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

International audienceRelationships between bacteria, microphytobenthos and extracellular polymeric substances (EPS) that makeup 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 × 108 cell.m−2 and 59.0 ± SD 10.42 mgchla.m−2) than in winter (3.7 × 108 ± SD1.9 × 108 cell.m−2 and 106.64 ± SD 11.29 mgchla.m−2), coinciding with a high abundance of the gastropodPeringia ulvae in summer, which subsequently impacted 1st-cm chl a concentration by intense grazing. Boundand colloidal EPS carbohydrate temporal patterns were similar in winter (5.71 ± SD 3.95 and 4.67 ± SD3.45 μg.g−1, respectively) butwere different in summer (14.9±SD 4.05 and 5.60±SD 4.50 μg.g−1, respectively).Carbohydrate colloidal EPS appeared to be related to light and salinity, while 1st-mm chl a concentration wasnegatively affected by strong salinities and predation pressure by P. ulvae. The fluctuations of colloidal carbohydrateswere remarkably similar in the two seasons with peaks just after spring tideswhen the highest irradiancewas received by microphytobenthic cells. Apparently, colloidal EPS carbohydrates can protect cells against thehigh salinity values ranging from 32.3 to 50.4 PSU. The presence of bound EPS carbohydrates may be linked tosediment colonization and resistance of biofilm activity. Proteins in EPS were absent in winter and representeda small proportion in summer (10%), but they appeared to be a good indicator of potential synergistic effects betweenMPB and bacteria in summer. Conversely, bound EPS carbohydrates reached high levels in winter, whilethe number of bacteria decreased simultaneously, suggesting a negative effect on bacterial growth in the absenceof 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