Abstract

Methane is a product of and substrate for microbial metabolisms in the deep subsurface, but little is known about microbial metabolisms in deep methane hydrate-bearing sediments. We analyzed microbial community diversity and function in subsurface sediments beneath Hydrate Ridge, offshore Oregon (ODP Leg 204 Site 1244). We targeted four geochemically distinct sediment zones: near surface (2 mbsf), sulfate-methane transition zone (4 and 8 mbsf), iron-manganese reduction zone (18 and 20 mbsf) and deep subsurface (35 and 69 mbsf). Reactive iron increased with depth from 2 mbsf (0.4%) to 21 mbsf (1.1%), where dissolved iron and methane concentrations also peaked. The proportion of archaeal sequences decreased with depth, with deeper sediments dominated by Atribacteria and Chloroflexi. There was a resurgence of uncultivated archaeal groups (10% SAGMEG-1 and 16% MBGB) in the iron-manganese reduction zone. Illumina HiSeq metagenomic sequencing of genomic DNA subjected to single cell multiple displacement amplification resulted in 336 million total and 33.7 million coding reads. The taxonomic affiliations of metagenomic sequences corroborated the trend of increasing Atribacteria genes with depth, but a higher percentage of Chloroflexi sequences. ESOM assembly yielded two Atribacteria genomes of ≤5% contaminationfrom 2 vs. 69 mbsf with 55% and 37% completeness, respectively. Genes for amino acid transport and peptide fermentation, as well as Ni-Fe hydrogenases and the Wood-Ljungdahl carbon fixation pathway, were present. This grant entrained two early-career female researchers not previously funded by C-DEBI, and resulted in eight new collaborations and eight research presentations. All data are available at http://www.bco-dmo.org/project/626690.