MICROBIAL COMMUNITIES OF TWO ARCHEAN OCEAN ANALOGS

Archean (4.0-2.5 Ga) oceans were anoxic, Fe-rich (ferruginous) and dominated by microbial life. Photoferrotrophs (bacteria that use light and Fe²⁺ to fix CO₂) and methanogens (archaea that utilize organic carbon to create methane) may have had significant impacts on biogeochemical cycling of carbon on early Earth. However, documenting the presence and activity of these microbes in the Archean sedimentary record is challenging due to post-depositional alterations, as well as poor preservation or extraction potential from iron-rich sediments. Modern permanently stratified (meromictic) analogs to Archean oceans are therefore useful to study the impact these microbes have on Fe and C cycles and their potential (bio)signatures on ferruginous systems.

Brownie Lake (MN) and Canyon Lake (MI) are two ferruginous, meromictic lakes in the Midwest, USA. Brownie Lake contains a shallow water column (13 m), which allows for light to penetrate to the chemocline (4.5 m). Large fluxes of methane were observed from Brownie Lake irrespective of the season, with the largest fluxes in the summer (13.98 mmol CH₄ m^-2 hr^-1). In contrast to anthropogenically-influenced Brownie Lake, pristine Canyon Lake is a north-south oriented lake with a maximum depth of 22 meters. Light penetrates to roughly 11 meters, with an extended suboxic zone present (< 32 μM) until the deep 18 m chemocline. However, the methane flux observed at Canyon Lake is low compared to Brownie Lake in the summer (0.04 mmol CH₄ m^-2 hr^-1).

16S rRNA amplicon sequencing of DNA extracted from multiple depths within each lake over various seasons will elucidate the microbial community composition in the oxic surface waters, at the chemocline, and in the anoxic bottom waters. Along with geochemical data, shifts in the microbial community due to seasonal change will be analyzed. We hypothesize that 1) photoferrotroph OTU abundance will be greater at the chemocline in Brownie Lake compared to Canyon Lake and 2) the relative abundance of methanogen OTUs will be greater below the chemocline at Brownie Lake compared to Canyon Lake.