Dissimilatory phosphite oxidation (DPO), a microbial metabolism by which phosphite (HPO) is oxidized to phosphate (PO), 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 CO reduction. We identified an uncultivated DPO bacterium, Phosphitivorax ( P.) anaerolimi strain Phox-21, that belongs to candidate order GW-28 within the , which has no known cultured isolates. Genes for phosphite oxidation and for CO reduction to formate were found in the genome of 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 P. anaerolimi whereby DPO drives CO reduction to formate, which is then assimilated into biomass via the reductive glycine pathway.

b Two (per)chlorate-reducing bacteria, strains CUZ and NSS, were isolated from marine sediments in Berkeley and San Diego, CA, respectively. Strain CUZ respired both perchlorate and chlorate [collectively designated (per)chlorate], while strain NSS respired only chlorate. Phylogenetic analysis classified both strains as close relatives of the gammaproteobacterium Sedimenticola selenatireducens. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) preparations showed the presence of rod-shaped, motile cells containing one polar flagellum. Optimum growth for strain CUZ was observed at 25 to 30°C, pH 7, and 4% NaCl, while strain NSS grew optimally at 37 to 42°C, pH 7.5 to 8, and 1.5 to 2.5% NaCl. Both strains oxidized hydrogen, sulfide, various organic acids, and aromatics, such as benzoate and phenylacetate, as electron donors coupled to oxygen, nitrate, and (per)chlorate or chlorate as electron acceptors. The draft genome of strain CUZ carried the requisite (per)chlorate reduction island (PRI) for (per)chlorate respiration, while that of strain NSS carried the composite chlorate reduction transposon responsible for chlorate metabolism. The PRI of strain CUZ encoded a perchlorate reductase (Pcr), which reduced both perchlorate and chlorate, while the genome of strain NSS included a gene for a distinct chlorate reductase (Clr) that reduced only chlorate. When both (per)chlorate and nitrate were present, (per)chlorate was preferentially utilized if the inoculum was pregrown on (per)chlorate. Historically, (per)chlorate-reducing bacteria (PRB) and chlorate-reducing bacteria (CRB) have been isolated primarily from freshwater, mesophilic environments. This study describes the isolation and characterization of two highly related marine halophiles, one a PRB and the other a CRB, and thus broadens the known phylogenetic and physiological diversity of these unusual metabolisms. P erchlorate (ClO 4Ϫ ) and chlorate (ClO 3 Ϫ ), collectively designated (per)chlorate, are oxyanions of chlorine that have both natural and anthropogenic sources (1-4). Perchlorate is used extensively in rocket fuel, pyrotechnics, lubricants, and paints (1, 5), while chlorate is used as an herbicide, a defoliant, and a bleaching agent in the paper industry (6, 7). Contamination of groundwater with perchlorate due to human activity is a major environmental concern, since perchlorate inhibits the uptake of iodine by the thyroid gland and may lead to hypothyroidism (8, 9). Chlorate exposure has been correlated with hemolytic anemia as well as with reproductive toxicity (10). The ability of microorganisms to respire (per)chlorate has been exploited recently as an effective method of bioremediation (11-13).(Per)chlorate-reducing bacteria (PRB) reduce (per)chlorate with the perchlorate reductase enzyme (Pcr), while chlorate-reducing bacteria (CRB) reduce only chlorate, using a distinct chlorate reductase enzyme (Clr) (11,(14)(15)(16). The product of both enzymatic reactions, chlorite (ClO 2 Ϫ ), is detoxified in both PRB and CRB i...