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-A2T and FCG-A23T, 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-A2T within the genus Pseudothioclava and strain FCG-A23T within the genus Terasakiella. 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-A2T grew at 15–35 °C (optimum 25–30 °C), at pH 5.0–10.0 (optimum pH 8.0), and with 1–4% (w/v) NaCl concentrations (optimum 3%). Strain FCG-A23T grew at 20–33 °C (optimum 25–30 °C), at pH 6.0–9.0 (optimum, pH 8.0), and with 2–6% (w/v) NaCl (optimum 2%). For both strains, ubiquinone-10 was the major respiratory quinone, and the predominant fatty acids were summed feature 3 (C16:1ω7c and/or C16:1ω6c) and summed feature 8 (C18:1ω7c and/or C18:1ω6c); strain FCG-A2T additionally contained C16:0 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-A2T than in FCG-A23T, implying a wider ecological distribution and greater opportunities for FCG-A2T to perform sulfur oxidation across habitats. The draft genomes had G + C contents of 62.09% (FCG-A2T) and 49.06% (FCG-A23T). Based on these results, we propose Pseudothioclava alba sp. nov. (type strain FCG-A2T = MCCC 1K08969T = KCTC 8462T) and Terasakiella sediminum sp. nov. (type strain FCG-A23T = MCCC 1K08972T = KCTC 8464T).