Spatial heterogeneity of bacterial populations along an environmental gradient at a shallow submarine hydrothermal vent near Milos Island (Greece)

The spatial heterogeneity of bacterial populations at a shallow-water hydro thermal vent in the Aegean Sea close to the island of Miles (Greece) was ex amined at two different times by using acridine orange staining for total c ell counts, cultivation-based techniques, and denaturing gradient gel elect rophoresis (DGGE) analysis of PCR-amplified 16S rRNA gene fragments. Concur rent with measurements of geochemical parameters, samples were taken along a transect from the center of the vent to the surrounding area. Most-probab le-number (MPN) counts of metabolically defined subpopulations generally co nstituted a minor fraction of the total cell counts; both counting procedur es revealed the highest cell numbers in a transition zone from the strongly hydrothermally influenced sediments to normal sedimentary conditions. Tota l cell counts ranged from 3.2 x 10(5) cells ml(-1) in the water overlying t he sediments to 6.4 x 10(8) cells g (wet weight) of sediment(-1). MPN count s of chemolithoautotrophic sulfur-oxidizing bacteria varied between undetec table and 1.4 x 10(6) cells g(-1) MPN counts for sulfate-reducing bacteria and dissimilatory iron-reducing bacteria ranged from 8 to 1.4 x 10(5) cells g(-1) and from undetectable to 1.4 x 10(6) cells g(-1) respectively. DGGE revealed a trend from a diverse range of bacterial populations which were p resent in approximately equal abundance in the transition zone to a communi ty dominated by few populations close to the center of the vent. Temperatur e was found to be an important parameter in determining this trend. However , at one sampling time this trend was not discernible, possibly due to stor m-induced disturbance of the upper sediment layers.