In intertidal marine sediments, characterized by rapidly fluctuating and often extreme light conditions, primary production is frequently dominated by diatoms. We performed a comparative analysis of photophysiological traits in 15 marine benthic diatom species belonging to the four major morphological growth forms (epipelon (EPL), motile epipsammon (EPM-M) and non-motile epipsammon (EPM-NM) and tychoplankton (TYCHO)) found in these sediments. Our analyses revealed a clear relationship between growth form and photoprotective capacity, and identified fast regulatory physiological photoprotective traits (that is, non-photochemical quenching (NPQ) and the xanthophyll cycle (XC)) as key traits defining the functional light response of these diatoms. EPM-NM and motile EPL showed the highest and lowest NPQ, respectively, with EPM-M showing intermediate values. Like EPL, TYCHO had low NPQ, irrespective of whether they were grown in benthic or planktonic conditions, reflecting an adaptation to a low light environment. Our results thus provide the first experimental evidence for the existence of a trade-off between behavioural (motility) and physiological photoprotective mechanisms (NPQ and the XC) in the four major intertidal benthic diatoms growth forms using unialgal cultures. Remarkably, although motility is restricted to the raphid pennate diatom clade, raphid pennate species, which have adopted a non-motile epipsammic or a tychoplanktonic life style, display the physiological photoprotective response typical of these growth forms. This observation underscores the importance of growth form and not phylogenetic relatedness as the prime determinant shaping the physiological photoprotective capacity of benthic diatoms.

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 ...

The gross primary production (GPP) of intertidal mudflat microphytobenthos supports important ecosystem services such as shoreline stabilization and food production, and it contributes to blue carbon. However, monitoring microphytobenthos GPP over a longterm and large spatial scale is rendered difficult by its high temporal and spatial variability. To overcome this issue, we developed an algorithm to map microphytobenthos GPP in which the following are coupled: (i) NDVI maps derived from high spatial resolution satellite images (SPOT6 or Pléiades), estimating the horizontal distribution of the microphytobenthos biomass; (ii) emersion time, photosynthetically active radiation (PAR), and mud surface temperature simulated from the physical model MARS-3D; (iii) photophysiological parameters retrieved from Production-irradiance (P-E) curves, obtained under controlled conditions of PAR and temperature, using benthic chambers, and expressing the production rate into mg C h −1 m −2 ndvi −1. The productivity was directly calibrated to NDVI to be consistent with remote-sensing measurements of microphytobenthos biomass and was spatially upscaled using satellite-derived NDVI maps acquired at different seasons. The remotely sensed microphytobenthos GPP reasonably compared with in situ GPP measurements. It was highest in March with a daily production reaching 50.2 mg C m −2 d −1 , and lowest in July with a daily production of 22.3 mg C m −2 d −1. Our remote sensing algorithm is a new step in the perspective of mapping microphytobenthos GPP over large mudflats to estimate its actual contribution to ecosystem functions, including blue carbon, from local and global scales.

International audienceThe strong biological production of estuarine intertidal flats is mainly supported by benthic diatoms in temperate areas. Their photosynthetic productivity is largely driven by changes in light intensity and temperature at the surface of sediment flats during emersion. The impact of an increase in salinity of the upper-layer sediment pore-water during emersion, which is often coupled with high light (HL), has been less studied. Furthermore, benthic diatoms show several growth forms which inhabit specific sediment types where the pore-water salinity can differentially vary due to the degree of cohesion of sediment grains. So far, no study explored if the main growth forms of benthic diatoms (i.e. epipelon, epipsammon and tychoplankton) show different photophysiological response to a combine high salinity-HL stress. Based on field monitoring, we compared the photophysiology (photosynthetic efficiency and photoprotection) of three representatives of the main growth forms during a short high salinity coupled with a moderate HL stress and stable optimal temperature, i.e. experimental conditions reproducing Spring environmental conditions in intertidal flats by the Atlantic French coast. Our results show that all growth forms reacted to HL exposure alone, as expected. While the epipelon representative was relatively insensitive to high salinity alone and combined with HL, the tychoplankton representative was highly sensitive to both, and the epipsammon representative was sensitive mainly to the stress combination. These specific responses fitted well with i) their natural habitat (i.e. more or less cohesive sediment) for which light climate and changes in salinity are different, ii) their growth form (i.e. motile, immotile or amphibious) which determines their probability to be confronted to a combined high salinity-HL stress. Hence, the negative effect of high salinity on photosynthetic efficiency of benthic diatoms appears to be mostly restricted to epipsammon and tychoplankton, and in field conditions, its effect probably remains negligible compared to HL stress

Barnett et al. Intertidal Microphytobenthos Light-Dependent Rhythmic Migration MPB surface accumulation, as compared to other wavelengths (white, green, and red) in patterns that were intensity-dependent and species-dependent. In particular, we found two species, Navicula spartinetensis and Gyrosigma fasciola, which strongly migrate up under blue light and could potentially be used as model species for further studying the light-responses of intertidal MPB.

The use of remote sensing techniques allows monitoring of photosynthesis at the ecosystem level and improves our knowledge of plant primary productivity. The main objective of the current study was to develop a remote sensing based method to measure microphytobenthos (MPB) primary production from intertidal mudflats. This was achieved by coupling hyperspectral radiometry (reflectance, ρ and second derivative, δδ) and PAM-fluorometry (non-sequential light curves, NSLC) measurements. The latter allowed the estimation of primary production using a light use efficiency parameter (LUE) and electron transport rates (ETR) whereas ρ allowed to estimate pigment composition and optical absorption cross-section (a*). Five MPB species representative of the main growth forms: epipelic (benthic motile), epipsammic (benthic motile and non motile) and tychoplanktonic (temporarily resuspended in the water column) were submitted to increasing light intensities from dark to 1950 μmol photons.m-2.s-1. Different fluorescence patterns were observed for the three growth-forms and were linked to their xanthophyll cycle (de-epoxydation state). After spectral reflectance measurements, a* was retrieved using a radiative transfer model and several radiometric indices were tested for their capacity to predict LUE and ETR measured by PAM-fluorometry. Only one radiometric index was not species or growth-form specific, i.e. δδ496/508. This index was named MPBLUE and could be used to predict LUE and ETR. The applicability of this index was tested with simulated bands of a wide variety of hyperspectral sensors at spectral resolutions between 3 and 15 nm of Full Width at Half Maximum (FWHM).

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