Summary auto-generated
Researchers isolated eleven novel bacteria from hypersaline lakes and salt marshes that perform an unusual metabolic process: oxidizing sulfur compounds while simultaneously reducing nitrate to nitrogen gas at high salt concentrations (up to 2 molar NaCl). These Gammaproteobacteria were cultured anaerobically using thiosulfate as their energy source and nitrate as their final electron acceptor, with sulfate as the end product. Ten isolates of rod-shaped bacteria form a single new species, Thiohalomonas denitrificans, while a single vibrio-shaped isolate represents a second species, Thiohalomonas nitratireducens. Both species are obligately chemolithoautotrophic, meaning they derive energy and carbon solely from inorganic sources. The bacteria thrive at neutral pH (7.3–7.8) and moderate halophilicity. Phylogenetic analysis based on 16S rRNA genes and nitrite reductase genes (nirS) confirmed they represent a distinct lineage within Gammaproteobacteria. This discovery demonstrates that autotrophic thiodenitrification—the coupling of sulfur oxidation to nitrate reduction—occurs naturally in hypersaline environments and provides insights into microbial adaptation to extreme salt concentrations.
Key findings
- Two new bacterial species, Thiohalomonas denitrificans and Thiohalomonas nitratireducens, catalyze thiodenitrification (sulfur oxidation coupled to nitrate reduction) at salt concentrations up to 2 M NaCl
- The bacteria are obligately chemolithoautotrophic and reduce nitrate completely to N2 gas, with nitrite and N2O as intermediates
- T. denitrificans strains have DNA G+C content of 58–60 mol% and show 97% 16S rRNA sequence identity with each other but form a separate lineage from known Gammaproteobacteria
- Both species carry cytochrome cd1-type nitrite reductase genes (nirS) essential for denitrification
- These bacteria were isolated from sediments of multiple hypersaline lakes across Siberia, Mongolia, Crimea, and a Mediterranean salt marsh, indicating widespread distribution in extreme environments
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Abstract
A novel group of moderately halophilic, obligately chemolithoautotrophic, sulfur-oxidizing Gammaproteobacteria was found in sediments of various inland hypersaline lakes and a solar saltern. These bacteria were enriched and isolated with thiosulfate as electron donor and nitrate as electron acceptor at 2 M NaCl. Ten isolates (HLD strains) were long non-motile rods. They grew anaerobically as complete denitrifiers, and aerobically under micro-oxic conditions. Sulfate was the final product of thiosulfate and sulfide oxidation, and nitrite and N2O were intermediates of nitrate reduction to N2. The HLD strains grew optimally at pH 7.37.8, and at NaCl concentrations of 1.52.0 M. On the basis of phenotypic and genetic analysis, the moderately halophilic, thiodenitrifying isolates are proposed to be assigned to a new genus and species, Thiohalomonas denitrificans gen. nov., sp. nov. The type strain is HLD 2T (=DSM 15841T=UNIQEM U222T ). A single strain, HRhD 3spT, with vibrio-shaped cells, was obtained from a co-culture capable of complete denitrification of nitrate in the presence of either thiocyanate or thiosulfate as electron donor. It grew anaerobically with thiosulfate, reducing nitrate to nitrite, or under micro-oxic conditions at 1.02.5 M NaCl with an optimum at 1.0 M. Strain HRhD 3spT was genetically related to the HLD strains at the level of a separate species and is described as Thiohalomonas nitratireducens sp. nov. The type strain is HRhD 3spT (=DSM 16925T=UNIQEM U248T).