The temperature ranges of growth of archaea are strongly correlated with the guanine-plus-cytosine (G+C) contents of their 16S rRNA sequences (P GC ). In order to estimate minimum (T min ), optimal (T opt ), and maximum (T max ) growth temperatures of uncultured archaea based on P GC , the 16S rRNA gene sequences of 207 archaeal species were collected from public databases, and their T min , T opt and T max were extracted from description papers and reviews. These values of growth temperatures were plotted againstP GC , and then the regression lines for estimating T min , T opt and T max were calculated. We PCR-amplified the archaeal 16S rRNA gene fragments from the hot water samples, cloned the fragments, and determined the sequences. Growth temperatures of environmental archaea were inferred from G+C content of the 16S rRNA gene sequences by the regression lines. In the terrestrial hot springs (74 • C and 85 • C), both estimated growth temperatures of archaea were higher than in situ temperatures of hot spring waters. Even from tepid hydrothermal fluid (40 • C) we obtained a significant number of archaeal genes indicating high growth temperatures. These results suggested that hot subsurface environments exist under those hydrothermal and geothermal regions. In this study, growth temperatures of uncultured archaea and in situ subsurface temperatures were roughly inferred from 16S rRNA gene sequences of archaea that were transported from the subsurface biosphere. This new method based on microbial molecular information may be applicable to temperature estimation of subsurface environments for We are grateful to the captains and crews of the R/V Yokosuka and Natsushima and to the operation teams of the Shinkai 6500 and Hyper-Dolphin for helping us to collect the deep-sea hydrothermal fluid samples. We thank Dr. Julia Maresca for comments on the manuscript.which it has been difficult to measure the actual temperature with appropriate instrumentation.

We studied planktonic bacterial population dynamics in response to the changing environment in a coastal system during an observation period of over 5 years using fluorescence in situ hybridization. To estimate the environmental constraint on the bacterial community, we focused on temperature, salinity, abundance of photoplankton (chlorophyll a), and dissolved organic carbon (DOC). The total number of bacteria (TDC) amounted to 3.0×10 5 to 5.0×10 6 cells mL −1 , with 1.0×10 5 to 1.0×10 6 cells mL −1 for Bacteria, accounting for 11.8 to 74.8% of TDC, and 1.0×10 4 to 1.0×10 5 cells mL −1 for Gammaproteobacteria, 1.0 to 20.8% of TDC. The abundance of Archaea, which contributed from 0.1 to 12% to TDC, ranged from 2.0×10 3 to 3.0×10 4 cells mL −1 . We found a positive relationship between environmental parameters such as temperature, salinity, chlorophyll a, and DOC and the abundance of total bacteria and Bacteria. The number of Gammaproteobacteria correlated with temperature, salinity, and chlorophyll a, but not with DOC. We suggest that increasing the temperature under eutrophic conditions will lead to high bacterial abundance and probably a change in the bacterial community.

A transect from the Tomoe River Mouth through Shimizu Port to Suruga Bay, Japan, was examined between 2005 and 2009 to reveal the population dynamics of Crenarchaeota and Euryarchaeota in an estuary environment. Crenarchaeota tended to increase in abundance in waters deeper than 100 m compared with Euryarchaeota, and comprised 11% of total direct counts. Archaeal abundance was highest in the Tomoe River Mouth, with a strong negative correlation between surface euryarchaeal abundance and salinity (P<0.001). The diversity index for the phylotypic archaeal community in the mouth was three times higher than that at sites St1-1m and St1-10m in the estuary, and OTUs represented most of the OTU groups at the sites. Three of the seven total OTUs, which comprised 83.6% of the 140 sequenced clones in the estuary, were related to the OTUs in the mouth with similarities higher than 97%. A significant proportion of the archaeal community appears to be derived from the Tomoe River. The two dominant phylotypes of the archaeal community in Shimizu Port, belonging to MGI and MGII, occurred ubiquitously.