Changes in microbial communities associated with gas hydrates in subseafloor sediments from the Nankai Trough

Abstract

Little is known about the microbial distribution patterns in subseafloor sediments. This study examines microbial diversity and activities in sediments of the Nankai Trough, where biogenic gas hydrates are deposited. Illumina sequencing of 16S rRNA genes revealed that the prokaryotic community structure is correlated with hydrate occurrence and depth but not with the sedimentary facies. The bacterial phyla 'Atribacteria' lineage JS1 and Chloroflexi dominated in all samples, whereas lower taxonomic units of Chloroflexi accounted for community variation related to hydrate saturation. In archaeal communities, 'Bathyarchaeota' was significantly abundant in the hydrate-containing samples, whereas Marine Benthic Group-B dominated in the upper sediments without hydrates. mcrA gene sequences assigned to deeply branching groups and ANME-1 were detected only in hydrate-containing samples. A predominance of hydrogenotrophic methanogens, Methanomicrobiales and Methanobacteriales, over acetoclastic methanogens was found throughout the depth. Incubation tests on hydrate-containing samples with a stable isotope tracer showed anaerobic methane oxidation activities under both low- and seawater-like salinity conditions. These results indicate that the distribution patterns of microorganisms involved in carbon cycling changed with gas hydrate occurrence, possibly because of the previous hydrate dissociation followed by pore water salinity decrease in situ, as previously proposed by a geochemical study at the study site.

Cultivation and biogeochemical analyses reveal insights into methanogenesis in deep subseafloor sediment at a biogenic gas hydrate site

“…1H1Hc7 and Methanoculleus sp. 25XMc2, were also found in mcrA clone libraries retrieved from the same DNA samples in this study [24].…”

“…The sequences of hydrogenotrophic Methanocalculus, Methanoculleus and Methanobacterium were also detected in clone libraries of the methyl-coenzyme M reductase (mcrA) genes [24], which were previously analyzed from the same DNA samples used for the analysis in this study. Consistent with this previous mcrA gene analysis [24] and the present radiotracer activity measurements, hydrogenotrophic methanogens dominated throughout the depths (62-97% of the total methanogenic sequences in a sediment sample), followed by methylotrophic methanogens (0-27%) and acetoclastic methanogens (0-14%) (provided that all of the sequences of Methanosarcina are classified into both methylotrophic and acetoclastic groups, and all sequences of Methanofastidiosales are classified into both hydrogenotrophic and methylotrophic groups). Among the methanogenic genera found in this study, only Methanoculleus and Methanosarcina were previously recovered from sediments at hydrate sites in the 16S rRNA gene sequencing analysis [43][44][45].…”

Low Energy Subsurface Environments as Extraterrestrial Analogs

“…In locations with significant organic carbon burial, typically on continental shelves, methane production from terminal electron accepting processes can lead to the formation of large quantities of dissolved and free methane gas. Some of this gas escapes the seafloor, supporting “cold seep” environments where methane oxidation supports chemotrophic communities, with the majority of the gas stored as gas hydrates or clathrate ices ( Whiticar, 1990 ; Clennell et al, 1999 ; Koh, 2002 ; Katayama et al, 2016 ).…”

Acetoclastic Methanosaeta are dominant methanogens in organic-rich Antarctic marine sediments

“…Even though CH 4 oxidizers consume most of this greenhouse gas (Reeburgh, 2007), it is important to understand all major subsurface sources. Sedimentary methanogens include representatives of the Methanosarcinales, Methanomicrobales, Methanococcales, Methanopyrales, Methanobacteriales and Methanomassiliicoccales orders (Sowers and Kastead 1984;Sowers and Ferry, 2003;Ferry and Kastead, 2007;Lang et al, 2015;Katayama et al, 2016). Collectively, these organisms compose a curiously small proportion of sequenced communities observed in CH 4 -rich, subsurface environments (Valentine, 2011).…”