Tidal and seasonal effects on the short-term temporal patterns of bacteria, microphytobenthos and exopolymers in natural intertidal biofilms (Brouage, France)

Relationships between bacteria, microphytobenthos and extracellular polymeric substances (EPS) that make up microbial biofilms over bare mudflats were investigated at an hourly frequency during two 14-day spring–neap cycles in winter and summer 2008. Bacterial abundance and total chl a concentration were lower in summer (0.78×108±SD 0.39×108cell.m−2 and 59.0±SD 10.42mgchla.m−2) than in winter (3.7×108±SD 1.9×108cell.m−2 and 106.64±SD 11.29mgchla.m−2), coinciding with a high abundance of the gastropod Peringia ulvae in summer, which subsequently impacted 1st-cm chl a concentration by intense grazing. Bound and colloidal EPS carbohydrate temporal patterns were similar in winter (5.71±SD 3.95 and 4.67±SD 3.45μg.g−1, respectively) but were different in summer (14.9±SD 4.05 and 5.60±SD 4.50μg.g−1, respectively). Carbohydrate colloidal EPS appeared to be related to light and salinity, while 1st-mm chl a concentration was negatively affected by strong salinities and predation pressure by P. ulvae. The fluctuations of colloidal carbohydrates were remarkably similar in the two seasons with peaks just after spring tides when the highest irradiance was received by microphytobenthic cells. Apparently, colloidal EPS carbohydrates can protect cells against the high salinity values ranging from 32.3 to 50.4PSU. The presence of bound EPS carbohydrates may be linked to sediment colonization and resistance of biofilm activity. Proteins in EPS were absent in winter and represented a small proportion in summer (10%), but they appeared to be a good indicator of potential synergistic effects between MPB and bacteria in summer. Conversely, bound EPS carbohydrates reached high levels in winter, while the number of bacteria decreased simultaneously, suggesting a negative effect on bacterial growth in the absence of proteins in EPS. There was a lower proportion (31%) of low molecular weight EPS in summer than in winter (83%), possibly in relation to desiccation. •Same response to tidal effects in summer and winter for colloidal carbohydrate EPS•Colloidal EPS secretion is stimulated by a salinity excess•Synergistic effects between benthic diatoms and bacteria in summer•Bound EPS carbohydrate have negative effects on bacteria abundances•LMW EPS represent the result of hydrolysis from HMW EPS