BIOMASS DEGRADATION UNDER FERRUGINOUS CONDITIONS

Photosynthetic iron oxidizers (photoferrotrophs) likely dominated the early Earth’s iron-rich oceans where they would have fuelled the early biosphere by providing energy to drive microbial growth and evolution. Photoferrotrophic activity, however, is rare in modern environments, leaving models for early life and ecology untested. Modern analogues like the stratified, iron-rich (ferruginous) Malili lake system, Indonesia and of Kabuno Bay, Democratic Republic of Congo are revealing the ecological interactions that underpin biogeochemical cycling in such environments. Current studies indicate that photoferrotrophs act as primary producers in these lakes, supplying carbon to heterotrophic microbial communities including iron reducers and methanogens, while producing Fe(III) as a by-product of their growth. Notably, some of the biomass produced through photoferrotrophy in these environments ends up fuelling methanogenesis in lieu of Fe(III) reduction. We have therefore conducted a suite of laboratory experiments to determine how biomass generated through photoferrotrophy is ultimately broken down into substrates that fuel microbial Fe(III) reduction and methanogenesis. Our overall objective is to assess how organic matter hydrolysis and fermentation might control the channelling of reductants sourced from photosynthetic biomass between Fe(III) reduction and methanogenesis. This new information on the biological controls on methane production, iron reduction, and the microbial interactions responsible for both metabolisms will help constrain models of banded iron formation deposition and the composition and evolution of Earth’s early atmosphere.