Microbial diversity in inactive chimney structures from deep-sea hydrothermal systems

Massive chimney structures, which arc characteristic of many hydrothermally active zones, harbor diverse Microbial communities containing both thermophilic and hyperthermophilic microbes. However, vent chimneys ultimately become hydrothermally inactive, and the changes that occur in the microbial communities upon becoming inactive have not been documented. We thus collected inactive chimneys from two geologically and geographically distinct hydrothermal fields, Iheya North in the western Pacific Ocean and the Kalrei field in the Indian Ocean. The chimneys displayed easily distinguishable strata, which were analyzed with regard to both mineralogical and microbiological properties. X-ray diffraction pattern and energy-dispersive spectroscopic analyses revealed that the main mineral components of the chimney substructures from Iheya North and the Kairei field were barite (BaSO4) and chalcopyrite (CuFeS2), respectively. Microbial cell densities in the substructures determined by DAPI counting ranged from 1.7 X 10(7) cells g(-1) to 3.0 X 10(8) cells g(-1). The proportions of archaeal rDNA in the whole microbial rDNA assemblages in all substructures were, at most, a few percent as determined by quantitative fluorogenic PCR. The microbial rDNA clone analysis and whole-cell fluorescence in Situ hybridization revealed a community that was decidedly different from any communities previously reported ill active chimneys. Curiously, both samples revealed the abundant presence of a group of Bacteria related to a magnetosome-bearing bacterium, "Magnetobacterium bavaricum" of the Nitrospirae division. These results suggest that inactive chimneys provide a distinct microbial habitat.