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© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Research on Kilauea and O‐yama Volcanoes has shown that microbial communities and their activities undergo major shifts in response to plant colonization and that molyb-denum‐dependent CO oxidizers (Mo‐COX) and their activities vary with vegetation and deposit age. Results reported here reveal that anaerobic CO oxidation attributed to nickel‐dependent CO oxidizers (Ni‐COX) also occurs in volcanic deposits that encompass different developmental stages. Ni‐COX at three distinct sites responded rapidly to anoxia and oxidized CO from initial concentrations of about 10 ppm to sub‐atmospheric levels. CO was also actively consumed at initial 25% concentrations and 25 °C, and during incubations at 60 °C; however, uptake under the latter conditions was largely confined to an 800‐year‐old forested site. Analyses of microbial communities based on 16S rRNA gene sequences in treatments with and without 25% CO incubated at 25 °C or 60 °C revealed distinct responses to temperature and CO among the sites and evidence for enrichment of known and potentially novel Ni‐COX. The results collectively show that CO uptake by volcanic deposits occurs under a wide range of conditions; that CO oxidizers in volcanic deposits may be more diverse than previously imagined; and that Ni‐dependent CO oxidizers might play previously unsuspected roles in microbial succession.

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