The abundance, diversity, activity, and composition of microbial communities in sulfide structures both of active and inactive vents were investigated by culture-independent methods. These sulfide structures were collected at four hydrothermal fields, both on-and off-axis of the back-arc spreading center of the Southern Mariana Trough. The microbial abundance and activity in the samples were determined by analyzing total organic content, enzymatic activity, and copy number of the 16S rRNA gene. To assess the diversity and composition of the microbial communities, 16S rRNA gene clone libraries including bacterial and archaeal phylotypes were constructed from the sulfide structures. Despite the differences in the geological settings among the sampling points, phylotypes related to the Epsilonproteobacteria and cultured hyperthermophilic archaea were abundant in the libraries from the samples of active vents. In contrast, the relative abundance of these phylotypes was extremely low in the libraries from the samples of inactive vents. These results suggest that the composition of microbial communities within sulfide structures dramatically changes depending on the degree of hydrothermal activity, which was supported by statistical analyses. Comparative analyses suggest that the abundance, activity and diversity of microbial communities within sulfide structures of inactive vents are likely to be comparable to or higher than those in active vent structures, even though the microbial community composition is different between these two types of vents. The microbial community compositions in the sulfide structures of inactive vents were similar to those in seafloor basaltic rocks rather than those in marine sediments or the sulfide structures of active vents, suggesting that the microbial community compositions on the seafloor may be constrained by the available energy sources. Our findings provide helpful information for understanding the biogeography, biodiversity and microbial ecosystems in marine environments.

Changes in the vertical distribution of dissolved CH, were monitored during the autumnal lake overturn period in mesotrophic Lake Nojiri, Japan (4.4 km* in area and 9.4 X 10' m3 in vol). A survey in 1992 revealed that the surface CH, concentration was highest in December, when the lake overturned. During the following two winters (1993-1994 and 1994-1995) we carried out detailed sampling during the autumnal overturn period. As a result of lake overturn in mid-December, CH, that had accumulated in the hypolimnion during the stratification period mixed rapidly throughout the water column. Increased CH, in the epilimnion quickly disappeared after the overturn as a result of the rising of CH, oxidation activity throughout the water column. The in situ-specific CH, oxidation rate peaked at 0.274, 0.235, and 1.01 d-' at 0.5, 20, and 36 m, respectively, during the overturn, and then declined the following month. During this period, the diffusive flux of methane across the air-water interface increased but was not the dominant sink (avg rate of 4.5 kg lake-' d-l); instead, methane oxidation in the water column was the dominant CH, sink (avg rate of 67.8 kg lake-' d-l), removing -94% of the CH, during the overturn period. A significant methane flux from the bottom sediments throughout the overturn period was confirmed but decreased gradually as the overturn proceeded. The production of organic carbon as a result of CH, oxidation in the water column by methanotrophs was comparable in extent to that generated by primary production at that time.

We sequenced the mitochondrial ND4 gene to elucidate the evolutionary processes of Bathymodiolus mussels and mytilid relatives. Mussels of the subfamily Bathymodiolinae from vents and seeps belonged to 3 groups and mytilid relatives from sunken wood and whale carcasses assumed the outgroup positions to bathymodioline mussels. Shallow water mytilid mussels were positioned more distantly relative to the vent/seep mussels, indicating an evolutionary transition from shallow to deep sea via sunken wood and whale carcasses. Bathymodiolus platifrons is distributed in the seeps and vents, which are approximately 1500 km away. There was no significant genetic differentiation between the populations. There existed high gene flow between B. septemdierum and B. brevior and low but not negligible gene flow between B. marisindicus and B. septemdierum or B. brevior, although their habitats are 5000-10 000 km away. These indicate a high adaptability to the abyssal environments and a high dispersal ability of Bathymodiolus mussels.

SummaryTo extend knowledge of subseafloor microbial communities within the oceanic crust, the abundance, diversity and composition of microbial communities in crustal fluids at back-arc hydrothermal fields of the Southern Mariana Trough (SMT) were investigated using culture-independent molecular techniques based on 16S rRNA gene sequences. Seafloor drilling was carried out at two hydrothermal fields, on-and off-ridge of the back-arc spreading centre of the SMT. 16S rRNA gene clone libraries for bacterial and archaeal communities were constructed from the fluid samples collected from the boreholes. Phylotypes related to Thiomicrospira in the Gammaproteobacteria (putative sulfide-oxidizers) and Mariprofundus in the Zetaproteobacteria (putative iron-oxidizers) were recovered from the fluid samples. A number of unique archaeal phylotypes were also recovered. Fluorescence in situ hybridization (FISH) analysis indicated the presence of active bacterial and archaeal populations in the fluids. The Zetaproteobacteria accounted for up to 32% of the total prokaryotic cell number as shown by FISH analysis using a specific probe designed in this study. Our results lead to the hypothesis that the Zetaproteobacteria play a role in iron oxidation within the oceanic crust.

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