Tianxiuia mixotrophica gen. nov., sp. nov., an anaerobic, facultative mixotrophic, iron(III)-reducing bacterium isolated from the Tianxiu hydrothermal field and the proposal of Fusibacteraceae fam. nov.

An anaerobic, salt-tolerant, rod-shaped bacterium, strain JL216-2^T, was isolated from the shell of the hydrothermal gastropod <i>Alviniconcha marisindica</i> in the Tianxiu hydrothermal field on the Carlsberg Ridge, Indian Ocean. Cells of strain JL216-2^T were motile and 0.5×3-6 µm in size. Strain JL216-2^T grew at 10-32 °C (optimal 28 °C), pH 6.0-9.0 (optimal pH 7.0) and 0.5-12.0% (w/v) sodium chloride(NaCl, optimal 2.5%). Strain JL216-2^T exhibited metabolic versatility, being capable of heterotrophic growth on complex organic compounds. It also performed autotrophic growth by utilizing carbon dioxide (CO₂) fixation or sodium bicarbonate (NaHCO₃) as the sole carbon source, driven by hydrogen oxidation coupled with Fe(III) reduction. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the strain was most closely related to <i>Helicovermis profundi</i> S502^T (90.43% sequence similarity). The genome size of strain JL216-2^T was 4.91 Mb, with a G+C content of 39.54 mol%. Phylogenomic analysis and overall genome relatedness indices values indicated a close relatedness of strain JL216-2^T to <i>H. profundi</i> S502^T and <i>Fusibacter</i> spp. The combined genotypic and phenotypic data showed that strain JL216-2^T represents a novel genus within the order <i>Eubacteriales</i>. The type strain is JL216-2^T (=MCCC M30262^T=KACC 23553^T).