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.

International audienceThe 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

The aim of this study was to contribute to a better understanding of turbid coastal ecosystems, focusing on spatio-temporal changes at the mesoscale in microphytobenthos structures in a macrotidal bay (Bourgneuf, France). This was based on pigment analysis by high-pressure liquid chromatography (HPLC), reinforced by microscopy observations, in relation to sediment characteris- tics. Microphytobenthos assemblages of this mudflat were 97% dominated by diatoms, confirmed by HPLC analysis, which showed biomarker pigments of this group: chlorophyll c, fucoxanthin, dia- dinoxanthin and diatoxanthin. The main assemblage is constituted of epipsammic species, like Plagiogrammopsis, Achnanthes and Cocconeis, characterising mixed sediments. However, this could be replaced by epipelic species, like Plagiotropis, Pleurosigma and Staurophora, during summer due to station silting. Microphytobenthic photosynthetic active biomass in the first 2 mm varied spatially and temporally from 12.2 to 186.9 mg chl a m-2. The predominance of a biomass spatial variability reflected spatial changes in sediment grain size and water content. Biomass in the first 2 mm repre- sented from 28 to 68% of the total amount in the first 10 mm. This indicated an accumulation of bio- mass in illuminated layers and a biomass decrease with depth associated with increasing values of the ratio of phaeopigments a and fucoxanthin to chlorophyll a (phaeo a/chl a and fuco/chl a). Both could exceed 1.00 in deeper layers. Temporal changes in microphytobenthos structure at the mesoscale were mainly linked to hydrodynamism. This is stronger in winter and responsible for allochthonous pigment input (lutein and phaeophytin b) and sediment mixing, as grazing activity by macro- and meiofauna, forming chlorophyll a breakdown products, is responsible for an increase in fuco/chl a and phaeo a/chl a values during spring/summer.