In intertidal mudflats, during low tide exposure, microphytobenthos (MPB) migrate vertically through the
surface sediment and form, with the heterotrophic bacteria, a transient biofilm. Inside this biofilm, multiple
interactions exist between MPB and bacteria. These micro-organisms secrete a wide range of extracellular
polymeric substances (EPS), which are major components of the biofilm matrix. In this study, we used a tidal
mesocosm experiment in order to decipher the interactions of the MPB–EPS–bacteria complex within the
biofilm. We tried to determine if the EPS could control bacterial activities and/or production and/or richness
according to the age of the biofilm and to the immersion/emersion period. The dynamics of biomasses of MPB
and prokaryotes, the bacterial production, the hydrolysis of predominating organic constituents in the dissolved
organic carbon (DOC) pool (i.e., carbohydrates and polypeptides), and the bacterial structure were studied in relation
to the different EPS fractions (carbohydrates and proteins: colloidal and bound) dynamics during 8 days.
Our experiment had emphasized the influence of the environmental conditions (light, immersion/emersion)
on the interactions within the biofilm and also on the effects on biofilm aging. Bacterial production was always
inhibited by the bound EPS-carbohydrate, especially during low tide. Our results suggest that the concentration
and composition of EPS had a major role in the bacterial/MPB interactions: these interactions can be either
positive or negative in order to regulate the productive phases of MPB and bacteria