In Lake Matano, Indonesia, the world's largest known ferruginous basin, more than 50% of authigenic organic matter is degraded through methanogenesis, despite high abundances of Fe (hydr)oxides in the lake sediments. Biogenic CH 4 accumulates to high concentrations (up to 1.4 mmol L ) and SO 4 2) in Lake Matano waters suggests that anaerobic methane oxidation may be coupled to the reduction of Fe (and ⁄ or Mn) (hydr)oxides. Thermodynamic considerations reveal that CH 4 oxidation coupled to Fe(III) or Mn(III ⁄ IV) reduction would yield sufficient free energy to support microbial growth at the substrate levels present in Lake Matano. Flux calculations imply that Fe and Mn must be recycled several times directly within the water column to balance the upward flux of CH 4 . 16S gene cloning identified methanogens in the anoxic water column, and these methanogens belong to groups capable of both acetoclastic and hydrogenotrophic methanogenesis. We find that methane is important in C cycling, even in this very Fe-rich environment. Such Fe-rich environments are rare on Earth today, but they are analogous to conditions in the ferruginous oceans thought to prevail during much of the Archean Eon. By analogy, methanogens and methanotrophs could have formed an important part of the Archean Ocean ecosystem.

Abstract. This study explores Mn biogeochemistry in a stratified, ferruginous lake, a modern analogue to ferruginous oceans. Intense Mn cycling occurs in the chemocline where Mn is recycled at least 15 times before sedimentation. The product of biologically catalyzed Mn oxidation in Lake Matano is birnessite. Although there is evidence for abiotic Mn reduction with Fe(II), Mn reduction likely occurs through a variety of pathways. The flux of Fe(II) is insufficient to balance the reduction of Mn at 125 m depth in the water column, and Mn reduction could be a significant contributor to CH 4 oxidation. By combining results from synchrotron-based X-ray fluorescence and X-ray spectroscopy, extractions of sinking particles, and reaction transport modeling, we find the kinetics of Mn reduction in the lake's reducing waters are sufficiently rapid to preclude the deposition of Mn oxides from the water column to the sediments underlying ferruginous water. This has strong implications for the interpretation of the sedimentary Mn record.

Previous investigations have shown that bacteria can derive benefits from elements within mineral assemblages associated with/or proximal to their point of attachment; however to date, no study has shown a direct connection between mineral dissolution and the biological oxidation of the greenhouse gas methane (CH 4 ). Here we investigate linkages between the Cu content of silicate glasses (Si-glasses) and CH 4 oxidation rates in Methylosinus trichosporiumOB3b, a type II methanotroph. A series of batch growth experiments were performed under varying solid phase Cu (0, 80, 200, 400, and 800 ppm Cu) and initial aqueous phase methane concentrations (40, 70, and 130 nM CH 4 ). Under near equilibrium conditions, the release of Cu from the glasses increased in association with M. trichosporium OB3b activity. The acquisition of Cu from the glasses directly influenced CH 4 oxidation rates of M. trichosporium OB3b, presumably by promoting the expression and activity of copper-containing particulate methane monooxygenase (pMMO), the most efficient MMO for methane oxidation produced. Highest CH 4 oxidation rates were observed in glasses with 80 ppm Cu. However, rates generally displayed an inverted-U relationship with solid-phase Cu concentration; i.e., rates were lower with solid phase Cu concentrations < 80 ppm or > 200 ppm. We conclude that in situ methanotroph activity in soils and sediments is strongly influenced by ambient solid-phase Cu concentrations, which has important implications to interpreting greenhouse gas flux data in geologic settings.

This study evaluates rates and pathways of methane (CH4) oxidation and uptake using 14C‐based tracer experiments throughout the oxic and anoxic waters of ferruginous Lake Matano. Methane oxidation rates in Lake Matano are moderate (0.36 nmol L−1 day−1 to 117 μmol L−1 day−1) compared to other lakes, but are sufficiently high to preclude strong CH4 fluxes to the atmosphere. In addition to aerobic CH4 oxidation, which takes place in Lake Matano's oxic mixolimnion, we also detected CH4 oxidation in Lake Matano's anoxic ferruginous waters. Here, CH4 oxidation proceeds in the apparent absence of oxygen (O2) and instead appears to be coupled to some as yet uncertain combination of nitrate (NO3−), nitrite (NO2−), iron (Fe) or manganese (Mn), or sulfate (SO42−) reduction. Throughout the lake, the fraction of CH4 carbon that is assimilated vs. oxidized to carbon dioxide (CO2) is high (up to 93%), indicating extensive CH4 conversion to biomass and underscoring the importance of CH4 as a carbon and energy source in Lake Matano and potentially other ferruginous or low productivity environments.

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