Antarctic microbial mats: A modern analog for Archean lacustrine oxygen oases

Abstract

The evolution of oxygenic photosynthesis was the most important geochemical event in
\nEarth history, causing the Great Oxidation Event (GOE) ~2.4 b.y. ago. However, evidence
\nis mixed as to whether O2 production occurred locally as much as 2.8 b.y. ago, creating O2
\noases, or initiated just prior to the GOE. The biogeochemical dynamics of possible O2 oases
\nhave been poorly constrained due to the absence of modern analogs. However, cyanobacteria
\nin microbial mats in a perennially anoxic region of Lake Fryxell, Antarctica, create a 1–2
\nmm O2-containing layer in the upper mat during summer, providing the first known modern
\nanalog for formation of benthic O2 oases. In Lake Fryxell, benthic cyanobacteria are present
\nbelow the oxycline in the lake. Mat photosynthesis rates were slow due to low photon flux rate
\n(1–2 μmol m-2 s-1) under thick ice cover, but photosynthetic O2 production was sufficient to
\nsustain up to 50 μmol O2 L-1, sandwiched between anoxic overlying water and anoxic sediments.
\nWe hypothesize that Archean cyanobacteria could have similarly created O2 oases in
\nbenthic mats prior to the GOE. Analogous mats may have been at least partly responsible
\nfor geological evidence of oxidative weathering prior to the GOE, and habitats such as Lake
\nFryxell provide natural laboratories where the impact of benthic O2 oases on biogeochemical
\nsignatures can be investigated.