Microbial community dynamics in Mediterranean nutrient-enriched seawater mesocosms: changes in abundances, activity and composition

Quantitative and qualitative changes in bacterial communities from the Medi terranean Sea were compared in duplicate batch mesocosms with or without ad dition of inorganic nutrients. Methods including traditional microbial ecol ogy techniques, molecular biology and flow cytometry were combined to deter mine abundances, production, cell size, activity, culturability and taxonom ic diversity of bacterial cells. Addition of nutrients and confinement resu lted in an increase of bacterial densities which were rapidly controlled by protozoan grazing. Changes in bacterial activity and morphology were obser ved during the growth phase of bacteria and under grazing pressure. The pro portion of medium-size and culturable cells increased during the growth pha se. These cells were preferentially consumed by grazers resulting in a stro ng limitation of bacterial production. As a consequence of the grazing pres sure, large cells were produced and contributed to the remaining bacterial productivity after grazing. Grazing had an effect on the taxonomic composit ion of bacterial communities by preferentially eliminating gamma -Proteobac teria, alpha -Proteubacteria were preserved, It seems that some species fro m the genera Ruegeria and Cytophaga may have developed defence strategies t o escape predation. (C) 2001 Federation of European Microbiological Societi es, Published by Elsevier Science B.V. All rights reserved.