Changes in photosynthetic yield (Fv/Fm) responses of salt-marsh microalgal communities along an osmotic gradient (Mont-Saint-Michel Bay, France)

The response of salt-marsh microphytobenthic communities to daily changes is characterized by high microalgal growth rates. In comparison, the irregular tidal submersions that prevail in the extreme macrotidal system of "La Baie du Mont-Saint-Michel" (France) could be considered as a large-scale disturbance. The physiological capacities of microphytobenthic communities along a salinity gradient were assessed for rapid response to osmotic stress, salinity tolerance and changes in response to short time submersion. The photosynthetic system response (Fv/Fm) of microphytobenthic communities sampled at different levels of the saltmarsh along a conductivity gradient was studied during spring-tide and neap-tide periods. All samples demonstrated a dominant adaptation of the microalgal community to coastal seawater salinity, but response heterogeneity ruled out euryhaline adaptation as the global response of microphytobenthic taxa. The higher and lower salinity tolerances at the seaward and uppermost stations, respectively, were probably due to the occurrence of oversalted or undersalted habitats during the neap tide period. Moreover, the seaward communities were characterized by successive stimulation and inhibition of Fv/Fm during the spring tide period. Such a phenomenon could result from the opposing influences of sea submersion, which supplies resources on the one hand and destabilizes the biofilm on the other hand. Despite the singular response of these saltmarsh microphytobenthic communities to a conductivity gradient, the rapid and irregular salinity changes due to seawater submersion cannot be considered as a stressor for the dominant taxa.