Tree phyllospheres are a habitat for diverse populations of CO-oxidizing bacteria

Carbon monoxide (CO) is both a ubiquitous atmospheric trace gas and an air pollutant. While aerobic CO-degrading microorganisms in soils and oceans are estimated to remove ~370 Tg of CO per year, the presence of CO-degrading microorganisms in above-ground habitats, such as the phyllosphere, and their potential role in CO cycling remains unknown. CO-degradation by leaf washes of two common British trees, Ilex aquifolium and Crataegus monogyna, demonstrated CO uptake in all samples investigated. Based on the analyses of taxonomic and functional genes, diverse communities of candidate CO-oxidizing taxa were identified, including members of Rhizobiales and Burkholderiales which were abundant in the phyllosphere at the time of sampling. Based on predicted genomes of phyllosphere community members, an estimated 21% of phyllosphere bacteria contained CoxL, the large subunit of CO-dehydrogenase. In support of this, data mining of publicly available phyllosphere metagenomes for genes encoding CO-dehydrogenase subunits demonstrated that, on average, 25% of phyllosphere bacteria contained CO-dehydrogenase gene homologues. A CO-oxidizing Phyllobacteriaceae strain was also isolated from phyllosphere samples which contains genes encoding both CO-dehydrogenase as well as a ribulose-1,5-bisphosphate carboxylase-oxygenase. These results suggest that the phyllosphere supports diverse and potentially abundant CO-oxidizing bacteria, which are a potential sink for atmospheric CO.