Carbohydrate production in relation to microphytobenthic biofilm development: an integrated approach in a tidal mesocosm

International audienceExperiments were performed to evaluate short-term changes in sediment extracellular carbohydratesfor a multispecific assemblage of benthic diatoms in relation to physiological status,endogenous migratory rhythms, and environmental conditions. For this purpose, a mesocosmwas used, which simulated both tidal and dark: light alternating cycles under controlled conditions.Scanning electronic microscopy in combination with picture analyses indicated thatnatural diatom migration patterns were reproduced in the mesocosm. Two EPS fractions wereoperationally separated in colloidal carbohydrate measurements: alcohol-soluble EPS (termed‘‘soluble EPS’’) and alcohol-insoluble EPS (termed ‘‘bound EPS’’). Microphytobenthic biomassfollowed a logistic-type curve and converged toward a maximal value termed the ‘‘biotic capacityof the local environment.’’ Both EPS fractions showed oscillations with production duringphotosynthetic periods and sharp decreases during night immersion periods. Productions ofboth EPS fractions increased with Chl a production during light periods suggesting a lightdependence in relation to migratory patterns. The decreases in both EPS fractions, which occurredduring night immersion periods suggest that carbohydrate hydrolysis and/or washawayaffected both EPS fractions similarly in benthic environments. Our results confirm the theoryaccording to which the two distinct fractions are under different metabolic controls. No changein soluble EPS release was obtained during the transition from logarithmic to stationary phase.On the other hand, a metabolism modification of microalgae, probably related to ammoniumdepletion, occurred when cells entered the stationary phase, since there was a high enhancementin bound EPS production. Mesocosm results can serve as a system of reference useful to characterizebiofilm development in field investigations and to revisit the effective implication ofeach EPS fraction in sediment stabilit