The possible occurrence of iron-dependent anaerobic methane oxidation in an Archean Ocean analogue

In the ferruginous and anoxic early Earth oceans, photoferrotrophy drove most of the biological production before the advent of oxygenic photosynthesis, but its association with ferric iron (Fe3+) dependent anaerobic methane (CH4) oxidation (AOM) has been poorly investigated. We studied AOM in Kabuno Bay, a modern analogue to the Archean Ocean (anoxic bottom waters and dissolved Fe concentrations > 600 µmol L−1). Aerobic and anaerobic CH4 oxidation rates up to 0.12 ± 0.03 and 51 ± 1 µmol L−1 d−1, respectively, were put in evidence. In the Fe oxidation–reduction zone, we observed high concentration of Bacteriochlorophyll e (biomarker of the anoxygenic photoautotrophs), which co-occurred with the maximum CH4 oxidation peaks, and a high abundance of Candidatus Methanoperedens, which can couple AOM to Fe3+ reduction. In addition, comparison of measured CH4 oxidation rates with electron acceptor fluxes suggest that AOM could mainly rely on Fe3+ produced by photoferrotrophs. Further experiments specifically targeted to investigate the interactions between photoferrotrophs and AOM would be of considerable interest. Indeed, ferric Fe3+-driven AOM has been poorly envisaged as a possible metabolic process in the Archean ocean, but this can potentially change the conceptualization and modelling of metabolic and geochemical processes controlling climate conditions in the Early Earth.

Complete Genome Sequence of the Photoautotrophic and Bacteriochlorophyll e-Synthesizing Green Sulfur Bacterium Chlorobaculum limnaeum DSM 1677T

ABSTRACT Chlorobaculum limnaeum DSM 1677T is a mesophilic, brown-colored, chlorophototrophic green sulfur bacterium that produces bacteriochlorophyll e and the carotenoid isorenieratene as major pigments. This bacterium serves as a model organism in molecular research on photosynthesis, sulfur metabolism, and bacteriochlorophyll biosynthesis. We report here the complete genome sequence.