To directly access the sub-seafloor microbial communities, seafloor drilling has been done in a deep-sea hydrothermal field of the Suiyo Seamount, Izu-Bonin Arc, Western Pacific. In the present study, crustal fluids were collected from the boreholes, and the bacterial and archaeal communities in the fluids were investigated by culture-independent molecular analysis based on 16S rRNA gene sequences. Bottom seawater, sands, rocks, sulfide mound, and chimneys were also collected around the boreholes and analyzed for comparisons. Comprehensive analysis revealed the characteristics of the microbial community composition in the crustal fluids. Phylotypes closely related to cultured species, e.g., Alteromonas, Halomonas, Marinobacter, were relatively abundant in some crustal fluid samples, whereas the phylotypes related to Pelagibacter and the SUP05-group were relatively abundant in the seawater samples. Phylotypes related to other uncultured environmental clones in Alphaproteobacteria and Gammaproteobacteria were relatively abundant in the sand, rock, sulfide mound, and chimney samples. Furthermore, comparative analysis with previous studies of the Suiyo Seamount crustal fluids indicates the change in the microbial community composition for 3 years. Our results provide novel insights into the characteristics of the microbial communities in crustal fluids beneath a deep-sea hydrothermal field.

To investigate the phylogenetic diversity of putative chemolithoautotrophs possessing the RubisCO form II gene (cbbM) in various environments, we designed a new PCR primer set targeting this gene. The primer set was designed to cover more diverse and longer sequences of cbbM genes than those reported previously. We analyzed various samples (i.e., benthic sands, basement rocks, sulfide chimneys, vent fluids and overlying bottom seawater) collected in a deep-sea hydrothermal field of the Suiyo Seamount, Izu-Bonin Arc, Western Pacific, by PCR-based analysis using the designed primer set. Most of the cbbM phylotypes recovered from the liquid samples were related to those of the SUP05 group that belongs to the Gammaproteobacteria and includes putative sulfide-oxidizing chemolithoautotrophs. In contrast, the cbbM phylotypes recovered from the solid samples were related to environmental clones with low similarity (74-90%) and not closely related to the SUP05 group (69-74%). The cbbM phylotypes recovered from the liquid samples were different from those of the solid samples. Furthermore, the cbbM phylotypes recovered from the solid samples were different from each other. Our results expand knowledge of the phylogenetic diversity and distribution of putative chemolithoautotrophs possessing RubisCO form II cbbM genes in deep-sea hydrothermal fields.

No abstract