Succession of pelagic marine bacteria during enrichment: a close look at cultivation-induced shifts

Enrichment experiments with North Sea bacterioplankton were performed to te st if rapid incubation-induced changes in community structure explain the f requent isolation of members of a few particular bacterial lineages or if r eadily culturable bacteria are common in the plankton but in a state of dor mancy. A metabolic inhibitor of cell division (nalidixic acid [NA]) was add ed to substrate-amended (S+) and unamended (S-) grazer-free seawater sample s, and shifts in community composition and per cell DNA and protein content were compared with untreated controls. In addition, starvation survival ex periments were performed on selected isolates. Incubations resulted in rapi d community shifts towards typical culturable genera rather than in the act ivation of either dormant cells or the original DNA-rich bacterial fraction . Vibrio spp. and members of the Alteromonas/Colwellia duster (A/C) were se lectively enriched in S+ and S-, respectively, and this trend,vas even magn ified by the addition of NA. These increases corresponded with the rise of cell populations with distinctively different but generally higher protein and DNA content in the various treatments. Uncultured dominant gamma -prote obacteria affiliating with the SAR86 cluster and members of the culturable genus Oceanospirillum were not enriched or activated, but there was no indi cation of substrate-induced cell death, either. Strains of Vibrio and A/C m aintained high ribosome levels in pure cultures during extended periods of starvation, whereas Oceanospirillum spp. did not. The life strategy of rapi dly enriched culturable gamma -proteobacteria could thus be described as a "feast and famine" existence involving different activation levels of subst rate concentration.