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Microphytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into the photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high‐resolution tri‐dimensional hydrodynamic model coupled to an MPB model with or without photoinhibition. Photoinhibition leads to a 20% (−0.79 × 103 t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (−29%) than on the lower shore (−5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (−22% and −23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter‐annual time scale. Plain Language Summary: Benthic micro‐algae or microphytobenthos (MPB) inhabiting the surficial sediment sustain the high biological production of intertidal mudflats. MPB achieve photosynthesis by aggregating into a dense biofilm at the mud surface during daytime low tides. As MPB can be exposed to short‐term variations and high light levels, they change their short‐term physiology and position within the sediment to protect themselves. However, such strategies can be outbalanced by a too long stressful light exposure. In this study, we explore with a numerical model the impact of photoinhibition on MPB primary production (PP) over a large and very productive mudflat (NW France). The model suggests that photoinhibition can strongly impact MPB PP. With photoinhibition, the yearly PP decreases by 20% over the whole mudflat. The model suggests MPB PP is sensitive to the photoacclimation status of MPB cells, that is, their light use efficiency at a given light level. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. Key Points: With or without photoinhibition, simulated yearly MPB PP is higher on the upper and middle shores than on the lower shorePhotoinhibition leads to a 20% (−0.79×103 t C yr−1) decrease of MPB PP over the entire mudflatThe photoinhibition process in the model is sensitive to the photoacclimation status of MPB cells [ABSTRACT FROM AUTHOR]

Highlights • Average growth rate is determined by the maximum and minimum light intensities in the PBR. • There exists an optical depth maximizing the average growth rate. • There is an optimal incident light intensity depending on the turbidity of the culture. • Incident light can inhibit the growth rate even in dense cultures. • Conditions for the occurrence of Allee effect are determined. Abstract Microalgae can be cultivated in closed or open photobioreactors (PBR). In these systems, light rapidly decreases as it passes through the culture due to the turbidity of the medium. Thus, microalgae experiment different light intensities depending on their position in the medium. In this paper, we study theoretically how the growth rate of microalgae is affected by different factors; incident light intensity, form of the PBR, microalgae population density, turbidity of non-microalgae components, and light path-length of the reactor. We show that for different types of PBR the average growth rate is completely determined by the incident light intensity and the optical depth. In the case of vertical cylindrical PBRs illuminated from above (e.g. race-way or panel-type reactors), we described (and we prove under general assumptions) in details the dependence of the AGR on the aforementioned factors. Finally, we discuss some implications of our analysis; the occurrence of the Allee effect, if light ostensibly limits or inhibits the growth rate in outdoor cultures, and how the geometry of the PBR affects microalgae growth rate and productivity. [ABSTRACT FROM AUTHOR]

Benthic diatoms are dominant primary producers in intertidal marine sediments, which are characterized by widely fluctuating and often extreme light conditions. To cope with sudden increases in light intensity, benthic diatoms display both behavioural and physiological photoprotection mechanisms. Behavioural photoprotection is restricted to raphid pennate diatoms, which possess a raphe system that enables motility and hence positioning in sediment light gradients (e.g. via vertical migration into the sediment). The main physiological photoprotection mechanism is to dissipate excess light energy as heat, measured as Non-Photochemical Quenching (NPQ) of chlorophyll fluorescence. A trade-off between vertical migration and physiological photoprotection (NPQ) in benthic diatoms has been hypothesized before, but this has never been formally tested. We exposed five epipelic diatom species (which move in between sediment particles) and four epipsammic diatom species (which live in close association with individual sand grains) to high light conditions, and characterized both NPQ and the relative magnitude of the migratory response to high light. Our results reveal the absence of a significant downward migratory response in an araphid diatom, but also in several raphid epipsammic diatoms, while all epipelic species showed a significant migratory response upon high light exposure. In all epipsammic species the upregulation of NPQ was rapid and pronounced; NPQ relaxation in low light conditions, however, occurred faster in the araphid diatom, compared with the raphid epipsammic species. In contrast, all epipelic species lacked a strong and flexible NPQ response and showed higher susceptibility to photodamage when not able to migrate. While overall our results support the vertical migration-NPQ trade-off, the lack of strong relationships between the capacity for vertical migration and NPQ within the epipsammic and epipelic groups suggests that other factors as well, such as cell size, substrate type and photoacclimation, may influence photoprotective strategies. [ABSTRACT FROM AUTHOR]

The susceptibility of photosynthesis to photoinhibition and its recovery were studied on cultures of the heterocystous filamentous cyanobacterium Nostoc muscorum grown under different light intensities and varying concentrations of cadmium (Cd). Various physiological parameters viz. growth, pigments content, photosynthetic oxygen evolution, photoinactivation and reactivation of photosynthesis were carried out. A general decline was observed in all these parameters with increasing concentration of Cd under normal and photoinhibitory light intensity. After the exposure of photoinhibitory light N. muscorum showed a rapid decrease in oxygen evolution followed by a quasi steady state rate of photosynthesis. It showed different manner, which became severe and prominent in the presence of Cd. Reactivation of photosynthesis in dim light after the photoinhibitory light treatment was rapid: 85 to 95 % necessary occurred within 2 h, which slowed down and stoped in the presence of cadmium. In the presence of the translation inhibitor, streptomycin (250 mg ml-1), no reactivation occurred. We also found that the damage increased dramatically when the culture was irradiated with high light exposed to streptomycin. A transcription inhibitor, rifampicin did not inhibit the reactivation process. Based on these observations, we conclude that the photoinhibitory damage observations is the net result of a balance between the photoinhibitory process and operation of the repairing mechanisms. Cadmium not only increased the photoinhibitory rate but also arrest recovery process. [ABSTRACT FROM AUTHOR]

Phaeocystis antarctica is an abundant phytoplankton species in the Southern Ocean, where growth is frequently limited by iron and light. Being able to grow under low iron conditions is essential to the species’ success, but there have been hints that this ability differs among clones. Here, we compare the growth, cell size and chlorophyll a concentrations of four P. antarctica clones cultured under different iron and light conditions. Iron was provided either as unchelated iron (Fe′) or bound to the bacterial siderophore desferrioxamine B, representing, respectively, the most and least bioavailable forms of iron which phytoplankton encounter in the marine environment. The growth rate data demonstrate that the clones vary in their ability to grow using organically bound iron, and that this ability is not related to their ability to grow at low inorganic iron concentrations. These results are consistent at low and high light. Physiologically, only three of the four clones shrink or decrease the concentration of chlorophyll a in response to iron limitation, and only one clone decreases colony formation. Together, our data show that P. antarctica clones 1) respond to the same degree of iron limitation using different acclimation strategies, and 2) vary in their ability to grow under the same external iron and light conditions. This physiological diversity is surprisingly large for isolates of a single phytoplankton species. [ABSTRACT FROM AUTHOR]

The springs of northern Florida have degraded dramatically over recent decades, and filamentous algal mats have replaced submersed macrophytes. The unique temporal stability of these spring ecosystems has prompted narratives about the mechanisms of change in which authors focused entirely on press disturbances, such as NO3- enrichment and flow reduction. We tested the hypothesis that pulse disturbances, in the form of episodic flow reversals, are a key component of spring degradation. During flow reversals, acidic floodwaters rich in dissolved organic C (DOC) from adjacent blackwater rivers dramatically alter the light environment, and DOC respiration depletes dissolved O2 (DO), thereby creating a trophic cascade wherein negative effects on algae consumers enable algal proliferation. Synthesis of river and aquifer stages from springs in the Suwannee River basin indicates that reversal events have become more frequent with time in response to aquifer declines attributed to climate variability and groundwater pumping. This increase in frequency and duration of flow reversal is remarkably concurrent with the emergence of nuisance algal mats. Furthermore, re-analysis of existing algal abundance data suggests that potential for flow reversal is a significant predictor, but only when conditioned on spring DO concentrations. In low-DO springs, algal cover is always high, and reversals have little effect. However, in high-DO springs, algal cover is low except where reversals are likely. Our results suggest that despite remarkable temporal stability, episodic flow reversals may play a key role in regulating the ecosystem state. Focusing on these pulse events may be critical to spring protection and restoration. [ABSTRACT FROM AUTHOR]

The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a Japanese brown alga, Ecklonia radicosa, was determined by using dissolved oxygen sensors and pulse-amplitude modulated (PAM) fluorometry. The effective quantum yield (ΦPSII) of the sporophytes, in a natural population in shallow water, showed a characteristic noontime decline that indicated the influence of high PAR. However, ΦPSII of sporophytes in deeper water (-10 to -20 m) did not appear to be affected and maintained relatively high values of ΦPSII. In laboratory measurements of ΦPSII, after 12 h of continuous low and high PAR (100 and 1000 µmol photon m-2 s-1), a relatively greater decrease in ΦPSII in the high PAR treatment was observed. Furthermore, despite an overnight dark acclimation, the maximum quantum yield (Fv/Fm) under the high PAR treatment did not fully recover. A net photosynthesis-irradiance (P-E) curve was generated at 20°C, which revealed low compensation and saturation irradiances (Ec, and Ek) of 14 and 45 µmol photon m-2 s-1, suggesting adaptation to a low PAR environment. Results from experiments to determine the net/gross photosynthesis and dark respiration rates revealed that the maximal gross photosynthetic rates occurred at 27.5°C and were 5.18 µg O2 gww -1min-1; whereas, the dark respiration rate increased exponentially with a mean value of 1.35 µg O2 gww-1 min-1 at 22°C. Fv/Fm was relatively stable at low temperature, and the highest value (0.73) occurred at 16.5°C. The field survey and laboratory experiments revealed that E. radicosa is well adapted to relatively low PAR and water temperatures typical of warm temperate coastal regions in Japan. The poor response to high PAR and the inability of sporophytes to fully recover overnight from high PAR exposure indicates that the vertical distribution of this species is driven by the PAR environment. [ABSTRACT FROM AUTHOR]

Microcystis are common cyanobacteria of eutrophic lakes that can form harmful blooms in warm seasons. The genus includes toxic and nontoxic species/strains, and they show competitive succession in natural environments that is not well understood. Here we show the effects of microcystin production on species competitiveness when cultured under different conditions. When phosphate is rich, toxic Microcystis grew faster than nontoxic Microcystis. However, phosphate limitation significantly promoted photosynthesis of nontoxic Microcystis. Under phosphate limitation, toxic Microcystis was more sensitive than nontoxic Microcystis, and the maximum relative electron transport rate (rETRmax) of nontoxic Microcystis was significantly higher than that of toxic Microcystis. Significantly higher light quantum efficiencies suggested that nontoxic Microcystis was more competitive than toxic Microcystis (P < 0.05) under light limitation. When in high light, the ratio of closed Photosystem II (PSI1) to total PSII was higher and more sensitive to light in nontoxic than toxic Microcystis. Intense non-photochemical quenching (

In this work, Nannochloropsis salina was cultivated in a continuous-flow flat-plate photobioreactor, working at different residence times and irradiations to study the effect of the specific light supply rate on biomass productivity and photosynthetic efficiency. Changes in residence times lead to different steady-state cell concentrations and specific growth rates. We observed that cultures at steady concentration were exposed to different values of light intensity per cell. This specific light supply rate was shown to affect the photosynthetic status of the cells, monitored by fluorescence measurements. High specific light supply rate can lead to saturation and photoinhibition phenomena if the biomass concentration is not optimized for the selected operating conditions. Energy balances were applied to quantify the biomass growth yield and maintenance requirements in N. salina cells. [ABSTRACT FROM AUTHOR]

Many members of the Klebsormidiophyceae typically occur in terrestrial habitats where they are exposed to ultraviolet radiation (UVR). To evaluate the extent of tolerance to this waveband, 32 strains of the generaEntransia,Hormidiella,InterfilumandKlebsormidiumwere exposed to identical artificial UVR. The respective response patterns were determined in terms of optimum quantum yield of PSII as well as the capability to synthesize and accumulate mycosporine-like amino acids (MAAs) as sunscreens. After exposure to UVR most of the investigated isolates ofHormidiella,InterfilumandKlebsormidiumexhibited only a moderate or no decrease in optimum quantum yield. In contrast, both isolates ofEntransiashowed strong photoinhibition. These photosynthetic activity data are well reflected in the biochemical capability to synthesize MAAs. With the exception of bothEntransiastrains, where not even traces of MAAs could be detected, all other isolates ofHormidiella,InterfilumandKlebsormidiumaccumulated UV-absorbing compounds after UVR exposure. WhileInterfilumandKlebsormidiumcontained the same putative MAA with an absorption maximum at 324 nm,Hormidiella attenuatashowed another putative MAA with an absorption maximum at 325 nm. In addition, both compounds exhibited different retention times during HPLC analysis, suggesting their chemical structure must be different. All data indicated a generally high UVR tolerance inHormidiella,InterfilumandKlebsormidiumwhich explains well their aeroterrestrial lifestyle. [ABSTRACT FROM PUBLISHER]

Photosynthetic acclimation of phytoplankton to lower irradiation can be met by several strategies such as increasing the affinity for light or increasing antenna size and stacking of the thylakoids. The latter is reflected by a higher proportion of polyunsaturated fatty acids (PUFAs). Additionally, photosynthetic capacity (Pmax), respiratory losses, and proton leakage can be reduced under low light. Here we consider the effect of light intensity and phosphorus availability simultaneously on the photosynthetic acclimation and fatty acid composition of four phytoplankters. We studied representatives of the Chlorophyceae, Cryptophyceae and Mediophyceae, all of which are important components of plankton communities in temperate lakes. In our analysis, excluding fatty acid composition, we found different acclimation strategies in the chlorophytesScenedesmus quadricauda, Chlamydomonas globosa, cryptophyteCryptomonas ovataand ochrophyteCyclotella meneghiniana. We observed interactive effects of light and phosphorus conditions on photosynthetic capacity inS. quadricaudaandCry. ovata. Cry. ovatacan be characterized as a low light-acclimated species, whereasS. quadricaudaandCyc. meneghinianacan cope best with a combination of high light intensities and low phosphorus supply. Principal component analyses (PCA), including fatty acid composition, showed further species-specific patterns in their regulation of Pmaxwith PUFAs and light. InS. quadricaudaandCyc. meneghiniana, PUFAs negatively affected the relationship between Pmaxand light. InChl. globosa, lower light coincided with higher PUFAs and lower Pmax, but PCA also indicated that PUFAs had no direct influence on Pmax. PUFAs and Pmaxwere unaffected by light inCry. ovata. We did not observe a general trend in the four species tested and concluded that, in particular, the interactive effects highlight the importance of taking into account more than one environmental factor when assessing photosynthetic acclimation to lower irradiation. [ABSTRACT FROM PUBLISHER]

Rock pools are dynamic and intermittently isolated habitats in the rocky intertidal. In this study, we assessed if the photosynthesis and physiological activity of the brown macroalga Cystoseira humilis and the red macroalga Digenea simplex in a rock pool at Lanzarote Island (eastern Atlantic) was affected due by their vertical distribution. Photosynthetic responses were measured at three depth levels (0.05–1 m, 0.4–2.5 and 3.5 m) through in vivo chlorophyll a fluorescence, in particular the maximal quantum yield ( F v / F m ) as an estimator of the physiological status and photoinhibition, and the electron transport rate (ETR) as an estimator of the photosynthetic capacity. Algal photoprotection and photodamage processes were related to algal zonation; shallow-water thalli had active mechanisms of dynamic photoinhibition compared to algae from the deeper level. The progressive increase of solar radiation during the day caused different responses for each macroalga, where C. humilis showed lower photoinhibition, higher ETR values than D. simplex. Algae from the shallow level had lower pigment content and higher resistance to high solar radiation through the accumulation of photoprotective compounds and higher antioxidant activity (DPPH) compared to thalli from the deeper level. In summary, this study corroborates that algae are vertically distributed inside rock pools according to their adaptive responses to light-induced stress conditions and that photoacclimation occur in a short-term period. [ABSTRACT FROM AUTHOR]

This paper presents a mathematical model capable of quantitative prediction of the state of the photosynthetic apparatus of microalgae in terms of their open, closed and damaged reaction centers under variable light conditions. This model combines the processes of photoproduction and photoinhibition in the Han model with a novel mathematical representation of photoprotective mechanisms, including qE-quenching and qI-quenching. For calibration and validation purposes, the model can be used to simulate fluorescence fluxes, such as those measured in PAM fluorometry, as well as classical fluorescence indexes. A calibration is carried out for the microalga Nannochloropsis gaditana , whereby 9 out of the 13 model parameters are estimated with good statistical significance using the realized, minimal and maximal fluorescence fluxes measured from a typical PAM protocol. The model is further validated by considering a more challenging PAM protocol alternating periods of intense light and dark, showing a good ability to provide quantitative predictions of the fluorescence fluxes even though it was calibrated for a different and somewhat simpler PAM protocol. A promising application of the model is for the prediction of PI-response curves based on PAM fluorometry, together with the long-term prospect of combining it with hydrodynamic and light attenuation models for high-fidelity simulation and optimization of full-scale microalgae production systems. [ABSTRACT FROM AUTHOR]

Background Red coralline algae are critical components of tropical reef systems, and their success and development is, at least in part, dependent on photosynthesis. However, natural variability in the photosynthetic characteristics of red coralline algae is poorly understood. This study investigated diurnal variability in encrusting Porolithon sp. and free-living Lithophyllum kotschyanum. Measured parameters included: photosynthetic characteristics, pigment composition, thallus reflectance and intracellular concentrations of dimethylsulphoniopropionate (DMSP), an algal antioxidant that is derived from methionine, an indirect product of photosynthesis. L. kotschyanum thalli were characterised by a bleached topside and a pigmented underside. Results Minimum saturation intensity and intracellular DMSP concentrations in Porolithon sp. were characterised by significant diurnal patterns in response to the high-light regime. A smaller diurnal pattern in minimum saturation intensity in the topside of L. kotschyanum was also evident. The overall reflectance of the topside of L. kotschyanum also exhibited a diurnal pattern, becoming increasingly reflective with increasing ambient irradiance. The underside of L. kotschyanum, which is shaded from ambient light exposure, exhibited a much smaller diurnal variability. Conclusions This study highlights a number of dynamic photoinhibition strategies adopted by coralline algae, enabling them to tolerate, rather than be inhibited by, the naturally high irradiance of tropical reef systems; a factor that may become more important in the future under global change projections. In this context, this research has significant implications for tropical reef management planning and conservation monitoring, which, if natural variability is not taken into account, may become flawed. The information provided by this research may be used to inform future investigations into the contribution of coralline algae to reef accretion, ecosystem service provision and palaeoenvironmental reconstruction. [ABSTRACT FROM AUTHOR]

Abstract: Chlamydomonas reinhardtii is a photoautotrophic green alga, which can be grown mixotrophically in acetate-supplemented media (Tris–acetate–phosphate). We show that acetate has a direct effect on photosystem II (PSII). As a consequence, Tris–acetate–phosphate-grown mixotrophic C. reinhardtii cultures are less susceptible to photoinhibition than photoautotrophic cultures when subjected to high light. Spin-trapping electron paramagnetic resonance spectroscopy showed that thylakoids from mixotrophic C. reinhardtii produced less 1O2 than those from photoautotrophic cultures. The same was observed in vivo by measuring DanePy oxalate fluorescence quenching. Photoinhibition can be induced by the production of 1O2 originating from charge recombination events in photosystem II, which are governed by the midpoint potentials (E m) of the quinone electron acceptors. Thermoluminescence indicated that the E m of the primary quinone acceptor (QA/QA −) of mixotrophic cells was stabilised while the E m of the secondary quinone acceptor (QB/QB −) was destabilised, therefore favouring direct non-radiative charge recombination events that do not lead to 1O2 production. Acetate treatment of photosystem II-enriched membrane fragments from spinach led to the same thermoluminescence shifts as observed in C. reinhardtii, showing that acetate exhibits a direct effect on photosystem II independent from the metabolic state of a cell. A change in the environment of the non-heme iron of acetate-treated photosystem II particles was detected by low temperature electron paramagnetic resonance spectroscopy. We hypothesise that acetate replaces the bicarbonate associated to the non-heme iron and changes the environment of QA and QB affecting photosystem II charge recombination events and photoinhibition. [Copyright &y& Elsevier]

It is well known that light and nutrients are essential to plants; however, there are few investigations in which these have been studied in combination on macroalgae, especially when solar ultraviolet radiation ( UVR) is concerned. We cultured the red alga Gracilaria lemaneiformis ( Bory) at different nitrate concentrations and light levels with or without UVR for 24 days. The results showed that nitrate supply markedly enhanced the growth and photosynthesis, increased the absorptivity of UV-absorbing compounds ( UVACs), and decreased photoinhibition in the presence of UVR. The thalli that received photosynthetically active radiation ( PAR) treatment exhibited higher growth rates than those that received PAR + UVR at ambient or enhanced nitrate concentrations. However, under PAR + UVR treatment, the absorptivity of UVACs was higher than that of PAR and fluctuated with light levels. UVR was found to reduce the maximal net photosynthetic rate, apparent photosynthetic efficiency and light-saturating irradiance while increasing the dark respiration rate, and inducing higher inhibition of growth and photosynthesis under high light versus under low light. Ultraviolet B significantly induced the synthesis of UVACs but led to higher inhibition on growth and photosynthesis than ultraviolet A. [ABSTRACT FROM AUTHOR]

Blooms of Microcystis aeruginosa (Kützing) Kützing occur frequently in many freshwater ecosystems around the world, but the role of environmental factors in promoting the growth and determining the proportion of toxic and non-toxic strains still requires more investigation. In this study, four strains (toxic CPCC299 & FACHB905 and non-toxic CPCC632 & FACHB315) were exposed to high light (HL) condition, similar to light intensity found at the surface of a bloom, to evaluate their sensitivity to photoinhibition. We also estimated their capacity to recover from this HL stress. For all strains, our results showed an increased inhibition of the photosynthetic activity with HL treatment time. When comparing the extent of photoinhibition between strains, both toxic strains were more resistant to the treatment and recovered completely their photosynthetic capacity after 3 h, while non-toxic strains needed more time to recover. For toxic strains, the rETR under HL was higher compared to the rETR under low light (LL) control condition despite 50% photoinhibition. This suggests that the detrimental effect of high light (HL; up to 2 h) is outweighed by their higher photosynthetic potential. This conclusion did not stand for non-toxic strains, and indicates their preference for LL environment. We also demonstrated that a LL/HL cycle induced a 259% increase in cell yield for a toxic strain and a decrease by 22% for a non-toxic strain. This also indicates that toxic strains have higher tolerance to HL in a fluctuating light environment. Our data demonstrated that difference of sensitivity to HL between strains can modify the competitive outcome between toxic and non-toxic strains and may affect bloom toxicity. [ABSTRACT FROM AUTHOR]