Vertical cell movement is a primary response of intertidal benthic biofilms to increasing light dose

International audience; Intertidal soft sediment microphytobenthic biofilms are often dominated by diatoms, which are able to regulate their photosynthesis by physiological processes (e.g. down-regulation through the xanthophyll cycle, referred to as non-photochemical quenching, NPQ) and behavioural processes (e.g. vertical cell movement in the sediment–biofilm matrix). This study investigated these 2 processes over a 6 h emersion period using chemical inhibitors under 2 light treatments (ambient and constant light at 300 µmol m–2 s–1). Latrunculin A (Lat A) was used to inhibit cell movement and dithiothreitol (DTT) to inhibit NPQ. HPLC analysis for chlorophyll a and spectral analysis (Normalised Difference Vegetation Index) indicated that Lat A significantly inhibited cell movement. Photosynthetic activity was measured using variable chlorophyll fluorescence and radiolabelled carbon uptake and showed that the non-migratory, Lat A-treated biofilms were severely inhibited as a result of the high accumulated light dose (significantly reduced maximum relative electron transport rate, rETRmax, and light utilisation coefficient, α, compared to the migratory DTT and control-treated biofilms). No significant patterns were observed for 14C data, although a decrease in uptake rate was observed over the measurement period. NPQ was investigated using HPLC analysis of xanthophyll pigments (diatoxanthin and the percentage de-epoxidation of diadinoxanthin), chlorophyll fluorescence (change in maximum fluorescence yield) and the 2nd order spectral derivative index (diatoxanthin index). Patterns between methods varied, but overall data indicated greater NPQ induction in the non-migratory Lat A treatment and little or no NPQ induction in the DTT and control treatments. Overall, the data resulted in 2 main conclusions: (1) the primary response to accumulated light dose was vertical movement, which when inhibited resulted in severe down-regulation/photoinhibition; (2) diatoms down-regulated their photosynthetic activity in response to accumulated light dose (e.g. over an emersion period) using a combination of vertical migration and physiological mechanisms that may contribute to diel and/or tidal patterns in productivity.