Multitude and temporal variability of ecological niches as indicated by the diversity of cultivated bacterioplankton.
The diversity of cultured planktonic bacteria was analyzed. Bacterial strains were isolated from a eutrophic lake (Zwischenahner Meer, Niedersachsen, Germany) at three different sampling dates (October 1997, April and May 1998). Phylogenetic diversity was assessed by polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), and sequencing of 16S rRNA gene fragments. Enterobacterial repetitive intergenic consensus (ERIC)-PCR revealed a high genomic diversity within the strain collections, which exceeded the diversity of the 16S rRNA gene sequences considerably. The composition of each of the three strain collections was unique since strains isolated at different dates always exhibited different ERIC-PCR fingerprints. Growth tests with 59 different carbon substrates demonstrated that even strains with identical ERIC-PCR fingerprints, isolated on one sampling date, differed in their physiology. The culturable fraction investigated in the present study constituted a relatively small fraction (</=15%) of the whole bacterioplankton assemblage. Nevertheless, the high physiological diversity in this fraction already indicates that a multitude of different ecological niches must exist in the planktonic environment. The majority of strains isolated in April prior to the decay of the phytoplankton bloom were members of the Cytophaga-Flavobacterium group. One month later, not a single strain of this group could be isolated. When a group-specific PCR-DGGE technique was employed, rapid shifts in the diversity of non-cultured Cytophaga-Flavobacteria also became evident. Based on the rapid shifts in the composition of cultivated as well as some non-cultivated bacteria, the ecological niches in the planktonic habitat must undergo rapid temporal changes.