Caldisalinibacter kiritimatiensis gen. nov., sp. nov., a Moderately Thermohalophilic Thiosulfate-Reducing Bacterium from a Hypersaline Microbial Mat

Abstract

A novel anaerobic bacterium, designated L21-TH-D2T was isolated from the anoxic zone of a microbial mat of the hypersaline Lake 21 on the Kiritimati Atoll (Republic of Kiribati, Central Pacific). Cells were Gram-positive, spore-forming and motile slender rods. The novel strain grew anaerobically by fermentation at temperatures between 40 and 65°C (optimum 50–55°C) and at pH between 5–9 (optimum 6.9–7.0). It required at least 20 g L−1 NaCl for growth and tolerated up to 150 g L−1 NaCl (optimum at 50 g L−1). In the presence of yeast extract or peptone the novel strain fermented glucose, fructose, maltose, mannose and pyruvate. The end-products from glucose fermentation were acetate, ethanol, CO2 and H2. Nitrate, nitrite, elemental sulfur, sulfate, and sulfite were not used as electron acceptors, while thiosulfate was reduced to sulfide. The cellular fatty acids pattern was dominated by iso-C15:0 and iso-C15:0 DMA. The major polar lipids were identified as phosphatidylglycerol, phosphatidylethanolamine, an unidentified phospholipid and two different unidentified glycolipids. No respiratory lipoquinones or cytochromes were detected. The DNA G + C content was 30.7 mol%. The determined draft genome sequence of strain L21-TH-D2T revealed a specialization on a saccharolytic fermentative metabolism. Besides enzymes for substrate-level phosphorylation, several membrane-bound enzyme complexes, like an ion-translocating RNF complex and an F-type ATP synthase, were annotated in the genome sequence and seem to be involved in the energy metabolism. Phylogenetic analyses based on a comparison of 16S rRNA gene sequences placed the novel isolate in a clade of anaerobic halophilic or thermohalophilic bacteria within the clostridial subcluster XII. Significant phenotypic and phylogenetic differences prevent inclusion of the novel isolate in existing genera within the family Clostridiaceae, so that the novel taxon Caldisalinibacter kiritimatiensis gen. nov., sp. nov., is proposed. The type strain is L21-TH-D2T (= DSM 26826T = JCM 18664T).