Methanogenesis in subglacial sediments

Abstract

Methanogenic archaea have a unique role in Earth's global carbon cycle as producers of the greenhouse gas methane (CH4 ). However, despite the fact that ice covers 11% of Earth's continental landmass, evidence for methanogenic activity in subglacial environments has yet to be clearly demonstrated. Here we present genetic, biochemical and geochemical evidence indicative of an active population of methanogens associated with subglacial sediments from Robertson Glacier (RG), Canadian Rockies. Porewater CH4 was quantified in two subglacial sediment cores at concentrations of 16 and 29 ppmv. Coenzyme M (CoM), a metabolic biomarker for methanogens, was detected at a concentration of 1.3 nmol g sediment(-1) corresponding to ∼3 × 10(3) active cells g sediment(-1) . Genetic characterization of communities associated with subglacial sediments indicated the presence of several archaeal 16S rRNA and methyl CoM reductase subunit A (mcrA) gene phylotypes, all of which were affiliated with the euryarchaeal order Methanosarcinales. Further, CH4 was produced at 9-51 fmol g dry weight sediment(-1) h(-1) in enrichment cultures of RG sediments incubated at 4°C. Collectively, these findings have important implications for the global carbon cycle in light of recent estimates indicating that the Earth's subglacial biome ranges from 10(4) to 10(6) km(3) sediment.

Ground-Level Concentrations of Atmospheric Methane in Southwest Greenland Evaluated Using Open-Path Laser Spectroscopy and Cavity-Enhanced Absorption Spectroscopy

“…However, our interpretation is supported by recent reports of elevated CH 4 concentrations and methanogenic Archaea in the discharge waters emanating from this same subglacial cave (Dieser et al, 2014). Other studies also report evidence for subglacial methane emissions (Walter Anthony et al, 2012;Boyd et al, 2010;Wadham et al, 2008;Christner et al, 2012). Further study of CH 4 concentrations and stable isotopic analyses of subglacial air are in progress and should yield valuable insight into CH 4 dynamics in the interior and at the base of ice sheets.…”

Microbial communities in the subglacial waters of the Vatnajökull ice cap, Iceland

“…Our failure to detect a significant archaeal population in these lakes stands in contrast to other subglacial environments in which archaea have been detected (Boyd et al, 2010;Stibal et al, 2012), and archaeal-dominated subsurface chemolithotrophic communities (Chapelle et al, 2002). In all, 9 reads out of B382 000 in our bacterial pyrosequence libraries assigned to the Archaea may be a result of nonspecific amplification, and it is possible that archaea are present in a benthic community we have not yet sampled.…”

Chemolithotrophic Primary Production in a Subglacial Ecosystem

“…Consistent with this calculation, numerous heterotrophic archaea, bacteria, and eukarya have been identified in subglacial ecosystems (10,(14)(15)(16)(17) where high concentrations of dissolved organic carbon (DOC) and particulate organic carbon (POC) are also observed (18)(19)(20)(21)(22)(23)(24). Previous studies that examined the composition of DOC pools in meltwaters sampled from the Greenland ice sheet during high discharge (July) revealed signatures corresponding primarily to lignin and relict plant material likely sourced from the flushing of overridden soils (25).…”

“…Using an estimate of 9.2 ϫ 10 7 16S rRNA gene templates/gdws (10) and making the assumptions that (i) proteobacterial cells on average harbor two 16S rRNA gene templates (42) and (ii) 50% of the active community is autotrophic or mixotrophic (an estimate based on the abundance of inferred autotrophic bacteria and autotrophic methanogens [10]), this rate converts to 0.6 Ϯ 0.3 ϫ 10 Ϫ17 mol DIC/cell per day. Intriguingly, this rate is only ϳ1 order of magnitude lower than the rate of DIC assimilation (6.7 Ϯ 3.9 ϫ 10 Ϫ17 mol/cell per day) previously estimated in deep marine sediments (43), which further highlights the similarities between the cold, dark, subsurface ecosystems in subglacial and marine environments with respect to metabolic transformations noted previously (23).…”