Effect of bacterial community dynamics on DOC seasonal changes in the north-western Mediterranean Sea

To better understand the seasonal variation in concentrations and accumulation of dissolved organic carbon (DOC) in north-western Mediterranean surface waters, we investigated changes in bacterial abundance, activity (production, BP; respiration, BR; growth efficiency, BGE) and DOC concentration at different depths during different periods: winter mixing, the phytoplankton bloom period and summer stratification. The highest DOC concentrations were observed during the stratified period, reaching 90 +/- 10 mu mol C L-1. During this period, DOC concentrations were negatively correlated with the BP and BGE values at 5 m and positively correlated with BR at 5 and 20 m. DOC concentrations were also negatively correlated with viral abundances at 5 and 20 m depths. Furthermore, viral abundances were negatively correlated with bacterial abundance, production and growth efficiency, thus suggesting a viral influence on bacterial DOC assimilation. Inorganic nutrient depletion observed during the summer period had no significant relationships with bacterial activity. Characterization of the bacterioplankton assemblages by denaturing gradient gel electrophoresis of 16S rRNA profiles revealed the existence of seasonal trends in phylogenetic identity of the bacterioplankton. During the summer stratification period, the relative intensity of specific phylotypes was correlated with DOC concentrations. Alphaproteobacteria and Synechococcus sp. phylotypes were highly correlated with chlorophyll a, BP and BGE