Two Novel Thiosulfate-Oxidizing Species from Coastal Sediments Reveal Distinct Ecological Strategies: Pseudothioclava alba sp. nov. and Terasakiella sediminum sp. nov.

Two sulfur-oxidizing bacterial strains, FCG-A2^T and FCG-A23^T, were isolated from coastal sediments collected in Fangchenggang, Guangxi Province, China. Phylogenetic analyses based on 16S rRNA gene and whole-genome sequences placed strain FCG-A2^T within the genus <i>Pseudothioclava</i> and strain FCG-A23^T within the genus <i>Terasakiella</i>. Genomic relatedness (ANI, AAI, dDDH, and POCP) to the closest described taxa was below the accepted species thresholds, demonstrating that both isolates represent novel species. Strain FCG-A2^T grew at 15-35 °C (optimum 25-30 °C), at pH 5.0-10.0 (optimum pH 8.0), and with 1-4% (<i>w</i>/<i>v</i>) NaCl concentrations (optimum 3%). Strain FCG-A23^T grew at 20-33 °C (optimum 25-30 °C), at pH 6.0-9.0 (optimum, pH 8.0), and with 2-6% (<i>w</i>/<i>v</i>) NaCl (optimum 2%). For both strains, ubiquinone-10 was the major respiratory quinone, and the predominant fatty acids were summed feature 3 (C_16:1<i>ω7c</i> and/or C_16:1<i>ω6c</i>) and summed feature 8 (C_18:1<i>ω7c</i> and/or C_18:1<i>ω6c</i>); strain FCG-A2^T additionally contained C_16:0 as a major fatty acid. Both strains oxidized thiosulfate to sulfate, consistent with the presence of genes encoding the Sox system and assimilatory sulfate reduction pathways. Comparative genome annotation further suggested a broader carbohydrate-degradation potential in FCG-A2^T than in FCG-A23^T, implying a wider ecological distribution and greater opportunities for FCG-A2^T to perform sulfur oxidation across habitats. The draft genomes had G + C contents of 62.09% (FCG-A2^T) and 49.06% (FCG-A23^T). Based on these results, we propose <i>Pseudothioclava alba</i> sp. nov. (type strain FCG-A2^T = MCCC 1K08969^T = KCTC 8462^T) and <i>Terasakiella sediminum</i> sp. nov. (type strain FCG-A23^T = MCCC 1K08972^T = KCTC 8464^T).