Evidence of methane cycling beneath the western margin of the Greenland Ice Sheet

Abstract

The greenhouse gas methane mainly regulated by microbial processes. Methanogenic archaea, a major biological source of methane, are a diverse group of organisms that produce methane as a metabolic byproduct. The generation of methane takes place in diverse anoxic environments using CO2 and other reduced substrates. The release of methane into oxic environments often is regulated by methanotrophic Proteobacteria and Archaea, specialized groups of bacteria that use methane as sole sources of carbon and energy. These organisms can play major roles in the regulation of methane transport into the atmosphere and make up one part of the methane cycle. Pools of are methane thought to be sequestered from interacting with the global methane cycle may exist beneath the world’s glaciers and ice sheets. These environments are cold, dark, and anoxic. Thereby these environments would be suitable for the growth and metabolism of methanogenic bacteria. Subglacial environments harbor active microbial ecosystems that potentially impact biogeochemical cycling and contribute significantly to global iron, sulfur, and carbon cycles. Recent molecular and biogeochemical evidence has suggested that subglacial environments are sources of the greenhouse gas methane. Here we present molecular and geochemical evidence of microbially derived subglacial methane being released at sites of subglacial discharge from a glacial terminus near Kangerlussuaq, Greenland. Dissolved methane in subglacial discharge was measured in samples collected during the summer of 2012 and ranged from 0.97 to 85 µM; δ13CH4 values for the methane indicated a biological origin. Duplicate samples that were not killed at time of collection showed depleted methane concentrations and heavy enrichment in δ13CH4 after four months of 4°C incubation. Molecular evidence, in the form of reverse transcribed pmoA mRNA and 16S rRNA, for active methanotrophs and recently active methanogens was found in these samples. Furthermore, a return visit to the subglacial site the following year provided additional evidence of an active methanotrophic community based on time series incubations (methane oxidation rates of 0.31 μM day-1). These data provide the first evidence suggesting that active microbial oxidation of subglacial methane is occurring at the western margin of the Greenland Ice Sheet.