Identification of Unknown Carboxydovore Bacteria Dominant in Deciduous Forest Soil via Succession of Bacterial Communities, coxL Genotypes, and Carbon Monoxide Oxidation Activity in Soil Microcosms.

Surveys of the coxL gene, encoding the large subunit of the CO dehydrogenase, are used as a standard approach in ecological studies of carboxydovore bacteria scavenging atmospheric CO. Recent soil surveys unveiled that the distribution of coxL sequences encompassing the atypical genotype coxL type I group x was correlated to the CO oxidation activity. Based on phylogenetic analysis including the available coxL reference genome sequences, this unusual genotype was assigned to an unknown member of the Deltaproteobacteria, with the coxL sequence from Haliangium ochraceum being the sole and closest reference sequence. Here we seek to challenge the proposed taxonomic assignation of the coxL group x genotype through the monitoring of CO consumption activity and microbial community successions during the colonization of sterile soil microcosms inoculated with indigenous microorganisms. In our study, we established that the estimated population density of Deltaproteobacteria was too small to account for the abundance of the coxL group x genotype detected in soil. Furthermore, we computed a correlation network to relate 16S rRNA gene profiles with the succession of coxL genotypes and CO uptake activity in soil. We found that most of the coxL genotypes for which the colonization profile displayed covariance with CO uptake activity were related to potential carboxydovore bacteria belonging to Actinobacteria and Alphaproteobacteria. Our analysis did not provide any evidence that coxL group x genotypes belonged to Deltaproteobacteria. Considering the colonization profile of CO-oxidizing bacteria and the theoretical energy yield of measured CO oxidation rates in soil microcosms, we propose that unknown carboxydovore bacteria harboring the atypical coxL group x genotype are mixotrophic K-strategists.