AN ANALOG TO ARCHEAN CYANOBACTERIAL CONTINENTAL COMMUNITIES IN MODERN LAKE FRYXELL, ANTARCTICA (Invited Presentation)

Modern continental environments host exceptionally diverse microbial ecosystems, ranging from nearly single species biofilms to heterogeneous communities, both shaped by complex chemical and physical interactions. Large variations in environmental conditions (relative to marine environments) over small spatial scales promote community diversity. This variability also allows modern microbial communities to have a significant impact on their local environment, providing small-scale analogs of how major evolutionary changes may have affected the geochemistry of ancient environments. For example, photosynthetic O₂ production can change the geochemistry of a still pond with a small water volume more easily than the same O₂ production in an agitated marine environment. Thus, continental systems of any age can provide insights into details of biogeochemical cycling that are difficult to record in marine environments.

The earliest production of O₂ by cyanobacteria is a particularly interesting question to address in continental settings. There is no clear record of Archean O₂ production in marine microbial mats, with the possible exception of fossil bubbles. However, local O₂ production and accumulation in continental settings may have induced oxidative weathering, affecting elemental fluxes to the oceans and thus measurable characteristics of ancient marine rocks. Modeling such processes requires understanding the complicated biogeochemical feedbacks that lead to local continental O₂ accumulation and mineral oxidation. Oxygen accumulates under anoxic, mildly sulfidic (<100 µM) water in modern Lake Fryxell, Antarctica, due to cyanobacterial photosynthesis. The seasonal “oxygen oasis” in Lake Fryxell contains enough O₂ (≤ 50 µM) to weather pyrite. The biogeochemical dynamics of Fryxell mats demonstrate that the feedbacks among biological and geochemical interactions are complex, leading to communities with a significantly different structure than those in more oxic environments. Specifically, without environmental O₂ to structure the communities, light and local heterogeneities shape the distributions of organisms. These mats require a new suite of biogeochemical models and developing such models will provide new insights into the ecology of the earliest O₂-producing communities.