The importance of being fast: comparative kinetics of vertical migration and non-photochemical quenching of benthic diatoms under light stress
Résumé
Sediment-inhabiting microphytobenthos (MPB) is dominated by raphid pennate diatoms that use vertical migration (VM) and the xanthophyll cycle (XC) as main photoprotective mechanisms against excess light. This study compares the short-term kinetics of activation of the two processes upon exposure to high light. The light response of VM was studied with temporal resolution of minutes, by using a new experimental approach (a `multiactinic imaging' system) based on the combination of (1) simultaneous application of multiple levels of incident light (50-1350 mu mol m(-2) s(-1)) to replicated samples, through the projection of spatially separated beams of actinic light, and (2) reflectance-based imaging of MPB surface biomass, using a modified chlorophyll fluorescence imaging system with red and infrared bandpass filters. The induction kinetics of the XC was inferred by measuring the activation of non-photochemical quenching (NPQ) of chlorophyll fluorescence, upon exposure to high light, on MPB suspensions. The possibility to trace VM with high temporal resolution allowed to find that the migratory photoresponse is much faster than previously reported, occurring at the scale of a few minutes. Surface biomass decreased by more than 30 % of total induced change during the first 2.5 min of light exposure and reached a steady state after 15 min. More importantly, it was found that the light induction of VM and NPQ largely overlaps in time, suggesting a complex interdependency in the regulation of the two mechanisms. These results further indicate that VM is fast enough to modulate the light regime experienced by the cells and to affect this photoacclimation state and their photoprotective capacity.
