-red light-adapted cells. Thus, rather than serving as an excitation energy trap, Chl f in far-red light-adapted H. hongdechloris cells is directly contributing to oxygenic photosynthesis at PSII.

Light-independent biosynthesis and assembly of the photosystem II complex in the diatom Chaetoceros gracilis

Diatoms can survive for long periods in the dark. However, how biosynthesis of photosynthetic proteins contributes to survival in the dark is poorly understood. Using a radiolabeling technique, we examined whether de novo biosynthesis and assembly of photosynthetic proteins differs in light-adapted vs. dark-adapted marine diatoms (Chaetoceros gracilis). In light-adapted cells, D1 protein was heavily radiolabeled owing to rapid turnover of photosystem II (PSII). In dark-adapted cells (>24h), the radiolabeling patterns of PSII components changed, but the PSII dimer still formed. Therefore, diatoms may regulate the biosynthesis of photosynthetic proteins for long-term survival in the dark.

Longitudinal profiles of the vacuolar pH in internally perfused cells of characean alga

Activities of ion pumps and H+-conducting channels in the plasmalemma of illuminated characean algae are distributed inhomogeneously along the internode, which accounts for the shifts of surface pH up to 3.5 units between various cells regions. Spatial variations in cytoplasmic properties provide the basis for uneven distribution of photosynthetic activity along the cell length and might affect the operation of H+-transporting systems at the tonoplast. In order to visualize the longitudinal distribution of the vacuolar pH in Chara corallina internodal cells, the pH microelectrode was inserted into the vacuole and the cell sap was gradually displaced along the cell during intracellular perfusion with an artificial medium. Fluorescein was added to the perfusion medium as a fluorescent marker to detect the arrival of the replacing medium into the area of pH and fluorescence measurements. In light-adapted cells, nonuniform longitudinal pH profiles were observed, with pH shifts as large as 2–2.5 units. In dark-adapted cells, the pH shifts in longitudinal profiles did not exceed 0.5 pH units. The occurrence of large pH changes within the vacuole of individual internodes indicates the possibility of nonuniform distribution of the tonoplast H+-transporting systems in different regions of the illuminated cell.

Distribution of phosphorylated protein kinase C alpha in goldfish retinal bipolar synaptic terminals: control by state of adaptation and pharmacological treatment

Protein kinase C (PKC) is a signalling enzyme critically involved in many aspects of synaptic plasticity. In cyprinid retinae, the PKC alpha isoform is localized in a subpopulation of depolarizing bipolar cells that show adaptation-related morphological changes of their axon terminals. We have studied the subcellular localization of phosphorylated PKC alpha (pPKC alpha) in retinae under various conditions by immunohistochemistry with a phosphospecific antibody. In dark-adapted retinae, pPKC alpha immunoreactivity is weak in the cytoplasm of synaptic terminals, labelling being predominantly associated with the membrane compartment. In light-adapted cells, immunoreactivity is diffusely distributed throughout the terminal. Western blot analysis has revealed a reduction of pPKC alpha immunoreactivity in cytosolic fractions of homogenized dark-adapted retinae compared with light-adapted retinae. Pharmacological experiments with the isoform-specific PKC blocker Goe6976 have shown that inhibition of the enzyme influences immunolabelling for pPKC alpha, mimicking the effects of light on the subcellular distribution of immunoreactivity. Our findings suggest that the state of adaptation modifies the subcellular localization of a signalling molecule (PKC alpha) at the ribbon-type synaptic complex. We propose that changes in the subcellular distribution of PKC alpha immunoreactivity might be one component regulating the strength of the signal transfer of the bipolar cell terminal.

Primary photoprocesses in oxyblepharismin interacting with its native protein partner

The primary photoprocesses in the photoreceptor for the step-up photophobic response of the light-adapted cells of Blepharisma japonicum (OBIP, OxyBlepharismin bInding Protein) have been studied by ultrafast UV–vis transient spectroscopy. The results are rationalized in terms of heterogeneity of the OBIP sample. Two independent classes of chromoprotein are proposed: a “reactive” species, which presents a specific 680-nm band decaying in 4 and 56 ps and a “non-reactive” one, which behaves like the free chromophore (OxyBP) in solution. A bimolecular photooxidation of OxyBP in the presence of 1,4-benzoquinone was performed to record the absorption spectrum of the OxyBP radical cation. Comparison with reactive OBIP suggests that an electron transfer could be involved in the primary photoprocesses of OBIP and possibly trigger the sensory transduction chain of B. japonicum. In addition, the specificity of the chromophore–protein interaction was investigated by studying the artificial complex that OxyBP forms with human serum albumin (HSA). OxyBP–HSA happens to be spectroscopically much closer to free OxyBP than to OBIP. This highlights the specific nature of the interaction between OxyBP and its native protein partner and further supports the proposal that OBIP is the actual photoreceptor for the photophobic response of B. japonicum.

A model for correcting the fluorescence signal from a free-falling depth profiler

Measuring chlorophyll fluorescence at five different wavelengths provides the discrimination of four phytoplankton groups. Here the problems associated with a free-falling depth profiler for phytoplankton discrimination are considered. When F 0, F, and F m are determined sequentially in the same measuring cell, then the algae inside the cell have a different light history. It depends on their different locations in the cell as caused by the induction curve of chlorophyll fluorescence. Mathematical algorithms are developed which enable the calculation of the concentrations of individual phytoplankton groups from the integral fluorescence signal (averaged for 1 s) for different velocities of the falling probe. The theory requires the knowledge of the fluorescence behaviour of phytoplankton in stationary suspensions. The predictions of the model are compared with measurements in flowing suspensions containing chlorophyta, cyanobacteria, cryptophyta and diatoms. The comparison shows the reliability of the algorithms. The application of the algorithms is indispensable for dark-adapted cells and is less important for light-adapted cells.

Biofilm formation by algae as a mechanism for surviving on mine tailings

Photosynthetic biofilms successfully colonize the sediments of a mine tailings reservoir (Guanajuato, Mexico) despite the high metal concentrations that are present. To elucidate the mechanisms of biofilm survival despite metal ores, experiments were performed to evaluate the response of seminatural biofilms to Cu, Zn, and a combination of both metals at concentrations observed in the field. The biofilms were composed mostly of the chlorophyte Chlorococcum sp. and the cyanobacterium Phormidium sp., and their response to the two added metals was described by measurements of extracellular polymeric substances (EPS) and in vivo fluorescence. The photosynthetic efficiency and the minimal chlorophyll fluorescence of dark-adapted cells were measured by multiwavelength pulse amplitude-modulated fluorometry. The photosynthetic efficiency of light-adapted cells (phi(PSII)) also was measured. Metal exposure increased the EPS production of biofilms, as visualized with confocal laser-scanning microscopy. Extracellular polymeric substances enhanced the extracellular metal accumulation from the first day of metal exposure. Metals provoked changes in the relative abundance of the dominant taxa because of a species-specific response to the metals when added individually. Metals affected the phi(PSII) less than the total biomass, suggesting ongoing activity of the surviving biofilms. Survival of individual biofilm photosynthetic cells was found to resume from the embedding in the mucilaginous structure, which immobilizes the metals extracellularly. The survival of biofilms under mixed-metal exposure has practical applications in the remediation of mine tailings.

In-line recording of PAM fluorescence of phytoplankton cultures as a new tool for studying effects of fluctuating nutrient supply on photosynthesis

Turbidostat cultures of Dunaliella salina (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) were grown at fluctuating concentrations of nitrate, phosphate and silicate. In-line measurements of PAM fluorescence were used to monitor the effects of fluctuating nutrient supply on the photochemical efficiency of photosystem II reaction centres of light-adapted cells (ΔF/F′m). Besides the maximal photochemical efficiency of photosystem II reaction centres of dark-adapted cells (F v/F m), chlorophyll a, particulate organic carbon, nitrogen and phosphorus, and the cell number were measured frequently during the experiments. Following nutrient-replete growth, the cells were supplied with medium from which either nitrate, phosphate or silicate was omitted. When significant effects of nutrient starvation were indicated by the fluorescence parameters, a pulse of the deficient nutrient was added to the cultures. Our experimental set-up for in-line fluorescence measurements provided sensitive and reproducible detection of the various fluorescence signals, revealing strong influences of nutrient supply on the photochemical efficiency of photosystem II. In general the fluorescence values changed substantially within 1–30 min after re-addition of the deficient nutrient. Addition of phosphate and silicate induced an immediate characteristic decrease in fluorescence, whereas nitrate addition was characterized by a strong, delayed increase in fluorescence. Complete recovery to pre-starvation fluorescence values took about 48 h in all experiments. The physiological background of nutrient uptake is used to explain the observed tight couplings between fluorescence responses and nutrient re-addition. Our study clearly demonstrates that in-line fluorescence measurements provide a new tool for the investigation of phytoplankton reactions to fluctuating nutrients and offer the possibility to detect nutrient starvation in the field.

In-line recording of PAM fluorescence of phytoplankton cultures as a new tool for studying effects of fluctuating nutrient supply on photosynthesis

Turbidostat cultures of Dunaliella salina (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) were grown at fluctuating concentrations of nitrate, phosphate and silicate. In-line measurements of PAM fluorescence were used to monitor the effects of fluctuating nutrient supply on the photochemical efficiency of photosystem II reaction centres of light-adapted cells (ΔF/F′m). Besides the maximal photochemical efficiency of photosystem II reaction centres of dark-adapted cells (F v/F m), chlorophyll a, particulate organic carbon, nitrogen and phosphorus, and the cell number were measured frequently during the experiments. Following nutrient-replete growth, the cells were supplied with medium from which either nitrate, phosphate or silicate was omitted. When significant effects of nutrient starvation were indicated by the fluorescence parameters, a pulse of the deficient nutrient was added to the cultures. Our experimental set-up for in-line fluorescence measurements provided sensitive and reproducible detection of the various fluorescence signals, revealing strong influences of nutrient supply on the photochemical efficiency of photosystem II. In general the fluorescence values changed substantially within 1–30 min after re-addition of the deficient nutrient. Addition of phosphate and silicate induced an immediate characteristic decrease in fluorescence, whereas nitrate addition was characterized by a strong, delayed increase in fluorescence. Complete recovery to pre-starvation fluorescence values took about 48 h in all experiments. The physiological background of nutrient uptake is used to explain the observed tight couplings between fluorescence responses and nutrient re-addition. Our study clearly demonstrates that in-line fluorescence measurements provide a new tool for the investigation of phytoplankton reactions to fluctuating nutrients and offer the possibility to detect nutrient starvation in the field.

Subcellular localization and light-dark control of ornithine decarboxylase in the unicellular green alga Chlamydomonas reinhardtii

The polyamines putrescine, spermidine and spermine are required for cell proliferation of both eukaryotic and prokaryotic organisms. In plant cells, putrescine, which is also the precursor of spermidine, is derived from ornithine by the action of ornithine decarboxylase (ODC, EC 4.1.1.17) or from arginine by arginine decarboxylase (ADC, EC 4.4.4.19). In the unicellular green alga Chlamydomonas reinhardtii, ODC activity exceeds ADC activity by orders of magnitude as shown in the present communication, indicating that biosynthesis of polyamines is controlled by ODC activity in this organism. ODC was localized in the cytosol and was found to consist of 50 kDa subunits as revealed by affinity labeling with [ 3 H]difluoromethylornithine and subsequent SDS-PAGE and fluorography. Its activity was much higher in light- than in dark-adapted (starved) cells. After transfer from darkness to light, a rapid increase in ODC activity was observed. This light-induced increase in ODC activity could be completely abolished by addition of cycloheximide, an inhibitor of protein biosynthesis, but not by actinomycin D inhibition of transcription. Up-regulation of ODC activity after onset of illumination could be prevented by addition of the photosystem II inhibitor 3-(3'-4'-dichlorophenyl)-1,1-dimethyl-urea. ODC activity was found to be correlated with the ATP level both in light- and dark-adapted cells. ODC half-lives of 42 and 83 min were determined for dark- and light-adapted cells, respectively.

Blue light-induced chloroplast relocation in Arabidopsis thaliana as analyzed by microbeam irradiation.

Chloroplast relocation in mesophyll cells of Arabidopsis thaliana was observed microscopically and analyzed by microbeam irradiation. Chloroplasts located along the anticlinal walls in dark-adapted cells. When part of a cell was irradiated with a microbeam of high fluence rate blue light (B) simultaneously with background red light (R) on the whole cell, the chloroplasts moved towards the B-irradiated area, but did not enter the beam. The background R illumination activated cytoplasmic motility as well as chloroplast movement. Without R illumination, there was little chloroplast relocation. In light-adapted cells in which the chloroplasts were spread over the cell surface perpendicular to the incident light, R-illumination had the same effect. Under background R, the chloroplasts moved out of the area irradiated with a B microbeam of 8 or 30 W m(-2) (avoidance response), but chloroplasts outside the beam moved towards the area irradiated with the B microbeam (accumulation response). These results suggest that the signals for accumulation and avoidance responses were generated in a single cell by high fluence rate B. cry1cry2, npq1 and nph1 mutants showed B-induced chloroplast relocation. Both the accumulation and avoidance responses were observed in all the mutants, although in the nph1 mutant, the sensitivity of accumulation movement was slightly lower than that of the wild type. We discuss the possible photoreceptor for B-induced chloroplast relocation.

Occupancy of the chromophore binding site of opsin activates visual transduction in rod photoreceptors.

The retinal analogue beta-ionone was used to investigate possible physiological effects of the noncovalent interaction between rod opsin and its chromophore 11-cis retinal. Isolated salamander rod photoreceptors were exposed to bright light that bleached a significant fraction of their pigment, were allowed to recover to a steady state, and then were exposed to beta-ionone. Our experiments show that in bleach-adapted rods beta-ionone causes a decrease in light sensitivity and dark current and an acceleration of the dim flash photoresponse and the rate constants of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that in bleach-adapted rods beta-ionone activates phototransduction in the dark. Control experiments showed no effect of beta-ionone in either fully dark-adapted or background light-adapted cells, indicating direct interaction of beta-ionone with the free opsin produced by bleaching. We speculate that beta-ionone binds specifically in the chromophore pocket of opsin to produce a complex that is more catalytically potent than free opsin alone. We hypothesize that a similar reaction may occur in the intact retina during pigment regeneration. We propose a model of rod pigment regeneration in which binding of 11-cis retinal to opsin leads to activation of the complex accompanied by a decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin and leads to the recovery of sensitivity. Our findings resolve the conflict between biochemical and physiological data concerning the effect of the occupancy of the chromophore binding site on the catalytic potency of opsin. We show that binding of beta-ionone to rod opsin produces effects opposite to its previously described effects on cone opsin. We propose that this distinction is due to a fundamental difference in the interaction of rod and cone opsins with retinal, which may have implications for the different physiology of the two types of photoreceptors.

Movement of retinal along cone and rod photoreceptors.

Single isolated photoreceptors can be taken through a visual cycle of light adaptation by bleaching visual pigment, followed by dark adaptation when supplied with 11-cis retinal. Light adaptation after bleaching is manifested by faster response kinetics and a permanent reduction in sensitivity to light flashes, presumed to be due to the presence of bleached visual pigment. The recovery of flash sensitivity during dark adaptation is assumed to be due to regeneration of visual pigment to pre-bleach levels. In previous work, the outer segments of bleached, light-adapted cells were exposed to 11-cis retinal. In the present work, the cell bodies of bleached photoreceptors were exposed. We report a marked difference between rods and cones. Bleached cones recover sensitivity when their cell bodies are exposed to 11-cis retinal. Bleached rods do not. These results imply that retinal can move freely along the cone photoreceptor, but retinal either is not taken up by the rod cell body or retinal cannot move from the rod cell body to the rod outer segment. The free transfer of retinal along cone but not along rod photoreceptors could explain why, during dark adaptation in the retina, cones have access to a store of 11-cis retinal which is not available to rods. Additional experiments investigated the movement of retinal along bleached rod outer segments. The results indicate that retinal can move along the rod outer segment, but that this movement is slow, occurring at about the same rate as the regeneration of visual pigment.

Chromoprotein- and pigment-dependent modeling of spectral light absorption in two dinoflagellates, Prorocentrum minimum and Heterocapsa pygmaea

Pigmentand chromoprotein-dependent spectral models, designed to accurately reconstruct whole cell absorption spectra for photosynthetic dinofldgellates, were assessed. Measured spectral absorption properties (400 to 700 nm) included signatures from whole cells, dispersed thylakoid fragments (unpacked absorption), isolated chromoproteins and individual pigments from high (500 pm01 m-2 S-') and low (35 pm01 m-2 S) light-adapted cells of the dinoflagellates Prorocentrum minimum and Heterocapsa pygmaea grown in continuous light at 15 OC. For model verification, we also developed a procedure to measure unpackaged cell absorption, free of solvent and light-scattering effects. Maximum measured chl a-spec~f~c absorption at 675 nm appears to be closer to 0.027 than a predicted value of 0.0203 m2 mg-l chl a based on absorption from chl a in 90% acetone. The percent fractional absorption of 'in vivo' welght-specific absorption coefficients of individual pigments relative to total weighted absorption (all pigments) was estimated to indicate the light-harvesting capabilities of the different pigments as a function of photoadaptive status and water color. Correspondingly, the weighted absorption of each pigment fraction has been estimated in theoretical white light and in 'clearest' green coastal and blue oceanic waters. Independent of water color, peridinin was by far the most important light-harvesting pigment, followed by chl c2 and chl a. The photoprotective diadinoxanthin absorbed most efficiently in the blue part of the visible spectrum. Results indicate that the chromoprotein model (1) overcame spectral distortions inherent in more general pigment-dependent models and, when combined with corrections for pigment packaging effects. (2) provided accurate spectral estimates of in vivo absorption coefficents and (3) worked equally well for dinoflagellate species with or without the major light-harvesting peridinin-chlorophyll-protein complex, PCP. Findings are discussed in the context of modeling of blo-optical characteristics in dinoflagellates, their photoecology and implications for the in situ optical monitoring of red tides.

Presence of Both Hemidiscoidal and Hemiellipsoidal Phycobilisomes in a Phormidium Species (Cyanobacteria)

Hemidiscoidal and hemiellipsoidal phycobilisomes have been determined in cells of the complementary chromatically adapting cyanobacterium Phormidium sp. C86 . They could be isolated from red and green light-adapted cells, respectively. Hemidiscoidal red light phycobilisomes show molar pigment ratios of allophycocyanin: phycocyanin of 1:4.5 with phycoerythrin lacking. Hemiellipsoidal phycobilisomes induced by green light present allophycocyanin: phycocyanin: phycoerythrin ratios of 1:1:6.8. The differences between the two phycobilisome types could additionally be demonstrated by their ultrastructure and sedimentation values. Isolated red light phycobilisomes have six rods, show dimensions of 70×30×15nm and a sedimentation value of 66 S whereas green light phycobilisomes are nearly twice larger. They contain ten rods and present dimensions of 70×40×25nm and a sedimentation value of 98 S. The number of phycobilisomes in red light cells is almost twice as large as in green light cells. There is evidence that cells grown under white light contain both types as well as “intermediate” forms.

Light and productivity of phytoplankton in polar marine ecosystems: a physiological view

This study deals with the modeling of photosynthesis and growth of polar phytoplankton and variations in relevant parameters. Polar regions are characterised by low sun elevations (< 40?50°), extreme seasonal variations in irradiance and day length, and low sea temperatures (?1.8 to 6°C). Due to the latter, maximum phytoplankton growth rates arc low (< 0.6d?1). Light absorption by phytoplankton is strongly dependent on spectral composition (blue oceanic versus green coastal waters), but absorption characteristics (and thus chlorophyll a-normalized photosynthetic efficiency ?B) do not differ appreciably between polar and other phytoplankton. The maximum chlorophyll-normalised photosynthetic rate PBm, is, however, lower and dependent on the irradiance to which the cells are adapted. Chla:C ratios vary widely, but within ranges known for other phytoplankton. The carbon-normalised coefficient PCm varies little with irradiance, but is clearly dependent on day length and nutrient supply. The corresponding coefficient ?C is somewhat higher in shade-adapted than in light-adapted cells. Polar species exhibit a high tolerance for strong light and long days in combination with low temperature relative to other species. The interpretation of P-I functions is discussed, and an empirical formulation is suggested that does not need the Chla:C ratio for predicting the light-limited gross growth rate of polar phytoplankton. Mathematical simulations of the spring bloom indicate that the depth of the mixed layer and the attenuation of light are the most important variables for determining the photosynthetic rate. The spectral composition of light is of particular importance in low light, e.g. in deeply mixed layers. Generally, the deeper the mixing, the more sensitive the development of a spring bloom becomes to any algal or environmental variable.

Modeling of light-dependent algal photosynthesis and growth: experiments with the Barents sea diatoms Thalassiosira nordenskioldii and Chaetoceros furcellatus

The models by Sakshaug et al (1989, Limnology and Oceanography, 34, 198–205) and Webb et al. (1974, Oecologia, 17, 281–291), for prediction of the gross growth rate of phytoplankton and short-term photosynthesis, respectively, have been modified on the basis of experiments with cultures of the centric diatoms Thalassiosira nordenskioeldii and Chaetoceros furcellatus grown at 0.5°C at combinations of two irradiances (25 and 400μmol m −2s −1) and two day-lengths (12 and 24 h). The models have one spectrum, °σ, which represents chlorophyll a (Chl a) specific absorption of photosynthetically usable light, and introduces a factor q which represents Chla per PSU, functionally defined. The models describe phytoplankton growth in terms of physiologically relevant coefficients. A properly scaled fluorescence excitation spectrum (° F) represents a more appropriate estimate for °σ than the Chl a-specific absorption spectrum ° a c judging from calculations of Φ max (= α B /° σ ). On the basis of ° F, Φ max is 0.04 g-at C(mol photons) −1 for gross growth and about 0.05–0.08 for short-term carbon uptake (unfiltered samples). Calculations based on ° a c yield values for Φ max which on average are 44% lower. P vs I (photosynthesis vs irradiance) parameters are relatively independent of day-length and highly dependent on growth irradiance. The product of q [mg Chl a (mol PSU) −1] and τ (the minimum turnover time of the photosynthetic unit, h) increases 2–3-fold from high to low irradiance, thus P m B (= Φ max / qτ) and I k(=1/ qτ° σ)decreased. ° F decreases from high to low irradiance. Carbon-specific dark respiration rates are <0.09 day −1. Pigment ratios vary inversely with irradiance and day-length. The Chl a:C ratio is particularly low under high, strong continuous light; Chl c: Cha a ratios are higher for shalde- than for light-adapted cells, while the converse is true for the ratio of the sum of the photoprotective pigments diadinoxanthin and diatoxanthin to Chl a. The fucoxanthin: Chl a ratio is virtually independent of the light regime. The two species are similar with respect to variations in growth rate (0.09–0.33 day −1 and I k (31–36 vs 49–100 μmol m −2 s −1 at low and high irradiance, respectively). P m B and a B for growth as well as ° F are systematically higher for C. furcellatus than for T. nordenskioeldii, while the product qτ is lower. C. furcellatus is considerably more plastic than T. nordenskioeldii with respect to pigment composition.

Light and productivity of phytoplankton in polar marine ecosystems: a physiological view

This study deals with the modeling of photosynthesis and growth of polar phytoplankton and variations in relevant parameters. Polar regions are characterised by low sun elevations (< 40?50°), extreme seasonal variations in irradiance and day length, and low sea temperatures (?1.8 to 6°C). Due to the latter, maximum phytoplankton growth rates arc low (< 0.6d?1). Light absorption by phytoplankton is strongly dependent on spectral composition (blue oceanic versus green coastal waters), but absorption characteristics (and thus chlorophyll a-normalized photosynthetic efficiency ?B) do not differ appreciably between polar and other phytoplankton. The maximum chlorophyll-normalised photosynthetic rate PBm, is, however, lower and dependent on the irradiance to which the cells are adapted. Chla:C ratios vary widely, but within ranges known for other phytoplankton. The carbon-normalised coefficient PCm varies little with irradiance, but is clearly dependent on day length and nutrient supply. The corresponding coefficient ?C is somewhat higher in shade-adapted than in light-adapted cells. Polar species exhibit a high tolerance for strong light and long days in combination with low temperature relative to other species. The interpretation of P-I functions is discussed, and an empirical formulation is suggested that does not need the Chla:C ratio for predicting the light-limited gross growth rate of polar phytoplankton. Mathematical simulations of the spring bloom indicate that the depth of the mixed layer and the attenuation of light are the most important variables for determining the photosynthetic rate. The spectral composition of light is of particular importance in low light, e.g. in deeply mixed layers. Generally, the deeper the mixing, the more sensitive the development of a spring bloom becomes to any algal or environmental variable.