Background. Life on early Earth was microbial and confined to iron-rich oceans, yet heavily influenced the world around it. Primary producers fixed carbon through anoxygenic photosynthesis. Other microbes produced gases, such as methane, which modulated Precambrian climate. All these biological processes still occur today, but in niche environments that are reminiscent of conditions on Earth billions of years ago.

Aims. The aims of this dissertation were to investigate microbes that influence iron and carbon cycling under ferruginous conditions. Specifically: (1) to characterize two lakes as geochemical analogs for early Earth oceans; (2) to explore factors that impact methane production, oxidation, and pathways of emission to the atmosphere from ferruginous water columns; and (3) to explore the capacity for Fe(II)-dependent anoxygenic photosynthesis.

Methods. These studies were carried out utilizing limnological techniques to document biogeochemical and physical trends. 16S rRNA amplicon sequencing was used to assess the abundance of depth-related microbes, augmented by culturing and physiological experiments.

Results. Below the chemocline, both Brownie Lake and Canyon Lake are anoxic and rich in ferrous iron which allows these lakes to be suitable geochemical analogs for early Earth oceans. A large reservoir of methane is actively being oxidized microbially within the water columns, yet a large methane flux still reaches the atmosphere. Non-diffusional transport of methane (e.g. ebullition) from Brownie Lake sediments and lateral transport of methane into pelagic waters from littoral sediments at Canyon Lake are the major methane emission pathways. A metabolically diverse photoferrotroph, present in Brownie Lake, was isolated, and utilizes bacteriochlorophyll c and chlorobactene to harvest light for photosynthesis.

Conclusions. Brownie Lake and Canyon Lake can be added to the catalog of ferruginous ocean analogs for exploring questions pertaining to early Earth. Non-diffusive pathways of methane emission can dominate from shallow, ferruginous water columns and these pathways should be considered when modeling early Earth methane fluxes. A 16S rRNA phylogenetic analysis of photoferrotroph strain BLA1 reveals that it is most genetically similar to other Fe(II)-oxidizing green sulfur bacteria and is given the name Chlorobium sp. strain BLA1. This isolate oxidizes iron with rates similar to other photoferrotrophs.