Phytoplankton adaptation to marine ecosystems: insights into photophysiological functional diversity

The aim of this study was to investigate the diversity of the photoprotective responses activated by phytoplankton at short temporal scales, i.e. the non-photochemical quenching of Chl a fluorescence (NPQ) associated with the xanthophyll cycle (XC) activity. In this work, we tested the hypothesis that the process of photoprotection, in terms of modulation, extent and efficiency, might be considered as a functional trait in phytoplankton. This ecophysiological work has been performed on four species belonging to the phylum of Stramenopiles. The ecological diversity has been taken into account by studying the photoresponses of species isolated either in surface or deeper water layers, as well as of species adapted to grow either in oceanic or coastal or upwelling ecosystems. Species distinct photoresponses have been also assessed in relation to cell size constraints, using one microplanktonic and three picoplanktonic species.The relation between phytoplankton ecophysiological functional diversity and ecological adaptation has been studied in three picoeukaryotes (< 3.0 μm) isolated from distinct ecosystems, through the analysis of the physiological response curves obtained by plotting a measured variable against a range of a factor (i.e. light) affecting that variable. The photophysiological flexibility as a functional trait has been discussed in relation to niche adaptation. Because of their peculiar set of biological and ecological features, related to their minute cell size, and because of their very narrow size range, picoeukaryotes are interesting model organisms to address (and answer) questions concerning the ecophysiology of algae.The integrative physiological processes (i.e. photoacclimation, photoregulation, photosynthesis, growth), which are in part activated or modified in relation to changing light conditions, have been deeply investigated in the microplanktonic coastal diatom, Pseudo-nitzschia multistriata (Bacillariophyceae). A great plasticity in functioning and regulation has been shown to characterize the photoprotective/photoacclimative processes (XC and NPQ) of this diatom, in agreement with its ecological properties.