Changes in marine bacterioplankton phylogenetic composition during incubations designed to measure biogeochemically significant parameters

Bottle incubations, during which the activity and growth of prokaryotes is monitored during several days, are frequently carried out to study function al aspects of marine prokaryotic assemblages. These experiments will relate directly to in situ activities if all populations grow harmonically during the incubation. We tested whether this was the case by analyzing the compo sition of bacterial assemblages at the beginning and at the end of the incu bation by denaturing gradient gel electrophoresis. Five experiments were do ne in different Antarctic regions. Bacterial assemblages north and south of the Polar Front were very different. In all cases, the final assemblages w ere very different from the initial ones, and these changes were often acco mpanied by a significant decrease of diversity indices. Our experiments inc luded treatments with different temperature and organic matter amendments. Whereas the increase in temperature tested had a minor effect on prokaryoti c growth rate and specific composition, the addition of organic matter stro ngly stimulated growth rate and selected a particular bacterial assemblage in some experiments but not in others. A significant component of bacterial assemblages From waters south of the Polar Front appeared to be Polari-bac ter franzmannii, a gas vacuolated bacterium of the Cytophaga-Flavobacterium -Bacteroides group that was originally isolated from Antarctic sea ice. Thi s phylotype was enriched and dominated in almost all final assemblages. Our results indicate that long-term bottle incubations mostly measure the acti vity of a few opportunistic bacteria and not that of the original assemblag e. This should be taken into account if data obtained in these experiments are used for balancing whole ecosystem carbon budgets and to derive biogeoc hemical conclusions.