Absorption Cross-section Of Photosystem Research Articles

Underwater light climate and wavelength dependence of microalgae photosynthetic parameters in a temperate sea.

Studying how natural phytoplankton adjust their photosynthetic properties to the quantity and quality of underwater light (i.e. light climate) is essential to understand primary production. A wavelength-dependent photoacclimation strategy was assessed using a multi-color pulse-amplitude-modulation chlorophyll fluorometer for phytoplankton samples collected in the spring at 19 locations across the English Channel. The functional absorption cross section of photosystem II, photosynthetic electron transport (PETλ) parameters and non-photochemical quenching were analyzed using an original approach with a sequence of three statistical analyses. Linear mixed-effects models using wavelength as a longitudinal variable were first applied to distinguish the fixed effect of the population from the random effect of individuals. Population and individual trends of wavelength-dependent PETλ parameters were consistent with photosynthesis and photoacclimation theories. The natural phytoplankton communities studied were in a photoprotective state for blue wavelengths (440 and 480 nm), but not for other wavelengths (green (540 nm), amber (590 nm) and light red (625 nm)). Population-detrended PETλ values were then used in multivariate analyses (partial triadic analysis and redundancy analysis) to study ecological implications of PETλ dynamics among water masses. Two wavelength ratios based on the microalgae saturation parameter Ek (in relative and absolute units), related to the hydrodynamic regime and underwater light climate, clearly confirmed the physiological state of microalgae. They also illustrate more accurately that natural phytoplankton communities can implement photoacclimation processes that are influenced by in situ light quality during the daylight cycle in temporarily and weakly stratified water. Ecological implications and consequences of PETλ are discussed in the context of turbulent coastal ecosystems.

Thylakoid membrane dynamics and state transitions in Chlamydomonas reinhardtii under elevated temperature.

Moderately elevated temperatures can induce state transitions in higher plants by phosphorylation of light-harvesting complex II (LHCII). In this study, we exposed unicellular algae Chlamydomonas reinhardtii to moderately elevated temperatures (38°C) for short period of time in the dark to understand the thylakoid membrane dynamics and state transition mechanism. Here we report that under elevated temperatures (1) LHCII gets phosphorylated similar to higher plants and (2) there is decreased absorption cross section of photosystem II (PSII), whereas (3) there is no change in absorption cross section of photosystem I (PSI) indicating that LHCII trimers are largely disconnected with both photosystems under moderately elevated temperatures and (4) on return to room temperature after elevated temperature treatment there is a formation of state transition complex comprising of PSII-LHCII-PSI. The temperature-induced state transition mechanism also depends on stt7 kinase-like in light-induced state transition. The protein content was stable at the moderately elevated temperature treatment of 40°C; however, at 45°C severe downregulation in photosynthetic performance and protein content was observed. In addition to the known changes to photosynthetic apparatus, elevated temperatures can destabilize the PSII-LHCII complex that can result in decreased photosynthetic efficiency in C. reinhardtii. We concluded that the membrane dynamics of light-induced state transitions differs considerably from temperature-induced state transition mechanisms in C. reinhardtii.

Carbon use efficiencies and allocation strategies in Prochlorococcus marinus strain PCC 9511 during nitrogen-limited growth.

We studied cell properties including carbon allocation dynamics in the globally abundant and important cyanobacterium Prochlorococcus marinus strain PCC 9511 grown at three different growth rates in nitrogen-limited continuous cultures. With increasing nitrogen limitation, cellular divinyl chlorophyll a and the functional absorption cross section of Photosystem II decreased, although maximal photosynthetic efficiency of PSII remained unaltered across all N-limited growth rates. Chl-specific gross and net carbon primary production were also invariant with nutrient-limited growth rate, but only 20% of Chl-specific gross carbon primary production was retained in the biomass across all growth rates. In nitrogen-replete cells, 60% of the assimilated carbon was incorporated into the protein pool while only 30% was incorporated into carbohydrates. As N limitation increased, new carbon became evenly distributed between these two pools. While many of these physiological traits are similar to those measured in other algae, there are also distinct differences, particularly the lower overall efficiency of carbon utilization. The latter provides new information needed for understanding and estimating primary production, particularly in the nutrient-limited tropical oceans where P. marinus dominates phytoplankton community composition.

Contrasting Photophysiological Characteristics of Phytoplankton Assemblages in the Northern South China Sea.

The growth of phytoplankton and thus marine primary productivity depend on photophysiological performance of phytoplankton cells that respond to changing environmental conditions. The South China Sea (SCS) is the largest marginal sea of the western Pacific and plays important roles in modulating regional climate and carbon budget. However, little has been documented on photophysiological characteristics of phytoplankton in the SCS. For the first time, we investigated photophysiological characteristics of phytoplankton assemblages in the northern South China Sea (NSCS) using a real-time in-situ active chlorophyll a fluorometry, covering 4.0 × 105 km2. The functional absorption cross section of photosystem II (PSII) in darkness (σPSII) or under ambient light (σPSII’) (A2 quanta-1) increased from the surface to deeper waters at all the stations during the survey period (29 July to 23 August 2012). While the maximum (Fv/Fm, measured in darkness) or effective (Fq’/Fm’, measured under ambient light) photochemical efficiency of PSII appeared to increase with increasing depth at most stations, it showed inverse relationship with depth in river plume areas. The functional absorption cross section of PSII changes could be attributed to light-adapted genotypic feature due to niche-partition and the alteration of photochemical efficiency of PSII could be attributed to photo-acclimation. The chlorophyll a fluorometry can be taken as an analog to estimate primary productivity, since areas of higher photochemical efficiency of PSII coincided with those of higher primary productivity reported previously in the NSCS.

Measurement of algae PSII photosynthetic parameters using high-frequency excitation flashes

We establish a system to measure the functional absorption cross section of photosystem II (PSII) (σPSII) and maximum quantum yield of photochemistry in PSII (Fv/Fm). The system utilizes a sequence of high-frequency excitation flashes at microsecond intervals to induce a microsecond-level fluorescence yield curve. Parameters σPSII and Fv/Fm are calculated by fitting the curve using nonlinear regression. Experimental results show that the relative standard deviation (RSD) of the system is less than 3%, and the correlation coefficient of Fv/Fm values measured by this system and those measured by pulse amplitude modulation method is 0.950.

Steady-State Phosphorylation of Light-Harvesting Complex II Proteins Preserves Photosystem I under Fluctuating White Light

According to the "state transitions" theory, the light-harvesting complex II (LHCII) phosphorylation in plant chloroplasts is essential to adjust the relative absorption cross section of photosystem II (PSII) and PSI upon changes in light quality. The role of LHCII phosphorylation upon changes in light intensity is less thoroughly investigated, particularly when changes in light intensity are too fast to allow the phosphorylation/dephosphorylation processes to occur. Here, we demonstrate that the Arabidopsis (Arabidopsis thaliana) stn7 (for state transition7) mutant, devoid of the STN7 kinase and LHCII phosphorylation, shows a growth penalty only under fluctuating white light due to a low amount of PSI. Under constant growth light conditions, stn7 acquires chloroplast redox homeostasis by increasing the relative amount of PSI centers. Thus, in plant chloroplasts, the steady-state LHCII phosphorylation plays a major role in preserving PSI upon rapid fluctuations in white light intensity. Such protection of PSI results from LHCII phosphorylation-dependent equal distribution of excitation energy to both PSII and PSI from the shared LHCII antenna and occurs in cooperation with nonphotochemical quenching and the proton gradient regulation5-dependent control of electron flow, which are likewise strictly regulated by white light intensity. LHCII phosphorylation is concluded to function both as a stabilizer (in time scales of seconds to minutes) and a dynamic regulator (in time scales from tens of minutes to hours and days) of redox homeostasis in chloroplasts, subject to modifications by both environmental and metabolic cues. Exceeding the capacity of LHCII phosphorylation/dephosphorylation to balance the distribution of excitation energy between PSII and PSI results in readjustment of photosystem stoichiometry.

Phytoplankton Productivity and Photophysiology in the Surf Zone of Sandy Beaches in North Carolina, USA

Measurements of primary production in surf-zone habitats are relatively rare and often utilize simulation approaches, owing to the physical challenges of working in surf. The study reported here examined primary production in situ at two open ocean sandy beaches in southeastern North Carolina during relatively calm summer conditions. In situ bottle incubations using 14C uptake methods were complemented by simultaneous measures of phytoplankton photo-physiology assessed by Fast Repetition Rate Fluorometry (FRRF) in flow-through mode at the two sites across a spring-neap tidal cycle in July, 2010. The surf-zone phytoplankton was dominated by small centric and pennate diatoms as well as cyanobacteria and chlorophytes with biomass concentrations of 3.63–9.23 mg chl a m−3. Primary productivity was relatively high, ranging from 31.5–88.0 mg C m−3 h−1 by 14C. Biomass-specific productivity averaged ∼9.4 mg C (mg chl a)−1 h−1 by 14C, indicating healthy phytoplankton populations. Measurements of the functional absorption cross section of photosystem II, σPSII, via FRRF were 327–380, comparable to values reported by other investigators of open ocean phytoplankton. Averaged values of the maximum effective quantum yield, Fv/Fm, corresponded to proportions of photochemically competent PSII reaction centers of 62.6 % to 72 %, indicating that the phytoplankton were nutrient-replete. These data suggest that the surf zone, although a spatially confined habitat, is a productive one that plays a significant role in coastal ocean ecology. Further investigation is needed to better understand primary productivity of phytoplankton in the surf zone and the effect of the dynamic environment on their physiological responses.

Fluoride affects distribution of absorbed excitation energy more in favour of photosystem 1

The effects of fluoride on the photosynthetic electron transport chain have been studied in spinach thylakoid membranes. Inhibition in photosystem (PS) 2 electron transport rates and a subsequent increase in PS 1 electron transport rate indicated a possibility of state transitions being a mechanism of fluoride action. This hypothesis was further confirmed by the increase in fluorescence emission F735/685 at 77 K, a decrease in variable to maximum fluorescence ratio (Fv/Fm) at room temperature and increase in the absorption cross section of PS 1 suggesting that fluoride affects distribution of the excitation energy in favour of PS 1 at the expense of PS 2.

Temporal and vertical variability in photosynthesis in the North Pacific Subtropical Gyre

In situ fast‐repetition‐rate fluorometric (FRRF) surveys were conducted in the North Pacific Subtropical Gyre (NPSG) at Station ALOHA (Sta. ALOHA, 22°459N, 158°009W), from September 2002 to December 2004, to assess temporal and vertical photosynthetic variability in relation to environmental conditions. The nighttime potential photosynthetic efficiency of the photoautotrophic microbial assemblage (given as the ratio of variable to maximal fluorescence, FV:FM) was low in the mixed layer and increased with depth. High FV:FM values were observed at and below the deep chlorophyll maximum layer (DCML); some values approached the theoretical maximum for prokaryotes grown under nutrient‐replete conditions in the laboratory (0.60). In contrast, the absorption cross section of photosystem II (ςPSII) was high at the surface and decreased with depth; minima (1,000 A° 2 quanta‐1) occurred around the DCML. These vertical patterns suggest photosynthetic stress conditions in surface photoautotrophic populations. No significant seasonal cycles were found for FV:FM, but surface ςPSII values peaked in winter and decreased during summer, suggesting that seasonal variations in light availability may influence the observed ςPSII variability. A significant correlation was found among surface FV:FM, ςPSII, and the distance from the mixed‐layer depth (MLD) to the top of the nutricline. Neither nutrient nor light variations were significantly related to FV:FM and ςPSII in the DCML. Within this layer, FV:FM variability was positively and negatively related to concentrations of chlorophyll b and zeaxanthin, respectively. Our results suggest that, at Sta. ALOHA, surface photosynthesis takes place under chronic nutrient limitation, while higher photosynthetic efficiency in the lower euphotic zone appears to be sensitive to community‐structure changes.

Photosynthetic efficiency as a function of thermal stratification and phytoplankton size structure in an oligotrophic alpine lake

Allometric relationships of phytoplankton communities were studied on the basis of a five-year data-set in a deep oligotrophic alpine lake in Austria. The seasonal phytoplankton succession in Mondsee is characterised by diatoms during winter mixing and a distinct metalimnetic population of Planktothrix rubescens during stratification in summer. The variation of phytoplankton photosynthetic efficiency between seasons was assessed using in situ carbon-uptake rates (5 years data) and Fast Repetition Rate Fluorometry (FRRF) (2 years data). The light-saturated, chlorophyll-specific rate of photosynthesis (P*max), irradiance at the onset of saturation (Ek) and maximum light-utilisation efficiency (α*) were determined for winter mixing and summer stratification. Fluorescence-based parameters as the functional absorption cross section of Photosystem II (σPSII) and the photochemical quantum yield (Fv/Fm) were additionally analysed in 2003 and 2004 to study the underlying physiological mechanisms for the variability in photosynthetic performance. Beyond their sensitivity to changing environmental conditions like thermal stratification, phytoplankton populations differ in their photosynthetic behaviour according to their size structure. Therefore Photosynthesis vs. Irradiance (P/E)-relationships were analysed in detail within a 1-year period from size fractionated cell counts, chlorophyll-a and carbon-uptake.

Iron Deficiency in Cyanobacteria Causes Monomerization of Photosystem I Trimers and Reduces the Capacity for State Transitions and the Effective Absorption Cross Section of Photosystem I in Vivo

The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.

Differences in photosynthetic activity between coral sections infested and not infested by boring spionid polychaetes

A SCUBA-based fast repetition rate fluorometer (FRRF) was used to study differences in the functional absorption cross-section of Photosystem II (σPSII) between areas of a coral colony of Astreoporamyriophthalma that were infested with spionid polychaetes vs areas lacking worms. The mean value of σPSII in infested areas (mean±SD=347.62±30.67 Å2) was significantly higher than in the areas that were not infested (316.32±17.49 Å2; P<0.0001). Several physiological mechanisms are discussed that may contribute to the observed differences.

Basin-scale variability of phytoplankton bio-optical characteristics in relation to bloom state and community structure in the Northeast Atlantic

Phytoplankton physiological data collected throughout the Iceland Basin and Rockall Trough during the North Atlantic spring bloom from May to June 2001 are presented. Physiological parameters including the maximum photochemical quantum efficiency ( F v /F m) and the functional absorption cross section of photosystem II ( σ PSII) were measured using fast repetition rate fluorometry. Information on the taxonomic and pigment characteristics of the phytoplankton populations was also collected, with pigment data being used to reconstruct absorption spectra. Significant changes in the physiological properties of PSII were found to be associated with the progression of the spring bloom from diatom to flagellate domination. Changes in both community composition and physiology were in turn correlated with environmental parameters. Lower F v /F m, higher σ PSII and corresponding decreases in cell size were associated with the observed decrease of nutrients that accompanied increasing stratification. Differences in σ PSII were primarily associated with the changing pigment composition of the phytoplankton populations, with the largest changes appearing to be governed by the amount of absorption by photosynthetic carotenoids. The physiological state of PSII was thus found to be an indicator of bloom status and community structure in this productive temperate region principally as a result of taxon specific variability.

Estimation of oxygen evolution by marine phytoplankton from measurement of the efficiency of Photosystem II electron flow

The relation between photosynthetic oxygen evolution and Photosystem II electron transport was investigated for the marine algae t Phaeodactylum tricornutum, Dunaliella tertiolecta, Tetraselmis sp., t Isochrysis sp. and t Rhodomonas sp.. The rate of Photosystem II electron transport was estimated from the incident photon flux density and the quantum efficiency of Photosystem II electron transport as determined by chlorophyll fluorescence. The relation between the estimated rate of Photosystem II electron transport and the rate of oxygen evolution was investigated by varying the ambient light intensity. At limiting light intensities a linear relation was found in all species. At intensities approaching light saturation, the relation was found to deviate from linearity. The slope of the line in the light-limited range is species dependent and related to differences in absorption cross-section of Photosystem II. The observed non-linearity at high irradiances is not caused by photorespiration but probably by a Mehler-type of oxygen reduction. The relationship could be modelled by including a redox-state dependent oxygen uptake. In the diatom t Phaeodactylum tricornutum, the photochemical efficiency of dark adapted open Photosystem II centers was found to be temperature-dependent with an optimum near 10°C.

Effect of iron limitation on photosynthesis in a marine diatom

The response of the marine diatom Phaeodactylum tricornutum to Fe deficiency was evaluated in the context of fundamental physiological models of growth and photosynthesis. Fe deficiency induced chlorosis, which decreased Chl a: C ratios and Chl a-specific light-saturated photosynthesis (PmB). In contrast to PmB, αb was slightly increased under Fe deficiency, and photosynthesis in Fedeficient cells became light-saturated at lower irradiances than in Fe-replete cells grown at the same irradiance. Fe deficiency increased the in vivo absorption cross section normalized to Chl a (a*), but decreased the maximum quantum yield of photosynthesis (ϕm). Thus, the product a* ϕm, which equals the Chl a-specific initial slope of the photosynthesis-irradiance curve (αB), was less sensitive to Fe limitation than was a* or ϕm alone. Using a pump-and-probe fluorometer, we found that Fe deficiency reduced the maximum fluorescence yield (Δϕsat), which is consistent with the reduction in ϕm, but increased the absorption cross section of photosystem 2(σPS2). Immunoassays of proteins separated electrophoretically indicated that the reduction in maximum fluorescence yields was accompanied by a reduction in the relative abundance of D1, the photosystem 2 reaction center protein. Light-harvesting chlorophyll proteins (LHCP) and the large and small subunits of ribulose bisphosphate carboxylase were not affected by Fe deficiency. Changes in the abundance of D1 relative to LHCP suggest an increase in the fraction of nonfunctional reaction centers under Fe-limited conditions. Fe-deficient cells, growing at <20% of their maximum growth rate, had reduced cellular C, N, and P contents, but maintained C: N: P ratios at the Redfield proportions. These results imply that C: N: P ratios do not provide an unequivocal index of relative growth rate.

Light 2 directed changes in the effective absorption cross-section of Photosystem II in Synechocystis 27170 are related to modified action on the donor side of the reaction center

Synechocystis 27170 was grown in green, orange and red light in order to adapt cells to light preferentially absorbed by phycoerythrin, phycocyanin and chlorophyll a, respectively. Chromatically adapted cells were used to study effective absorption cross-sections of RCII, σeff, in state 1 and state 2. σeff for state 1 and state 2 in orange and green light grown cells was approximately twice that for red light grown cells. Light 1 and 2 both induced increased σeff in an intensity dependent way. The effect saturated at 40 μ mol quanta m−2 s−1 of light 2. Increased σeff invoked by light 1 was related to PSI activity only, since light 1 did not close significant numbers of RCII. Light 2 also effected an increased σeff, but in an inverse relationship with the fraction of open RCII traps. These data support the model of modulated energy transfer from PSII chlorophyll a to PSI as the mechanism for state 1 > 2 transitions in cyanobacteria. Although matching each other in light 1, PSII fluorescence yields deviated substantially from O2-flash yields in light 2 over a wide intensity range. Since the half-time of Q−A was equal in state 1 and 2, it was concluded that light 2 has a modifying effect on the RCII donor side, probably causing longer half-time of P+680, which delays the reopening of closed RCII. These results were interpreted as a feature of the RCII related to an increased half-time of an exciton in the PSII chlorophyll a antenna. This phenomenon is thought to be functional in facilitating energy transfer from PSII chlorophyll a to PSI.

Photoinhibition in Chlamydomonas reinhardtii: Effect on state transition, intersystem energy distribution and Photosystem I cyclic electron flow

The energy distribution, state transitions and photosynthetic electron flow during photoinhibition of Chlamydomonas reinhardtii cells have been studied in vivo using photoacoustics and modulated fluorescence techniques. In cells exposed to 2500 W/m(2) light at 21 °C for 90 min, 90% of the oxygen evolution activity was lost while photochemical energy storage as expressed by the parameter photochemical loss (P.L.) at 710-720 nm was not impaired. The energy storage vs. modulation frequency profile indicated an endothermic step with a rate constant of 2.1 ms. The extent of the P.L. was not affected by DCMU but was greatly reduced by DBMIB. The regulatory mechanism of the state 1 to state 2 transition process was inactivated and the apparent light absorption cross section of photosystem II increased during the first 20 min of photoinhibition followed by a significant decrease relative to that of photosystem I. These results are consistent with the inactivation of the LHC II kinase and the presence of an active cyclic electron flow around photosystem I in photoinhibited cells.

Trapping and annihilation in the antenna system of Photosystem I

The primary charge separation in Photosystem I of pea chloroplasts was measured as a photovoltage in the pico- and nanosecond time range by applying laser flashes at 532 nm of variable energy and different duration (12 ns and 30 ps, respectively). Contributions to the photovoltage from Photosystem II was eliminated by addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination. The dependence of the photovoltage amplitude on the excitation energy could be described by an exponential saturation law when the excitation flash had a duration of 12 ns. Nearly the same dependence was found when the excitation source was the train of a mode-locked laser (approx. ten 30-ps flashes spaced by 7 ns; highest energy of a single flash, 80 μJ / cm −2). Even with single 30-ps flashes the photovoltage was only slightly smaller than the one elicited by 12-ns flashes of the same energy. These findings demonstrate that trapping of excitation energy by the reaction center of Photosystem I is much more effective than losses by annihilation and other loss processes. The photovoltage yield was nearly independent of the fraction of closed traps, thus demonstrating that the absorption cross section of Photosystem I is not altered by the closing of its reaction centers. By recording the rise time of the photovoltage with our highest time resolution we found that the trapping rate of the excitation energy in Photosystem I depended on the energy of the 30-ps flashes: at low excitation energies (less than 10 14 photons / cm 2 per pulse) trapping occurred within 90 ± 15 ps and at high excitation energy (10 15 photons / cm 2 per pulse) trapping and charge stabilization occurred within the time resolution of the apparatus, i.e., up to 50 ps. The trapping rate at low energies is in agreement with the one determined by fluorescence decay kinetics. Up to 50 ns there was no further detectable electrogenic phase (neither forward nor backward reactions). This demonstrates that all the electrogenicity, produced by the charge separation, takes place in less than 50 ps.