Benthic diatoms are dominant primary producers in intertidal marine sediments and are roughly divided into two main growth forms: the epipelon comprises mainly larger raphid motile diatoms and dominates silty sediments, whereas the epipsammon mainly consists of small motile and non-motile species that live in close association with single sand particles. As intertidal sediments are characterized by rapidly fluctuating and often extreme light conditions, benthic diatoms display behavioural as well as physiological photoprotection mechanisms. Vertical migration into the sediment (behavioural photoprotection), however, is largely restricted to epipelic diatoms, whereas epipsammic diatoms have to undergo changes in light conditions. As in natural communities (in situ studies) it is hard to characterize the photoprotective strategies of diatom growth forms (as natural communities can contain both epipelic and epipsammic growth forms), we studied the photoprotection capacity of unialgal isolates belonging to the main growth forms under controlled lab conditions.v One of the major physiological photoprotection mechanisms is to dissipate excess light energy as heat which can be measured as Non-Photochemical Quenching (NPQ). The capacity of this mechanism is mainly defined by the xanthophyll cycle (XC) pigment diatoxanthin and Light-harvesting Complex X (LHCX) proteins. We show that epipelic and epipsammic diatoms show fundamentally different photoprotective responses: epipsammic diatoms have a higher NPQ and associated XC capacities compared to epipelic diatoms. In the latter group, the behavioural response (vertical migration) is more important which may alleviate the need for strong physiological photoprotection. The regulation and performance of NPQ was further studied using model representatives of each functional group during and after exposure to high light. The epipsammic species Opephora guenter-grassii could rapidly switch NPQ on and off by relying on fast XC kinetics. This species also demonstrated high de novo synthesis of xanthophylls within a relatively short period of time (1 h), including significant amounts of zeaxanthin, a feature not observed before in other diatoms. In contrast, the epipelic representative Seminavis robusta showed slower NPQ and associated XC kinetics, partly relying on NPQ conferred by de novo synthetized diatoxanthin molecules and synthesis of Light-Harvesting Complex X (LHCX) isoforms. The genome of S. robusta contains fourteen LHCX genes. For eight LHCX genes we could show distinct upregulation during (strong) light exposure. While overall our results support the a trade-off between behavioural and physiological photoprotection mechanisms other factors besides growth form, such as environmental factors, cell size, substrate type and photoacclimation, may influence photoprotective strategies and explain species-specific photoregulation traits in intertidal benthic diatoms.
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