Changes in bacterial community structure in seawater mesocosms differing in their nutrient status

Quantitative and qualitative changes in bacterial communities from the Mediterranean Sea were analysed under eutrophication conditions simulated in batch mesocosms (addition of inorganic nutrients or phytoplanktonic lysate). A wide variety of methods including traditional microbial ecology techniques, molecular biology and flow cytometry were combined to determine abundances, production, cell size, activity, culturability, and genetic and taxonomic diversity. In all mesocosms, the increase in biomass was rapidly controlled by protozoan grazing. Morphological and physiological changes were observed during the growth phase of bacteria and under grazing pressure. The proportion of medium-size and culturable cells increased during the growth phase. Grazing eliminates preferentially active and medium-sized cells within communities regulating bacterial productivity. Small and large cells were produced as a consequence of grazing pressure, and the large active cells contributed to the remaining productivity after grazing. Although grazing had an effect on the genetic diversity of bacterial communities by eliminating some populations, other species were preserved. It seems that some species such as Alteromonas macleodii may have developed defence strategies to escape predation. We hypothesize that such species may escape grazing by producing small and/or large cells during their growing phase.