81-4 Linking benthic microbial mats to exported biomass using depth-dependent genetic and isotopic signatures in ice-covered Lake Fryxell, Antarctica

Abstract:

Perennially ice-covered lakes in the McMurdo Dry Valleys of Antarctica serve as sources of organic matter to surrounding soils, which are among the most carbon-poor ecosystems on Earth. At Lake Fryxell, buoyant mobilization of benthic microbial mats (liftoff) beneath the ice drives biomass export. Bubbles nucleate within mats, enabling mats to float to and freeze into the overlying ice cover. Once within the ice, seasonal cycles of surface ice ablation and basal freezing cause the mats to move vertically through the ice, whence they are exported up to 10 years later.

We have shown that liftoff is driven by photosynthetically active radiation (PAR), which supports oxygen production and bubble formation. Consequently, the extent of the “liftoff zone” fluctuates with changes in PAR. During 2022-2025, we documented a large-scale disturbance event, with liftoff mats observed to 8.5 m depth and a doubling of the surface area supporting mobilization relative to 1980-1981 when lift-off was not observed below 8.0 m depth. To link benthic mats with exported biomass, we characterized microbial mat composition (16S rRNA) and stable isotopic signatures (δ¹³C, δ¹⁵N) along a depth transect and compared them to exported biomass. Principal coordinate analyses reveal distinct microbial communities between shallow (4.3-6.1 m) and deeper (>7.9 m) environments. Isotope signatures show that contemporary exported mats overlap with shallow benthic and moat mats, but not deeper environments.

These results show that contemporary biomass flux from lake to soil is sourced primarily from under-ice habitats shallower than ~6 m and the moat. The deeper mats are as yet unrepresented in exported material, which we infer to reflect the approximately one-decade lag for liftoff to migrate to the ice surface. Years with unusually large liftoff events are thus likely to enhance nutrient flux to surrounding soils, albeit some years later. Interannual variability in mat mobilization therefore represents a key process linking irradiance and organic carbon flux across these polar cryo-ecosystems.

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