Microbial manganese(II) oxidation: Biogeochemistry of a deep-sea hydrothermal plume, enzymatic mechanism, and genomic perspectives

Manganese (Mn) oxides are highly reactive minerals that play an important role in elemental biogeochemical cycles, controlling the speciation and availability of many metals and organic compounds. Microbes catalyze the transformation of soluble Mn(II) into solid-phase Mn(III/IV) oxides, however the identity of the organisms responsible and mechanism of this biomineralization are unknown. Field work was carried out in deep-sea hydrothermal vent plumes at Guaymas Basin (GB) in the Gulf of California, where previous studies suggested that microbes play a major role in the oxidation precipitation of Mn. Mn(II) oxidation rates were measured using 54Mn as a radioactive tracer and were found to be the fastest ever recorded in a deep-sea hydrothermal plume. This rapid Mn(II) oxidation is catalyzed by microbial enzymes and is specific to the GB plume, suggesting that a distinct community of microbes is present in the plume compared to background deep-seawater. However, molecular methods revealed that there are only subtle differences in the microbial community in the plume versus background deep-seawater, indicating that the unique biogeochemistry of the plume is due to unique activities of plume microbes rather than unique types of plume microbes. Mn(II)-oxidizing isolates were not well represented in plume clone libraries, suggesting that the organisms responsible for Mn(II) oxidation in the plume have not yet been identified. Laboratory investigations of model Mn(II)-oxidizers in the lab further elucidated the mechanism of Mn(II) oxidation. The Mn-oxidizing enzyme from spores of a marine Bacillus species was partially purified and identified by tandem mass spectrometry (MS/MS), providing a conclusive match to the protein MnxG, a multicopper oxidase. These results demonstrate that MnxG directly catalyzes the oxidation of Mn(II) to Mn(IV) oxides, a novel biochemical reaction for a multicopper oxidase. Genome sequence analysis of the Mn(II)-oxidizing alpha-proteobacterium strain SI85-9A1 provided genomic insights into Mn(II) oxidation. The genome encodes metabolic versatility, including pathways for heterotrophy, lithotrophy (on sulfur, methanol, CO), and authotrophy (via the Calvin cycle). Genes encoding the putative Mn(II) oxidase were identified and found to be widespread in completed proteobacterial genomes and in environmental datasets, suggesting that Mn(II) oxidation is more widespread that previously recognized.