AME 19:255-267 (1999)  -  doi:10.3354/ame019255

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

Philippe Lebaron1,*, Pierre Servais2, Marc Troussellier3, Claude Courties4, Josep Vives-Rego5, Gerard Muyzer6,**, Laetitia Bernard1, Teresa Guindulain5, Hendrik Schäfer6,**, Erko Stackebrandt7

1Observatoire Océanologique, Université Pierre et Marie Curie, UMR-7621 CNRS, Institut National des Sciences de l'Univers, BP 44, 66651 Banyuls-sur-Mer Cedex, France
2Groupe de Microbiologie des Milieux Aquatiques, Université Libre de Bruxelles, Campus de la Plaine, CP 221, boulevard du Triomphe, 1050 Bruxelles, Belgium
3Laboratoire d'Hydrobiologie Marine - UMR CNRS 5556, Université Montpellier II, 34095 Montpellier Cedex 05, France
4Observatoire Océanologique, Université Pierre et Marie Curie, UMR-7628 CNRS, BP 44, 66651 Banyuls-sur-Mer Cedex, France
5Departament de Microbiologia, Universitat de Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain
6Max-Planck-Institut für Marine Mikrobiologie, Fahrenheitstraße 1, 28359 Bremen, Germany
7Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (DSMZ), Mascheroder Weg 1B, 38124 Braunschweig, Germany
**Present address: Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, The Netherlands

ABSTRACT: 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.

KEY WORDS: Mesocosm · Eutrophication · Bacteria · Biomass · Production · Diversity · Cell-specific activity · Grazing

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