Characterization of free-living and attached bacteria in sediments colonized by Hediste diversicolor

The dynamics of free-living and attached bacteria populations were studied in salt marsh sediments that were extensively colonized by the polychaete Hediste (Nereis) diversicolor (O. F. Muller). Bioturbation by H. diversicolor affected the density and community structure of free-living (FLB) and attached (AB) bacteria assemblages. Passage through the polychaete gut resulted in an 81 to 88% decrease of both bacterial groups and shift in favor of FLB in the hindgut (HG). In surficial sediments outside and inside the worm bed, attached bacteria represented 77 to 99% of the total bacterial densities. Although FLB were minor in terms of biomass, their densities strongly increased in January and May and peaked from 9.6 to 16.2 x 10(8) cells g(-1). AB peaked in August and October with densities from 205.8 to 283.6 x 10(8) cells g(-1). AB densities were significantly higher in burrow wall sediment than in surrounding sediment. The density of FLB and AB was reduced in surficial sediment inside the worm bed compared to sediments not colonized by H. diversicolor. Community fingerprints of environmental 5S rRNA showed that there was no significant difference between the structure of the 2 bacterial assemblages. However, in August 1995, the 2 assemblages were highly divergent in the sediment outside the worm bed. The structure of FLB and AB from inside the worm bed was significantly different from the structure of bacterial assemblages outside the sediments colonized by H. diversicolor.