ANAEROBIC OXIDATION OF METHANE COUPLED TO METAL REDUCTION IN AN ARCHAEAN OCEAN ANALOGUE

Methane is an important greenhouse gas and methane-based microbial metabolisms likely evolved billions of years ago. Prior to the Great Oxidation Event, the major sink for methane is postulated to have been microbial anaerobic oxidation of methane (AOM). In modern marine sediments, AOM is generally coupled to reduction of sulfate, an electron acceptor that was extremely scarce in the ancient ocean. While microorganisms that couple AOM to reduction of nitrate, nitrite and sulfate have been characterized, and geochemical data suggests that AOM coupled to Fe(III) and Mn(IV) reduction (Fe- and Mn-AOM) is thermodynamically favorable and occurring in diverse marine and freshwater environments, the organisms mediating Fe- and Mn-AOM remain unknown. Lake Matano, Indonesia is an ideal ecosystem to enrich Fe- and Mn-AOM microbes because its anoxic ferruginous deep waters and sediments contain abundant Fe(III), Mn(IV) and methane, and extremely low sulfate and nitrate. Our research aims to isolate and characterize the microbes mediating Fe- and Mn-AOM from Lake Matano sediments through serial enrichment cultures in minimal media lacking nitrate and sulfate. 16S amplicon sequencing of sediment inoculum revealed the presence of the Fe(III)-reducing bacterium Geobacter as well as a number of Euryarchaeota implicated in AOM including ANME-1 and 2d and methanogenic Methanosarcinales. After 90 days of primary enrichment, all three sediment layers showed high levels of Fe(III) reduction (60-90 uM Fe(II) d^-1) in the presence of methane compared to no methane and heat killed controls. Secondary enrichments showed highest Mn(IV) reduction (40-90 uM Mn(IV) d^-1) in the presence of methane and anthraquinone-2,6-disulfonate (AQDS), a humic acid analogue and electron shuttle. Comparison of the phylogenetic composition of microbial communities in inoculum and Fe- and Mn-AOM enrichment and quantification of AOM rates are underway.