Antarctic subglacial lakes are largely unexplored frontiers that play host to dynamic microbial ecosystems. Among the organisms present are several that either produce or consume CH4 as an important energy vector. New research explores subglacial biogeochemical cycling by analysing CH4 isotopologues and bacterial genes and species from Subglacial Lake Whillans (SLW), which sits beneath the West Antarctic Ice Sheet (WAIS). A team led by Alexander Michaud reports its findings in Nature Geoscience, revealing that a microbial community beneath SLW acts as a CH4 sink that might mitigate large-scale release of this greenhouse gas as ice sheets retreat.

Mapping the biogeochemical CH4 cycle required Michaud and colleagues to probe below SLW. “We had never drilled through 800 m of ice before,” stresses Priscu, the Chief Scientist for SLW sampling efforts. “Once we had determined that there was life, it was time to address how microbial processes beneath large ice sheets participate in biogeochemical cycles,” adds Michaud. In the anoxic sediments beneath SLW, they found that hydrogenotrophic methanogenesis — the reduction of CO2 to CH4 using H2 — is prevalent. Sediment 39 cm below the lake contains significantly more CH4 (300 μM) than does the lake itself (0.024 μM), such that 6.8 ± 1.8 mmol CH4 m−2 yr−1 pass upward through the sediments to the lake water. The authors measured the relative isotopic abundances in SLW sedimentary CH4, and the natural variations provided a ‘fingerprint’ that revealed clues regarding its origin. The CH4 concentrations, as well as δ13C–CH4 and δ2H–CH4 values (the extent to which the quotients 13C/12C and 2H/1H in CH4, respectively, depart from standard values), provide evidence for hydrogenotrophic methanogenesis. The resulting CH4 diffuses upwards and is oxidized by bacteria at the sediment/water interface.

Credit: Rachael Tremlett/Macmillan Publishers Limited

it was time to address how microbial processes beneath large ice sheets participate in biogeochemical cycles

Michaud and colleagues calculate that >99% of sub-ice-sheet CH4 is oxidized at the sediment/water interface, a finding supported by a large positive shift in δ13C–CH4. Their results challenge earlier models that emphasized the significance of a possible subglacial CH4 flux into the atmosphere. The authors attribute CH4 consumption to oxidation by aerobic methanotrophs in surficial sediment, with O2 sourced from basal melting of the WAIS. These active SLW methanotrophs may be a globally important CH4 sink; conversion of sedimentary CH4 efflux into CO2 and biomass may act as a ‘buffer’ and lessen the warming potential of subglacial gases that escape as ice sheets retreat.

Having established the flow of CH4, the team sought to find out which organisms were trading this currency. “Molecular microbiological data complement the geochemical analyses,” says Michaud. “Together, these methods allow us to say that methane is being consumed by known methane-oxidizing bacteria.” pmoA genes, which encode particulate methane monooxygenases (enzymes that enable methanotrophy in the O2-rich SLW regions), were found in the SLW sediment. Chemical affinity calculations also implicate aerobic CH4 oxidation as the most energetically favourable process for sustaining microorganisms in SLW surficial sediment (whereas the oxidation of both FeS2 and NH4+ is more favorable in the SLW water column). The pmoA sequences and community analysis of 16S rRNA genes indicated that SLW, particularly in surficial sediments, is home to Methylobacter tundripaludum and other methanotrophs. M. tundripaludum is an aerobic cold-adapted bacterium that oxidizes CH4 to eventually produce CO2 and biomass, with the product ratio being dependent on O2 supply and energy requirements.

Given the role of CH4 in climate forcing, understanding its sources, sinks and feedbacks is paramount. Priscu is now leading a second project (SALSA) — one that will involve drilling beneath Subglacial Lake Mercer, a deeper and larger system that may reveal more about biogeochemical cycling beneath the Antarctic ice sheet. “I was not surprised that we found life in Subglacial Lake Whillans. I was surprised, however, by the diversity of life we found and the biogeochemical transformations associated with this life,” says Priscu.

WEB SITES

Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project: http://www.wissard.org/

Subglacial Antarctic Lakes Scientific Access (SALSA) project: https://salsa-antarctica.org/