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

Quantitative and qualitative changes in bacterial communities from the Medi terranean Sea were analysed under eutrophication conditions simulated in ba tch mesocosms (addition of inorganic nutrients or phytoplanktonic lysate). A wide variety of methods including traditional microbial ecology technique s, molecular biology and flow cytometry were combined to determine abundanc es, production, cell size, activity, culturability, and genetic and taxonom ic diversity. In all mesocosms, the increase in biomass was rapidly control led by protozoan grazing. Morphological and physiological changes were obse rved during the growth phase of bacteria and under grazing pressure. The pr oportion of medium-size and culturable cells increased during the growth ph ase. 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 c ontributed to the remaining productivity after grazing. Although grazing ha d an effect on the genetic diversity of bacterial communities by eliminatin g some populations, other species were preserved. It seems that some specie s such as Alteromonas macleodii may have developed defence strategies to es cape predation. We hypothesize that such species may escape grazing by prod ucing small and/or large cells during their growing phase.