Thiosulfate Reduction Coupled with Anaerobic Ammonium Oxidation by Ralstonia sp. GX3-BWBA

Thiosulfate reduction coupled with anaerobic ammonium oxidation (Sammox) has been recently reported as an ancient metabolic pathway. Therefore, the Sammox microbes are well worth discovering. In this study, we isolated and identified a mixtrophic Sammox bacterium, Ralstonia sp. GX3-BWBA. Nitrite was an intermediate product of both thiosulfate- and sulfate-fueled Sammox metabolism, while nitrogen gas was the end product. Ralstonia sp. GX3-BWBA prefers thiosulfate to sulfate as an electron acceptor for coupling anaerobic ammonium oxidation. Genomic analysis of Ralstonia sp. GX3-BWBA implied that this ancient metabolism in modern microbes might contain two stages according to ammonium transformation, oxidation of ammonium to nitrite and denitrification. The incomplete reductive tricarboxylic acid cycle and reductive acetyl-CoA pathway were all identified Ralstonia sp. GX3-BWBA metabolic networks, which were responsible for chemolithotrophic metabolism. Ralstonia sp. GX3-BWBA contains no ATP sulfurylase (Sat)-coding gene, suggesting that sulfate reduction in sulfate-fueled Sammox is different from dissimilatory sulfate reduction. Ferredoxin oxidoreductase or other iron–sulfur enzymes encoded by genes with unknown sequences may be involved in thiosulfate reduction in the Sammox process. Our findings regarding the Sammox microbes provide microbiological evidence for this unique feature of life.