Bacteria and Archaea physically associated with Gulf of Mexico gas hydrates

Although there is significant interest in the potential interactions of mic robes with gas hydrate, no direct physical association between them has bee n demonstrated. We examined several intact samples of naturally occurring g as hydrate from the Gulf of Mexico for evidence of microbes. All samples we re collected from anaerobic hemipelagic mud within the gas hydrate stabilit y zone, at water depths in the ca. 540- to 2,000-m range. The delta C-13 of hydrate-bound methane varied from -45.1 parts per thousand Peedee belemnit e (PDB) to -74.7 parts per thousand PDB, reflecting different gas origins. Stable isotope composition data indicated microbial consumption of methane or propane in some of the samples. Evidence of the presence of microbes was initially determined by 4,6-diamidino 2-phenylindole dihydrochloride (DAPI ) total direct counts of hydrate-associated sediments (mean = 1.5 X 10(9) c ells g(-1)) and gas hydrate (mean = 1.0 X 10(6) cells ml(-1)). Small-subuni t rRNA phylogenetic characterization was performed to assess the compositio n of the microbial community in one gas hydrate sample (AT425) that had no detectable associated sediment and showed evidence of microbial methane con sumption. Bacteria were moderately diverse within AT425 and were dominated by gene sequences related to several groups of Proteobacteria, as well as A ctinobacteria and low-G + C Firmicutes. In contrast, there was low diversit y of Archaea, nearly all of which were related to methanogenic Archaea, wit h the majority specifically related to Methanosaeta spp. The results of thi s study suggest that there is a direct association between microbes and gas hydrate, a finding that may have significance for hydrocarbon flux into th e Gulf of Mexico and for life in extreme environments.