The Selenoproteome as a dynamic response mechanism to oxidative stress in methanogenic communities

Methanogenesis is a critical process in the carbon cycle that is applied industrially in anaerobic digestion and biogas production. While naturally occurring in diverse environments, methanogenesis requires anaerobic and reduced conditions though varying degrees of oxygen tolerance have been described. Micro-aeration is touted as the next step to increase methane production and improve hydrolysis in digestion processes; therefore, a deeper understanding of the methanogenic response to oxygen stress is needed. In order to explore the drivers of oxygen tolerance in methanogenesis, two parallel enrichments were performed in an environment without reducing agents and in a redox-controlled environment by adding redox mediator AQDS. The cellular response to oxidative conditions is mapped using proteomic analysis. The resulting community showed remarkable tolerance to high-redox environments and was unperturbed in its methane production. Next to expressing pathways to mitigate reactive oxygen species, the higher redox potential environment showed an increased presence of selenocysteine and selenium-associated pathways. By including sulfur-to-selenium mass shifts in a proteomic database search, we provide the first evidence of dynamic and large-scale incorporation of selenocysteine as a response to oxidative stress by methanogenic organisms and the presence of a dynamic selenoproteome.