Do tree microbiomes clean up urban air pollution?

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Overview Carbon monoxide (CO) is a ubiquitous trace gas in the atmosphere that is produced by natural processes. It is also a significant anthropogenic air pollutant produced by combustion processes. As a toxic gas, it contributes to the high burden of premature mortality and morbidity caused by air pollution. Microorganisms play a crucial role in the removal of CO from the atmosphere and have previously been identified as abundant members of soil microbial communities (Cordero et al., 2019, King and Weber, 2007). Surprisingly, we recently found that CO-degrading bacteria are also found in the phyllosphere of trees (Palmer et al., 2021), which is defined as the above ground parts of trees and constitutes a huge microbial habitat. We showed that leaf washings from two common UK tree species, hawthorn and holly, had the potential to degrade CO, indicating the presence of CO-degrading microorganisms. We identified a wide diversity of candidate CO-oxidising bacteria in the phyllosphere community using cultivation independent approaches of microbial community analysis using 16S rRNA as a taxonomic marker and the gene encoding the large subunit of the carbon monoxide dehydrogenase (CODH, encoded by coxL) as a functional marker. Data mining of environmental phyllosphere metagenomes suggests that the bacteria containing genes for CO oxidation could constitute up to 25% of the phyllosphere microbial community. These findings suggest that phyllosphere microbiota with the potential to degrade CO could drive a significant and previously unrealised sink in the global CO cycle (Palmer et al., 2021). This project will build on those findings and aim to characterise CO degrading microorganisms more widely in above ground habitats including trees and mosses. You will assess the abundance and activity of CO-degrading bacteria associated in these habitats, test the potential for CO degradation and assess whether CO-degrading bacteria are active in situ. This will provide fundamental new insights into the distribution of CO-degrading bacteria in above ground habitats and advance our understanding of the CO biogeochemical cycle. It may also lay the foundation for a mechanistic understanding of how phyllosphere microbiota may affect air quality, how air quality affects microbial activity, and it will potentially provide a basis for informed selection of tree species that could maximise microbial ecosystem services for mitigation of air pollution in urban areas.