Functional Absorption Cross Section Research Articles

The Fitting of the OJ Phase of Chlorophyll Fluorescence Induction Based on an Analytical Solution and Its Application in Urban Heat Island Research

Chlorophyll (Chl) fluorescence induction (FI) upon a dark–light transition has been widely analyzed to derive information on initial events of energy conversion and electron transfer in photosystem II (PSII). However, currently, there is no analytical solution to the differential equation of QA reduction kinetics, raising a doubt about the fitting of FI by numerical iteration solution. We derived an analytical solution to fit the OJ phase of FI, thereby yielding estimates of three parameters: the functional absorption cross-section of PSII (σPSII), a probability parameter that describes the connectivity among PSII complexes (p), and the rate coefficient for QA− oxidation (kox). We found that σPSII, p, and kox exhibited dynamic changes during the transition from O to J. We postulated that in high excitation light, some other energy dissipation pathways may vastly outcompete against excitation energy transfer from a closed PSII trap to an open PSII, thereby giving the impression that connectivity seemingly does not exist. We also conducted a case study on the urban heat island effect on the heat stability of PSII using our method and showed that higher-temperature-acclimated leaves had a greater σPSII, lower kox, and a tendency of lower p towards more shade-type characteristics.

Fluorescence-based primary productivity estimates are influenced by non-photochemical quenching dynamics in Arctic phytoplankton.

Chlorophyll fluorescence-based estimates of primary productivity typically include dark or low-light pre-treatments to relax non-photochemical quenching (NPQ), a process that influences the relationship between PSII photochemistry and fluorescence yields. The time-scales of NPQ relaxation vary significantly between phytoplankton taxa and across environmental conditions, creating uncertainty in field-based productivity measurements derived from fluorescence. To address this practical challenge, we used fast repetition rate fluorometry to characterize NPQ relaxation kinetics in Arctic Ocean phytoplankton assemblages across a range of hydrographic regimes. Applying numerical fits to our data, we derived NPQ relaxation life times, and determined the relative contributions of various quenching components to the total NPQ signature across the different assemblages. Relaxation kinetics were best described as a combination of fast-, intermediate- and slow-relaxing processes, operating on time-scales of seconds, minutes, and hours, respectively. Across sampling locations and depths, total fluorescence quenching was dominated by the intermediate quenching component. Our results demonstrated an average NPQ relaxation life time of 20 ± 1.9 min, with faster relaxation among high light acclimated surface samples relative to lowlight acclimated sub-surface samples. We also used our results to examine the influence of NPQ relaxation on estimates of photosynthetic electron transport rates (ETR), testing the commonly held assumption that NPQ exerts proportional effects on light absorption (PSII functional absorption cross section, σPSII) and photochemical quantum efficiency (FV/FM). This assumption was violated in a number of phytoplankton assemblages that showed a significant decoupling of σPSII and FV/FM during NPQ relaxation, and an associated variability in ETR estimates. Decoupling of σPSII and FV/FM was most prevalent in samples displaying symptoms photoinhibition. Our results provide insights into the mechanisms and kinetics of NPQ in Arctic phytoplankton assemblages, with important implications for the use of FRRF to derive non-invasive, high-resolution estimates of photosynthetic activity in polar marine waters.

In situ phytoplankton photosynthetic characteristics and their controlling factors in the eastern Indian Ocean

Photosynthesis is the most important bioenergy conversion process on Earth. Capturing instantaneous changes in in situ photosynthesis in open ocean ecosystems remains a major challenge. In this study, fast repetition rate fluorometry (FRRF), which can obtain nondestructive, real-time and in situ estimates of photosynthetic parameters, was used for the first time to continuously observe the spatial variation in in situ photosynthetic parameters in the eastern Indian Ocean (EIO). We further formulated new insights regarding abiotic and biotic factors of potential importance in determining photosynthetic performance. First, we found that the distributions of micro/nano- and picophytoplankton were opposite under the control of nutrient concentrations. Micro/nanophytoplankton had higher cell abundances in the nearshore and upwelling regions, while picophytoplankton had higher abundances in the open ocean, and Prochlorococcus was the dominant group. Second, based on the FRRF technology, we obtained the high-precision and high-density vertical profile map of photosynthetic parameters in the euphotic layer. It was observed that values of the maximum photochemical efficiency (Fv/Fm; 0.14–0.55, unitless) and the functional absorption cross-section of PSII (σPSII; 1.71–4.90 nm2 RCII−1) increased with increasing depth, while high values of the photosynthetic electron transfer rates (ETRRCII; 0.0019–17.0292 mol e− mol RCII−1 s−1) and the nonphotochemical quenching (NPQNSV; 0.35–7.26, unitless) occurred in the shallow 50 m layer, and the values decreased as the depth increased. Finally, we discussed limiting factors that regulated the distribution of photosynthetic parameters and concluded that optical properties varied significantly with changes in the ocean physico-chemical parameters and taxonomic composition of phytoplankton assemblages in the EIO. Picophytoplankton (especially cyanobacteria), rather than the micro/nanophytoplankton community, was the dominant factor influencing photosynthesis. Among abiotic factors, photosynthetically active radiation (PAR) was the proximal limiting factor affecting photosynthetic efficiency, followed by temperature and dissolved inorganic nitrogen (DIN). Consequently, phytoplankton photosynthetic parameters exhibited great variability, allowing rapid responses to environmental condition changes. In this study, we established the basis for detecting future changes in primary production in this oligotrophic area.

Stimulating and toxic effect of chromium on growth and photosynthesis of a marine chlorophyte.

Marine phytoplankton can interchange trace metals in various biochemical functions, particularly under metal-limiting conditions. Here, we investigate the stimulating and toxicity effect of chromium (Cr) on a marine Chlorophyceae Osetreococcus tauri under Fe-replete and Fe-deficient conditions. We determined the growth, photosynthesis, and proteome expressions of Osetreococcus tauri cultured under different Cr and Fe concentrations. In Fe-replete conditions, the presence of Cr(VI) stimulated significantly the growth rate and the maximum yield of photochemistry of photosystem II (Fv /Fm ) of the phytoplankton, while the functional absorption cross-section of photosystem II (σPSII ) did not change. Minor additions of Cr(VI) partially rescued phytoplankton growth under Fe-limited conditions. Proteomic analysis of this alga grown in Fe-replete normal and Fe-replete with Cr addition media (10 μM Cr) showed that the presence of Cr significantly decreased the expression of phosphate-transporting proteins and photosynthetic proteins, while increasing the expression of proteins related to carbon assimilation. Cr can stimulate the growth and photosynthesis of O. tauri, but the effects are dependent on both the Cr(VI) concentration and the availability of Fe. The proteomic results further suggest that Cr(VI) addition might significantly increase starch production and carbon fixation.

Study on Calibration Method for the Count of Living Algal Cells Detection Based on Variable Fluorescence in Ballast Water.

In the monitoring the discharge of ballast water, the count of living algal cells is of utmost significant. Variable fluorescence, denoted as Fv, stands as an optimal parameter for photosynthetic fluorescence, efficiently charactering the living algal cells count, unaffected by the ballast waters' complex background fluorescence environment. This study deeply investigates the quantitative relationship between Fv and the count of living algal cells. Observations indicate that single cell fluorescence yield (abbreviated as SCF) varies significantly across different algae species, leading to considerable errors in quantifying living algal cell count in ballast water with unknown components using the calibration relationship between Fv and the cell count. Thus, correcting SCF prior to calibration becomes necessary. The paper proposes an innovative SCF correction method based on cell cross-sectional area and an eμ factor (where μ is the expected value of the functional absorption cross-section of PSII) This method mitigates the influence of cell size and species differences on quantifying the living algal cell count. Correction operation trials revealed that dividing the SCF measurement by cell cross-sectional area and multiplying by eμ enhanced the correction effect. Comparative experiments demonstrated marked improvement: Relative errors (REs) for Chlorella pyrenoidosa and Chlorella marine, both belonging to the Chlorophyta group, fell from 92.1% and 90.6% to 37.2% and 9.5% respectively post-correction. Similarly, REs for Thalassiosira weissflogii and Nitzschia closterium minutissima, from the Bacillariophyta group, decreased from 74.7% and 68.1% to 14.3% and 19.1% respectively. The RE of Peridinium from the Pyrrophyta group dropped from 28.4% to 12.1%. The results underscore the effectiveness of cell cross-sectional area and eμ in correcting SCF, thus offering a novel correction method for swift and precise measurement of living algal cell count in ballast water, based on variable fluorescence.

Phytoplankton photophysiology across tropical eddies: Deconvolving nutrient, light, and community signals

Fast repetition rate fluorometry (FRRf) based on active chlorophyll fluorescence is a powerful, noninvasive tool for studying phytoplankton physiological status at high spatial and temporal resolution. The South China Sea (SCS) is one of the largest tropical–subtropical marginal seas in the world, which plays an important role in modulating regional carbon budget and climate. In this study, underway in situ FRRf measurements were carried out throughout the outer continental shelf of the northern SCS, the basin of the northern SCS, the cyclonic eddy influenced domain in the western SCS, and the basin of the southeastern SCS. Pronounced diurnal variability of FRRf-derived parameters were observed, characterized by a large midday depression and slight nocturnal depression of the maximum quantum yield of photosystem II (Fv/Fm) and a slight increase in the functional absorption cross-section of photosystem II photochemistry (σPSII) at noon. Fv/Fm at the surface was typically as low as 0.1–0.3 and exhibited higher values (~0.4) where internal waves occurred. The cyclonic eddy increased Fv/Fm slightly, implying that it had a limited impact on surface phytoplankton photophysiology. With proper interpretation, FRRf has been a powerful tool to assess the physiological status of phytoplankton in the sea and to correlate that to ocean dynamics in an unprecedented fine scale.

Investigating the effect of nickel concentration on phytoplankton growth to assess potential side-effects of ocean alkalinity enhancement

Abstract. Ocean alkalinity enhancement (OAE) is a proposed method for removing carbon dioxide (CO2) from the atmosphere by the accelerated weathering of (ultra-)basic minerals to increase alkalinity – the chemical capacity of seawater to store CO2. During the weathering of OAE-relevant minerals relatively large amounts of trace metals will be released and may perturb pelagic ecosystems. Nickel (Ni) is of particular concern as it is abundant in olivine, one of the most widely considered minerals for OAE. However, so far there is limited knowledge about the impact of Ni on marine biota including phytoplankton. To fill this knowledge gap, this study tested the growth and photo-physiological response of 11 marine phytoplankton species to a wide range of dissolved Ni concentrations (from 0.07 to 50 000 nmol L−1). We found that the phytoplankton species were not very sensitive to Ni concentrations under the culturing conditions established in our experiments, but the responses were species-specific. The growth rates of 6 of the 11 tested species showed generally limited but still significant responses to changing Ni concentrations (36 % maximum change). Photosynthetic performance, assessed by measuring the maximum quantum yield (Fv/Fm) and the functional absorption cross-section (σPSII) of photosystem II (PSII), was sensitive to changing Ni in 3 out of 11 species (35 % maximum change) and 4 out of 11 species (16 % maximum change), respectively. The limited effect of Ni may be partly due to the provision of nitrate as the nitrogen source for growth as previous studies suggest higher sensitivities when urea is the nitrogen source. Furthermore, the limited influence may be due to the relatively high concentrations of synthetic organic ligands added to the growth media in our experiments. These ligands are commonly added to control trace metal bioavailability and therefore for example “free Ni2+” concentrations by binding the majority of the dissolved Ni. Our data suggest that dissolved Ni does not have a strong effect on phytoplankton under our experimental conditions, but we emphasize that a deeper understanding of nitrogen sources, ligand concentrations, and phytoplankton composition is needed when assessing the influence of Ni release associated with OAE.

Exploring Variability of Trichodesmium Photophysiology Using Multi-Excitation Wavelength Fast Repetition Rate Fluorometry.

Fast repetition rate fluorometry (FRRf) allows for rapid non-destructive assessment of phytoplankton photophysiology in situ yet has rarely been applied to Trichodesmium. This gap reflects long-standing concerns that Trichodesmium (and other cyanobacteria) contain pigments that are less effective at absorbing blue light which is often used as the sole excitation source in FRR fluorometers—potentially leading to underestimation of key fluorescence parameters. In this study, we use a multi-excitation FRR fluorometer (equipped with blue, green, and orange LEDs) to investigate photophysiological variability in Trichodesmium assemblages from two sites. Using a multi-LED measurement protocol (447+519+634 nm combined), we assessed maximum photochemical efficiency (Fv/Fm), functional absorption cross section of PSII (σPSII), and electron transport rates (ETRs) for Trichodesmium assemblages in both the Northwest Pacific (NWP) and North Indian Ocean in the vicinity of Sri Lanka (NIO-SL). Evaluating fluorometer performance, we showed that use of a multi-LED measuring protocol yields a significant increase of Fv/Fm for Trichodesmium compared to blue-only excitation. We found distinct photophysiological differences for Trichodesmium at both locations with higher average Fv/Fm as well as lower σPSII and non-photochemical quenching (NPQNSV) observed in the NWP compared to the NIO-SL (Kruskal–Wallis t-test df = 1, p < 0.05). Fluorescence light response curves (FLCs) further revealed differences in ETR response with a lower initial slope (αETR) and higher maximum electron turnover rate () observed for Trichodesmium in the NWP compared to the NIO-SL, translating to a higher averaged light saturation EK (= /αETR) for cells at this location. Spatial variations in physiological parameters were both observed between and within regions, likely linked to nutrient supply and physiological stress. Finally, we applied an algorithm to estimate primary productivity of Trichodesmium using FRRf-derived fluorescence parameters, yielding an estimated carbon-fixation rate ranging from 7.8 to 21.1 mgC mg Chl-a–1 h–1 across this dataset. Overall, our findings demonstrate that capacity of multi-excitation FRRf to advance the application of Chl-a fluorescence techniques in phytoplankton assemblages dominated by cyanobacteria and reveals novel insight into environmental regulation of photoacclimation in natural Trichodesmium populations.

Photosynthetic Characteristics of Smaller and Larger Cell Size-Fractioned Phytoplankton Assemblies in the Daya Bay, Northern South China Sea.

Cell size of phytoplankton is known to influence their physiologies and, consequently, marine primary production. To characterize the cell size-dependent photophysiology of phytoplankton, we comparably explored the photosynthetic characteristics of piconano- (<20 µm) and micro-phytoplankton cell assemblies (>20 µm) in the Daya Bay, northern South China Sea, using a 36-h in situ high-temporal-resolution experiment. During the experimental periods, the phytoplankton biomass (Chl a) in the surface water ranged from 0.92 to 5.13 μg L−1, which was lower than that in bottom layer (i.e., 1.83–6.84 μg L−1). Piconano-Chl a accounted for 72% (mean value) of the total Chl a, with no significant difference between the surface and bottom layers. The maximum photochemical quantum yield (FV/FM) of Photosystem II (PS II) and functional absorption cross-section of PS II photochemistry (σPS II) of both piconano- and micro-cells assemblies varied inversely with solar radiation, but this occurred to a lesser extent in the former than in the latter ones. The σPS II of piconano- and micro-cell assemblies showed a similar change pattern to the FV/FM in daytime, but not in nighttime. Moreover, the fluorescence light curve (FLC)-derived light utilization efficiency (α) displayed the same daily change pattern as the FV/FM, and the saturation irradiance (EK) and maximal rETR (rETRmax) mirrored the change in the solar radiation. The FV/FM and σPS II of the piconano-cells were higher than their micro-counterparts under high solar light; while the EK and rETRmax were lower, no matter in what light regimes. In addition, our results indicate that the FV/FM of the micro-cell assembly varied quicker in regard to Chl a change than that of the piconano-cell assembly, indicating the larger phytoplankton cells are more suitable to grow than the smaller ones in the Daya Bay through timely modulating the PS II activity.

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.

Low Fe Availability for Photosynthesis of Sea-Ice Algae: Ex situ Incubation of the Ice Diatom Fragilariopsis cylindrus in Low-Fe Sea Ice Using an Ice Tank

Sea-ice algae play a crucial role in the ecology and biogeochemistry of sea-ice zones. They not only comprise the base of sea-ice ecosystems, but also seed populations of extensive ice-edge blooms during ice melt. Ice algae must rapidly acclimate to dynamic light environments, from the low light under sea ice to high light within open waters. Recently, iron (Fe) deficiency has been reported for diatoms in eastern Antarctic pack ice. Low Fe availability reduces photosynthetic plasticity, leading to reduced ice-algal primary production. We developed a low-Fe ice tank to manipulate Fe availability in sea ice. Over 20 days in the ice tank, the Antarctic ice diatomFragilariopsis cylindruswas incubated in artificial low-Fe sea ice ([total Fe] = 20 nM) in high light (HL) and low light (LL) conditions. Melted ice was also exposed to intense light to simulate light conditions typical for melting icein situ. When diatoms were frozen in, the maximum photochemical quantum efficiency of photosystem II (PSII),Fv/Fm, was suppressed by freezing stress. However, the diatoms maintained photosynthetic capability throughout the ice periods with a stableFv/Fmvalue and increased photoprotection through non-photochemical quenching (NPQ) via photoprotective xanthophyll cycling (XC) and increased photoprotective carotenoid levels compared to pre-freeze-up. Photoprotection was more pronounced in the HL treatment due to greater light stress. However, the functional absorption cross section of PSII, σPSII, inF. cylindrusconsistently increased after freezing, especially in the LL treatment (σPSII&amp;gt; 10 nm2PSII–1). Our study is the first to report such a large σPSIIin ice diatoms at low Fe conditions. When the melted sea ice was exposed to high light,Fv/Fmwas suppressed. NPQ and XC were slightly upregulated, but not to values normally observed when Fe is not limiting, which indicates reduced photosynthetic flexibility to adapt to environmental changes during ice melt under low Fe conditions. Although ice algae can optimize their photosynthesis to sea-ice environments, chronic Fe starvation led to less flexibility of photoacclimation, particularly in low light conditions. This may have detrimental consequences for ice algal production and trophic interactions in sea-ice ecosystems if the recent reduction in sea-ice extent continues.

Dynamic Photophysiological Stress Response of a Model Diatom to Ten Environmental Stresses.

Stressful environmental conditions can induce many different acclimation mechanisms in marine phytoplankton, resulting in a range of changes in their photophysiology. Here we characterize the common photophysiological stress response of the model diatom Thalassiosira pseudonana to ten environmental stressors and identify diagnostic responses to particular stressors. We quantify the magnitude and temporal trajectory of physiological parameters including the functional absorption cross-section of PSII (σPSII ), quantum efficiency of PSII, non-photochemical quenching (NPQ), cell volume, Chl a, and carotenoid (Car) content in response to nutrient starvation (nitrogen (N), phosphorus (P), silicon (Si), and iron (Fe)), changes in temperature, irradiance, pH, and reactive oxygen species (ROS) over 5 time points (0, 2, 6, 24, 72h). We find changes in conditions: temperature, irradiance, and ROS, often result in the most rapid changes in photophysiological parameters (<2h), and in some cases are followed by recovery. In contrast, nutrient starvation (N, P, Si, Fe) often has slower (6-72h) but ultimately larger magnitude effects on many photophysiological parameters. Diagnostic changes include large increases in cell volume under Si-starvation, very large increases in NPQ under P-starvation, and large decreases in the σPSII under high light. The ultimate goal of this analysis is to facilitate and enhance the interpretation of fluorescence data and our understanding of phytoplankton photophysiology from laboratory and field studies.

Inhibition of non-photochemical quenching increases functional absorption cross-section of photosystem II as excitation from closed reaction centres is transferred to open centres, facilitating earlier light saturation of photosynthetic electron transport.

Induction of non-photochemical quenching (NPQ) of chlorophyll fluorescence in leaves affords photoprotection to the photosynthetic apparatus when, for whatever reason, photon capture in the antennae of photosystems exceeds their capacity to utilise this excitation in photochemistry and ultimately in CO2 assimilation. Here we augment traditional monitoring of NPQ using the fast time resolution, remote and relatively non-intrusive light induced fluorescence transient (LIFT) technique (Kolber et al . 2005 ; Osmond et al . 2017 ) that allows direct measurement of functional (σ'PSII ) and optical cross-sections (a 'PSII ) of PSII in situ , and calculates the half saturation light intensity for ETR (E k ). These parameters are obtained from the saturation and relaxation phases of fluorescence transients elicited by a sequence of 270, high intensity 1 μs flashlets at controlled time intervals over a period of 30 ms in the QA flash at intervals of a few seconds. We report that although σ'PSII undergoes large transient increases after transfer from dark to strong white light (WL) it declines little in steady-state as NPQ is induced in shade- and sun-grown spinach and Arabidopsis genotypes Col , OEpsbs , pgr 5bkg , stn 7 and stn 7/8. In contrast, σ'PSII increases by ~30% when induction of NPQ in spinach is inhibited by dithiothreitol and by inhibition of NPQ in Arabidopsis npq 1, npq 4 and pgr 5. We propose this increase in σ'PSII arises as some excitation from closed PSII reaction centres is transferred to open centres when excitation partitioning to photochemistry (Y II ) and NPQ (Y NP ) declines, and is indicated by an increased excitation dissipation from closed PSII centres (Y NO , including fluorescence emission). Although E k increases following dissipation of excitation as heat when NPQ is engaged, it declines when NPQ is inhibited. Evidently photochemistry becomes more easily light saturated when excitation is transferred from closed RCIIs to open centres with larger σ'PSII . The NPQ mutant pgr 5 is an exception; E k increases markedly in strong light as electron transport QA → PQ and PQ → PSI accelerate and the PQ pool becomes strongly reduced. These novel in situ observations are discussed in the context of contemporary evidence for functional and structural changes in the photosynthetic apparatus during induction of NPQ.

A multi-spectral fluorescence induction and relaxation (FIRe) technique for physiological and taxonomic analysis of phytoplankton communities

Phytoplankton are extraordinarily diverse, comprising 13 phylogenetic groups, with diatoms, dinoflagellates, and haptophytes among the most prominent eukaryotes in the ocean. Development of sensor technologies for rapid taxonomic and physiological analysis of phytoplankton communities is crucial for ecological monitoring programs in the global ocean. We describe a novel, ultra-sensitive, multi-spectral fluorescence induction and relaxation instrument (a mini-FIRe) and examine its analytical capability of rapidly determining phytoplankton taxonomic groups, as well as physiological characteristics and photosynthetic rates. We collected and analyzed the database of spectral and photosynthetic properties of major taxonomic groups of phytoplankton. We revealed that the spectral shape of the functional absorption cross-section of Photosystem II (PSII), sPSII(lex), is remarkably constrained within each major phylogenetic group of eukaryotic phytoplankton, including diatoms, haptophytes, dinoflagellates, and chlorophytes. Variability in sPSII(lex) within each group was significantly smaller than the difference between groups. We also examined the classical excitation spectra of chl a fluorescence yields, Fm(lex). Our comparative analysis revealed that sPSII(lex) is a better and more specific proxy for taxonomic analysis. For instance, our developed sPSII-based algorithm correctly identified 90% of experimental data, compared to 77% identified by the Fm-based algorithm. Our results suggest that the multi-color variable fluorescence analysis offers a tool for combined physiological and taxonomic analysis, including identification of major phyla within the ‘red’ lineage of eukaryotic phytoplankton.

A Chlorophyte Alga Utilizes Alternative Electron Transport for Primary Photoprotection.

The green alga Desmodesmus armatus is an emerging biofuel platform that produces high amounts of lipids and biomass in mass culture. We observed D. armatus in light-limiting, excess-light, and sinusoidal-light environments to investigate its photoacclimation behaviors and the mechanisms by which it dissipates excess energy. Chlorophyll a/b ratios and the functional absorption cross section of PSII suggested a constitutively small light-harvesting antenna size relative to other green algae. In situ and ex situ measurements of photo-physiology revealed that nonphotochemical quenching is not a significant contributor to photoprotection; however, cells do not suffer substantial photoinhibition despite its near absence. We performed membrane inlet mass spectrometry analysis to show that D. armatus has a very high capacity for alternative electron transport (AET) measured as light-dependent oxygen consumption. Up to 90% of electrons generated at PSII can be dissipated by AET in a water-water cycle during growth in rapidly fluctuating light environments, like those found in industrial-scale photobioreactors. This work highlights the diversity of photoprotective mechanisms present in algal systems, indicating that nonphotochemical quenching is not necessarily required for effective photoprotection in some algae, and suggests that engineering AET may be an attractive target for increasing the biomass productivity of some strains.

Iron Availability Influences the Tolerance of Southern Ocean Phytoplankton to Warming and Elevated Irradiance

The Southern Ocean is responsible for approximately 40% of oceanic carbon uptake through biological and physical processes. In the Southern Ocean, phytoplankton growth is limited by low iron (Fe) and light supply. Climate model projections for the Southern Ocean indicate that temperature, underwater irradiance and Fe supply are likely to change simultaneously in the future due to increasing anthropogenic carbon dioxide emissions. The individual effects of these environmental properties on phytoplankton physiology have been extensively researched, and culturing studies using Southern Ocean phytoplankton have shown that temperature and Fe will play a key role on setting growth under future conditions. To explore the potential responses of Southern Ocean phytoplankton to these environmental changes, we cultured the haptophyte Phaeocystis antarctica and the diatoms Chaetoceros flexuosus, Proboscia inermis, and Thalassiosira antarctica under two light and iron combinations and over a range of temperatures. Our study revealed that the thermal response curves of key Southern Ocean phytoplankton are diverse, with the highest growth rates measured at 5°C (the annual temperature range at the isolation sites is currently 1–4°C). Warming had species-specific effects on the photochemical efficiency of photosystem II (PSII; Fv/Fm), the functional absorption cross-section of PSII (σPSII), carbon:nitrogen ratio and cellular Chlorophyll a concentrations. Iron availability increased species’ ability to tolerate warmer conditions by increasing the upper limit for growth and subsequently increasing the thermal niche that each species inhabit.

Lhcx proteins provide photoprotection via thermal dissipation of absorbed light in the diatom Phaeodactylum tricornutum

Diatoms possess an impressive capacity for rapidly inducible thermal dissipation of excess absorbed energy (qE), provided by the xanthophyll diatoxanthin and Lhcx proteins. By knocking out the Lhcx1 and Lhcx2 genes individually in Phaeodactylum tricornutum strain 4 and complementing the knockout lines with different Lhcx proteins, multiple mutants with varying qE capacities are obtained, ranging from zero to high values. We demonstrate that qE is entirely dependent on the concerted action of diatoxanthin and Lhcx proteins, with Lhcx1, Lhcx2 and Lhcx3 having similar functions. Moreover, we establish a clear link between Lhcx1/2/3 mediated inducible thermal energy dissipation and a reduction in the functional absorption cross-section of photosystem II. This regulation of the functional absorption cross-section can be tuned by altered Lhcx protein expression in response to environmental conditions. Our results provide a holistic understanding of the rapidly inducible thermal energy dissipation process and its mechanistic implications in diatoms.

Structural and functional analyses of photosystem II in the marine diatom Phaeodactylum tricornutum

A descendant of the red algal lineage, diatoms are unicellular eukaryotic algae characterized by thylakoid membranes that lack the spatial differentiation of stroma and grana stacks found in green algae and higher plants. While the photophysiology of diatoms has been studied extensively, very little is known about the spatial organization of the multimeric photosynthetic protein complexes within their thylakoid membranes. Here, using cryo-electron tomography, proteomics, and biophysical analyses, we elucidate the macromolecular composition, architecture, and spatial distribution of photosystem II complexes in diatom thylakoid membranes. Structural analyses reveal 2 distinct photosystem II populations: loose clusters of complexes associated with antenna proteins and compact 2D crystalline arrays of dimeric cores. Biophysical measurements reveal only 1 photosystem II functional absorption cross section, suggesting that only the former population is photosynthetically active. The tomographic data indicate that the arrays of photosystem II cores are physically separated from those associated with antenna proteins. We hypothesize that the islands of photosystem cores are repair stations, where photodamaged proteins can be replaced. Our results strongly imply convergent evolution between the red and the green photosynthetic lineages toward spatial segregation of dynamic, functional microdomains of photosystem II supercomplexes.

Assessment of phytoplankton photo-physiological status from a tropical monsoonal estuary

Assessment of phytoplankton physiological responses to abiotic changes provides valuable insights into the health of an ecosystem. The variable fluorescence (Fv = Fm-F0), photosynthetic efficiency (Fv/Fm), and functional absorption cross-section estimated using active fluorometry, are the commonly used parameters to gauge the physiological status or stress or photosynthetic efficiency (PE). However, in general, physiological assessments at longer time scales (seasonal to annual) are scarce but no information from monsoon-influenced estuarine systems in the tropical environment. Here phytoplankton physiological responses to abiotic changes were evaluated based on daily observations (October-2008 to May-2010) of variable fluorescence, chlorophyll and environmental parameters from a monsoon-influenced Dona Paula Bay (west-coast of India). Minimum fluorescence (F0) was an excellent proxy to track biomass changes. During monsoon (June-September) and non-monsoon (October-May), four (lasting up to 6-days) and two (lasting up to 8-days) bloom respectively were observed. For physiological assessment following scenarios low (<0.30), moderate (0.30–0.50) and high (0.51–0.65) levels of Fv/Fm suggesting stressed, transitional and growing conditions, respectively for phytoplankton were adapted. Low-to-moderate and moderate-to-high Fv/Fm were observed during monsoon and non-monsoon seasons, respectively. The dominance of photosynthetically inactive cells due to benthic-resuspension, bloom-phases (lag/decline), salinity-stress, and high-turbulence resulted in reduced Fv/Fm during monsoon. Photoinhibition and low-nutrient concentrations resulted in reduced Fv/Fm during non-monsoon. On an annual scale, Fv/Fm was moderate and was in accordance with the oceanic measurements suggesting that the phytoplankton community operates approximately half of maximum PE as reported for upper oceans. However, unlike oceans, the influencing factors may not be limited to nutrient-limitation. Nevertheless, the incorporation of such physiological scaling in monitoring programs from the perspective of the impact of variable monsoons/environment and anthropogenic activities will be crucial in ecological surveillance.

Probing functional and optical cross-sections of PSII in leaves during state transitions using fast repetition rate light induced fluorescence transients.

Plants adjust the relative sizes of PSII and PSI antennae in response to the spectral composition of weak light favouring either photosystem by processes known as state transitions (ST), attributed to a discrete antenna migration involving phosphorylation of light-harvesting chlorophyll-protein complexes in PSII. Here for the first time we monitored the extent and dynamics of ST in leaves from estimates of optical absorption cross-section (relative PSII antenna size; aPSII). These estimates were obtained from in situ measurements of functional absorption cross-section (σPSII) and maximum photochemical efficiency of PSII (φPSII); i.e. aPSII = σPSII/φPSII (Kolber et al. 1998) and other parameters from a light induced fluorescence transient (LIFT) device (Osmond et al. 2017). The fast repetition rate (FRR) QA flash protocol of this instrument monitors chlorophyll fluorescence yields with reduced QA irrespective of the redox state of plastoquinone (PQ), as well as during strong ~1 s white light pulses that fully reduce the PQ pool. Fitting this transient with the FRR model monitors kinetics of PSII → PQ, PQ → PSI, and the redox state of the PQ pool in the 'PQ pool control loop' that underpins ST, with a time resolution of a few seconds. All LIFT/FRR criteria confirmed the absence of ST in antenna mutant chlorina-f2 of barley and asLhcb2-12 of Arabidopsis, as well as STN7 kinase mutants stn7 and stn7/8. In contrast, wild-type barley and Arabidopsis genotypes Col, npq1, npq4, OEpsbs, pgr5 bkg and pgr5, showed normal ST. However, the extent of ST (and by implication the size of the phosphorylated LHCII pool participating in ST) deduced from changes in a'PSII and other parameters with reduced QA range up to 35%. Estimates from strong WL pulses in the same assay were only ~10%. The larger estimates of ST from the QA flash are discussed in the context of contemporary dynamic structural models of ST involving formation and participation of PSII and PSI megacomplexes in an 'energetically connected lake' of phosphorylated LHCII trimers (Grieco et al. 2015). Despite the absence of ST, asLhcb2-12 displays normal wild-type modulation of electron transport rate (ETR) and the PQ pool during ST assays, reflecting compensatory changes in antenna LHCIIs in this genotype. Impaired LHCII phosphorylation in stn7 and stn7/8 accelerates ETR from PSII →PQ, over-reducing the PQ pool and abolishing the yield difference between the QA flash and WL pulse, with implications for photochemical and thermal phases of the O-J-I-P transient.