Successional changes in the genetic diversity of a marine bacterial assemblage during confinement

The successional changes in the genetic diversity of Mediterranean bacterio plankton subjected to confinement were studied in an experimental 300 1 sea water enclosure. Five samples were taken at different times and analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGG E) fingerprinting to rapidly monitor changes in the bacterial genetic diver sity. DGGE analysis clearly showed variations between the samples. Three of the five samples, with different DGGE banding patterns, were further analy zed by cloning and sequencing of 16S rRNA genes. Comparative sequence analy sis indicated a shift from a mixed bacterial assemblage to a community domi nated by bacteria closely affiliated to a single genus, Alteromonas. Sequen ces obtained at the start of the experiment were affiliated with two alpha- proteobacterial and three gamma-proteobacterial lineages known from other s tudies of marine picoplankton. One sequence was affiliated with the Verruco microbiales. After 161 h of incubation two sequences represented a gamma-pr oteobacterial lineage also present at 0 h, but the majority of sequences cl ustered around that of Alteromonas macleodii. After 281 h only the dominant Alteromonas-like bacteria and bacteria distantly related to Legionella wer e found by cloning and sequencing. Mortality rates of bacteria indicated th at grazing was the dominant mortality process when heterotrophic protozoa w ere abundant. Hence, changes in the genetic diversity of bacteria were part ly influenced by the differential mortality of bacterial populations during the course of incubation.