Metagenomics-guided analysis of microbial chemolithoautotrophic phosphite oxidation yields evidence of a seventh natural CO ₂ fixation pathway

Dissimilatory phosphite oxidation (DPO), a microbial metabolism by which phosphite (HPO₃^2-) is oxidized to phosphate (PO₄^3-), is the most energetically favorable chemotrophic electron-donating process known. Only one DPO organism has been described to date, and little is known about the environmental relevance of this metabolism. In this study, we used 16S rRNA gene community analysis and genome-resolved metagenomics to characterize anaerobic wastewater treatment sludge enrichments performing DPO coupled to CO₂ reduction. We identified an uncultivated DPO bacterium, <i>Candidatus</i> Phosphitivorax (<i>Ca.</i> P.) anaerolimi strain Phox-21, that belongs to candidate order GW-28 within the <i>Deltaproteobacteria</i>, which has no known cultured isolates. Genes for phosphite oxidation and for CO₂ reduction to formate were found in the genome of <i>Ca.</i> P. anaerolimi, but it appears to lack any of the known natural carbon fixation pathways. These observations led us to propose a metabolic model for autotrophic growth by <i>Ca.</i> P. anaerolimi whereby DPO drives CO₂ reduction to formate, which is then assimilated into biomass via the reductive glycine pathway.