DOMINANCE OF ONE BACTERIAL PHYLOTYPE AT A MID-ATLANTIC RIDGE HYDROTHERMAL VENT SITE

Microbial community structure in natural environments has remained lar gely unexplored yet is generally considered to be complex. It is shown here that in a Mid-Atlantic Ridge hydrothermal vent habitat, where fo od webs depend on prokaryotic primary production, the surface microbia l community consists largely of only one bacterial phylogenetic type ( phylotype) as indicated by the dominance of a single 16S rRNA sequence . The main part of its population occurs as an ectosymbiont on the dom inant animals, the shrimp Rimicaris exoculata, where it grows as a mon oculture within the carapace and on the extremities, However, the same bacteria are also the major microbial component of the free-living su bstrate community, Phylogenetically, this type forms a distinct branch within the epsilon-Proteobacteria. This is different from all previou sly studied chemoautotrophic endo- and ectosymbioses from hydrothermal vents and other sulfidic habitats in which all the bacterial members cluster within the gamma-Proteobacteria.