Although estuarine microphytobenthos (MPB) is frequently exposed to excessive light and temperature conditions, little is known on their interactive effects on MPB primary productivity. Laboratory and in situ experiments were combined to investigate the short-term joint effects of high light (HL) and high temperature (37 °C versus 27 °C) on the operating efficiency of photoprotective processes [vertical migration versus non-photochemical quenching (NPQ)] exhibited by natural benthic diatom communities from two intertidal flats in France (FR) and Portugal (PT). A clear latitudinal pattern was observed, with PT biofilms being more resistant to HL stress, regardless the effect of temperature, and displaying a lower relative contribution of vertical migration to photoprotection and a stronger NPQ in situ. However, higher temperature leads to comparable effects, with photoinhibition increasing to about three times (i.e. from 3% to 10% and from 8% to 22% in PT and FR sites respectively). By using a number of methodological novelties in MPB research (lipid peroxidation quantification, Lhcx proteins immunodetection), this study brings a physiological basis to the previously reported depression of MPB photosynthetic productivity in summer. They emphasize the joint role of temperature and light in limiting, at least transiently (i.e. during emersion), MPB photosynthetic activity in situ.

The capacity of microphytobenthos to withstand the variable and extreme conditions of the intertidal environment, prone to cause photoinhibition, has been attributed to particularly efficient photoprotection. However, little is known regarding the capacity of this protection against photoinhibition or the mechanisms responsible for it. The present study quantified the photoprotective capacity and the extent of photoinhibition under excess light, estimated the contribution of vertical migration and the xanthophyll cycle to overall photoprotection, and evaluated the effects of photoacclimation. A new experimental protocol combined (1) chlorophyll fluorescence imaging, for the simultaneous measurement of replicates and experimental treatments, (2) specific inhibitors for vertical migration and for the xanthophyll cycle, to quantify the relative contribution of each process, and (3) recovery kinetics analysis of photosynthetic activity during light stress-recovery experiments, to distinguish rapidly reversible photochemical down-regulation from photoinhibition. The results show a high photoprotective capacity in 2 study periods, May and October, with photoinhibition rates below 20%. A clear change in photoacclimation state was observed, with acclimation to lower irradiances in autumn associated with higher susceptibility to photoinhibition. In May, vertical migration and the xanthophyll cycle provided comparable protection against photoinhibition; in October, the former predominated. The sum of their contributions was ~20% in both months, suggesting that other processes also contribute to photoprotection.KEY WORDS: Microphytobenthos · Photoinhibition · Photoprotection · Xanthophyll cycle · Vertical migration · Non-photochemical quenching · Chlorophyll fluorescence · Diatoms Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 67: [161][162][163][164][165][166][167][168][169][170][171][172][173][174][175] 2012 damaging to the photo synthetic apparatus when acting individually, the combined effects of all of these factors likely coalesce in the photoinhibition of photosynthesis of benthic microalgae. Of particular importance is the exposure to direct sunlight, which can result in excessive reductant pressure and in the formation of intracellular reactive oxygen species (ROS; Roncarati et al. 2008, Waring et al. 2010. High levels of ROS cause the permanent inactivation of photosystem II (PSII) protein D1, negatively im pacting photosynthetic yield and primary productivity (Nishiyama et al. 2006).Despite these harsh conditions, microphytobenthos of intertidal flats typically exhibit high growth rates, forming dense and diverse sedimentary biofilms, and are recognized as a major contributor to ecosystemlevel carbon fixation and primary productivity (Underwood & Kromkamp 1999). Furthermore, an apparent lack of photoinhibition in microphytobenthic biofilms has been repeatedly reported (Blanchard & Cariou-LeGall 1994, Kromkamp et al. 1998, Underwood 2002, Blanchard et ...

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Light-response curves (LCs) of chlorophyll fluorescence are widely used in plant physiology. Most commonly, LCs are generated sequentially, exposing the same sample to a sequence of distinct actinic light intensities. These measurements are not independent, as the response to each new light level is affected by the light exposure history experienced during previous steps of the LC, an issue particularly relevant in the case of the popular rapid light curves. In this work, we demonstrate the proof of concept of a new method for the rapid generation of LCs from nonsequential, temporally independent fluorescence measurements. The method is based on the combined use of sample illumination with digitally controlled, spatially separated beams of actinic light and a fluorescence imaging system. It allows the generation of a whole LC, including a large number of actinic light steps and adequate replication, within the time required for a single measurement (and therefore named "singlepulse light curve"). This method is illustrated for the generation of LCs of photosystem II quantum yield, relative electron transport rate, and nonphotochemical quenching on intact plant leaves exhibiting distinct light responses. This approach makes it also possible to easily characterize the integrated dynamic light response of a sample by combining the measurement of LCs (actinic light intensity is varied while measuring time is fixed) with induction/relaxation kinetics (actinic light intensity is fixed and the response is followed over time), describing both how the response to light varies with time and how the response kinetics varies with light intensity.

The symbiotic association between the acoel flatworm Symsagittifera roscoffensis and the prasinophyte microalgae Tetraselmis convolutae was studied regarding its photophysiology and photobehaviour. The photoacclimation status and the photophysiological responses to high light of the algal endosymbiont were studied non-destructively on individual S. roscoffensis using pulse amplitude modulated fluorometry. Specimens collected in an intertidal sandy shore were characterized regarding the maximum quantum yield of photosystem II (PSII), F v / F m , and the light response of photosynthetic activity, by constructing rapid light-response curves of the relative electron transport rate of PSII, rETR. The studied population could be considered as high light-acclimated when compared with other intertidal photosynthetic organisms (e.g. macroalgae), with the light-saturation parameter E k averaging 250 mmol m 22 s 21 . Light stress experiments showed S. roscoffensis to be able to withstand the exposure to high light without displaying signs of photoinhibition, suggesting the operation of efficient physiological photoprotective processes. The photobehaviour of S. roscoffensis was studied by characterizing the distribution of the flatworms under a light gradient, using a custom-made photoaccumulation chamber. The results showed a photoaccumulation pattern evidencing a clear avoidance of extreme low or high light levels, and with maximum photoaccumulation values being found for a range of irradiances (150 -400 mmol m 22 s 21 ) that generally coincided with the optima for photosynthetic activity. This matching between the optimum light levels for photosynthetic activity and photoaccumulation suggested that S. roscoffensis may use vertical migration as a form of behavioural photoprotection. This behavioural response may be used to rapidly and flexibly control light exposure, avoiding photodamage to the endosymbiont photosynthetic apparatus by direct exposure to sunlight.