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Relative abundance of Archaea and Bacteria along a thermal gradient of a shallow-water hydrothermal vent quantified by rRNA slot-blot hybridization

Departments of Microbiology and Biogeochemistry, and Molecular Ecology Group, Max-Planck-Institute for Marine Microbiology, Celsiusstr. 1, D-28359 Bremen, Germany¹²³

Author for correspondence: Stefan M. Sievert. Tel: +49 421 2028738. Fax: +49 421 2028580. e-mail: ssievert{at}mpi-bremen.de

Slot-blot hybridization of rRNA with domain-specific oligonucleotide probes targeting the 16S rRNA of Archaea and Bacteria was utilized to assess the relative abundance of these domains along a thermal gradient at a shallow submarine hydrothermal vent near Milos Island (Greece). The highest prokaryotic rRNA concentrations (defined as the sum of bacterial and archaeal rRNA) were found in the uppermost sediment surface (0–20 mm), decreasing strongly with depth. This indicates that the microbial activity was mainly occurring in the surface layer of this hydrothermal vent. Furthermore, rRNA concentrations were higher in regions closer to the vent, suggesting that the hydrothermal activity stimulated microbial activity. Archaea seemed to be a minor component of the microbial community at this vent site, even in the zones with higher temperatures. Bacteria made up at least 78% (mean 95%) of the prokaryotic rRNA. However, along the steepest temperature gradient, the proportion of archaeal rRNA increased. Nevertheless, even in the hottest sediment layer where a quantification was possible (in situ temperature 82 °C) archaeal rRNA made up only 11·9% of the prokaryotic rRNA. This suggests that Archaea were generally of minor importance at this vent site and were probably restricted to a narrow niche. The factors that allow Bacteria to dominate in a high temperature environment that was once believed to be the realm of Archaea remain elusive.

Keywords: rRNA slot-blot hybridization, Archaea and Bacteria, microbial activity, hydrothermal vent, Milos Island (Aegean Sea, Greece)

^a Present address: Technical University Hamburg-Harburg, Technical Microbiology (AB 2-09), Denickestrasse 15, D-21071 Hamburg.

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