Metabolic adaptation of a methanogen to a ferruginous environment associated with intracellular Fe- and S-rich...

Across the great oxidation event (GOE) period, the oceans experienced a major transition from ferruginous (Fe²⁺-rich) to localized euxinic (HS^--rich) conditions. In ferruginous oceans, sulfur is mostly sequestered into metastable aqueous Fe-S clusters (FeS_aq) and sulfide minerals like mackinawite (FeS_Mack). Similarly, in euxinic oceans, iron is sequestered in such clusters and/or sulfide minerals. However, it is unclear how ancient methanogens, with high demands for sulfur and iron, adapted to this critical transition. We hypothesize that FeS_aq and FeS_Mack could serve as sources of sulfur and iron to support cellular growth and methanogenesis. A model ancient methanogen Methanococcus maripaludis S2 (MmS2) was inoculated in a growth medium under either Fe²⁺-rich or HS^--rich condition. Biological utilization of FeS_Mack was imaged using transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS). Transcriptomic and proteomic analyses were performed to characterize the cellular and biochemical responses of MmS2 under contrasting Fe-S conditions. The results revealed that MmS2 maintained a consistent methanogenic activity and growth irrespective of different Fe-S conditions. Under high-Fe²⁺ but limited HS^- conditions, Fe- and S-rich particles accumulated inside cells, indicating utilization of FeS_aq and/or FeS_Mack as sulfur sources. Intracellular Fe- and S-rich particles accumulation was accompanied by distinct transcriptomic and proteomic responses involving cellular metabolism and energy conservation. These findings provide new insights into the adaptive mechanisms of methanogens in response to evolving iron and sulfur environments during early earth, shedding light on the co-evolution of life and environment over geological history.