Summary: Sulfurimonas species (class Campylobacteria , phylum Campylobacterota) were globally distributed and especially predominant in deep‐sea hydrothermal environments. They were previously identified as chemolithoautotrophic sulfur‐oxidizing bacteria (SOB), whereas little is known about their potential in sulfur reduction. In this report, we found that the elemental sulfur reduction is quite common in different species of genus Sulfurimonas . To gain insights into the sulfur reduction mechanism, growth tests, morphology observation, as well as genomic and transcriptomic analyses were performed on a deep‐sea hydrothermal vent bacterium Sulfurimonas sp. NW10. Scanning electron micrographs and dialysis tubing tests confirmed that elemental sulfur reduction occurred without direct contact of cells with sulfur particles while direct access strongly promoted bacterial growth. Furthermore, we demonstrated that most species of Sulfurimonas probably employ both periplasmic and cytoplasmic polysulfide reductases, encoded by genes psrA ₁ B ₁ CDE and psrA ₂ B ₂ , respectively, to accomplish cyclooctasulfur reduction. This is the first report showing two different sulfur reduction pathways coupled to different energy conservations could coexist in one sulfur‐reducing microorganism, and demonstrates that most bacteria of Sulfurimonas could employ both periplasmic and cytoplasmic polysulfide reductases to perform cyclooctasulfur reduction. The capability of sulfur reduction coupling with hydrogen oxidation may partially explain the prevalenceof Sulfurimonas in deep‐sea hydrothermal vent environments.

Chemolithoautotrophic Campylobacterota are widespread and predominant in worldwide hydrothermal vents, and they are key players in the turnover of zero-valence sulfur. However, at present, the mechanism of cyclooctasulfur activation and catabolism in Campylobacterota bacteria is not clearly understood. Here, we investigated these processes in a hydrothermal vent isolate named Sulfurovum indicum ST-419. A transcriptome analysis revealed that multiple genes related to biofilm formation were highly expressed during both sulfur oxidation and reduction. Additionally, biofilms containing cells and EPS coated on sulfur particles were observed by SEM, suggesting that biofilm formation may be involved in S⁰ activation in Sulfurovum species. Meanwhile, several genes encoding the outer membrane proteins of OprD family were also highly expressed, and among them, gene IMZ28_RS00565 exhibited significantly high expressions by 2.53- and 7.63-fold changes under both conditions, respectively, which may play a role in sulfur uptake. However, other mechanisms could be involved in sulfur activation and uptake, as experiments with dialysis bags showed that direct contact between cells and sulfur particles was not mandatory for sulfur reduction activity, whereas cell growth via sulfur oxidation did require direct contact. This indirect reaction could be ascribed to the role of H₂S and/or other thiol-containing compounds, such as cysteine and GSH, which could be produced in the culture medium during sulfur reduction. In the periplasm, the sulfur-oxidation-multienzyme complexes soxABXY₁Z₁ and soxCDY₂Z₂ are likely responsible for thiosulfate oxidation and S⁰ oxidation, respectively. In addition, among the four psr gene clusters encoding polysulfide reductases, only psrA₃B₃C₃ was significantly upregulated under the sulfur reduction condition, implying its essential role in sulfur reduction. These results expand our understanding of the interactions of Campylobacterota with the zero-valence sulfur and their adaptability to deep-sea hydrothermal environments.

Two novel marine hydrogen- and sulfur-oxidizing bacteria, designated HSL1-7^T and HSL3-1^T, were isolated from mangrove sediments from Fujian Province, China. Strain HSL1-7^T exhibited Gram-negative, rod-shaped to slightly curved morphology with polar flagellum-driven motility, whereas strain HSL3-1^T was Gram-negative, rod-shaped and non-motile. Strain HSL1-7^T and HSL3-1^T were obligate chemolithoautotrophs, capable of using molecular hydrogen and thiosulfate as an energy source, and molecular oxygen and elemental sulfur as the electron acceptors for growth. Cellular fatty acid profiles revealed similar predominant components (C_16:1ω7c, C_16:0, C_18:1ω7c, and C_14:0) in both strains. Strains HSL1-7^T and HSL3-1^T were strongly diazotrophic, as demonstrated by ¹⁵N₂ fixation when a fixed nitrogen source was absent from the growth medium. The DNA G+C contents of strains HSL1-7^T and HSL3-1^T were determined to be 36.1% and 57.3%, respectively. Based on the 16S rRNA gene sequences, strains HSL1-7^T and HSL3-1^T exhibited the highest sequence similarities with Sulfurimonas marina B2^T (98.5% and 94.45%, respectively). Notably, the 16S rRNA gene sequence similarity between strains HSL1-7^T and HSL3-1^T was 93.19%, indicating that they represent distinct species within the genus Sulfurimonas. Comparative genomic analyses revealed the presence of diverse metabolic profiles in strains HSL1-7^T and HSL3-1^T, including carbon fixation, hydrogen oxidation, sulfur oxidation, and nitrogen fixation. The combined phenotypic, chemotaxonomic, and phylogenetic evidence, including average nucleotide identity and in silico DNA-DNA hybridization values, shows that strains HSL1-7^T and HSL3-1^T represent two novel species of the genus Sulfurimonas for which the names Sulfurimonas microaerophilic sp. nov. and Sulfurimonas diazotrophicus sp. nov. are proposed, with the type strains HSL1-7^T (=MCCC 1A18899^T = KCTC 25640^T) and HSL3-1^T (=MCCC 1A18844^T), respectively.

The escalating global demand for large-scale, cost-effective, and sustainable high-quality protein has positioned single-cell protein (SCP) production from one-carbon (C1) gases as a highly promising solution. In this study, eight chemolithoautotrophic hydrogen-oxidizing bacteria (HOB) were isolated from mangrove sediments. Based on the 16S rRNA gene sequence analysis, they belonged to genera Sulfurimonas, Sulfurovum, Thiomicrolovo, and Marinobacterium. Among these, Thiomicrolovo sp. ZZH C-3 was identified as the most promising candidate for SCP production based on the highest biomass and protein content, and was selected for further characterization. Strain ZZH C-3 is a Gram-negative, short rod-shaped bacterium with multiple flagella. It can grow chemolithoautotrophically by using molecular hydrogen as an energy source and molecular oxygen as an electron acceptor. Genomic analysis further confirmed that ZZH C-3 harbors a complete reverse tricarboxylic acid (rTCA) cycle gene set for carbon fixation, and diverse hydrogenases (Group I, II, IV) for hydrogen oxidation. Subsequently, its cultivation conditions and medium composition for SCP production were systematically optimized using single-factor experiments and response surface methodology (RSM). Results showed that the optimal growth conditions were 28 °C, pH 7.0, and with 1 g/L (NH₄)₂SO₄ as the nitrogen source, 5–10% oxygen concentration, 9.70 mg/L FeSO₄·7H₂O, 0.17 g/L CaCl₂·2H₂O, and 1.90 mg/L MnSO₄·H₂O. Under the optimized conditions, strain ZZH C-3 achieved a maximum specific growth rate of 0.46 h⁻¹. After 28 h of cultivation, the optical density at 600 nm (OD₆₀₀) reached 0.94, corresponding to a biomass concentration of 0.60 g/L, and the protein content ranked at 73.56%. The biomass yield on hydrogen (Y_H2) was approximately 3.01 g/g H₂, with an average H₂-to-CO₂ consumption molar ratio of about 3.78. Compared to the model HOB Cupriavidus necator, strain ZZH C-3 exhibited a lower H₂/CO₂ consumption ratio, superior substrate conversion efficiency, and high protein content. Overall, this study not only validated the potential of mangrove HOB for SCP production but also offers new insights for future metabolic engineering strategies designed to enhance CO₂-to-biomass conversion efficiency.

Chemolithoautotrophic members from the Campylobacteria class are dominant key players in sulfidic habitats, where they make up a stable portion of sulfide-oxidizing bacterial communities. Nevertheless, few isolates have so far been cultivated and studied in situ , and most are derived from chemosynthetic ecosystems, limiting our understanding of their physiological and metabolic features as well as ecological roles in the global marine environments. In this study, seven potentially new species were successfully isolated from mangrove sediments and further diverged into three potentially new genera within the class Campylobacteria . These isolates were obligate chemolithoautotrophs that could grow through hydrogen oxidation as well as sulfur oxidation, reduction, and disproportionation. Metabolic reconstructions revealed that these isolates contained diverse sulfide:quinone oxidoreductase and flavocytochrome c sulfide dehydrogenase for sulfide oxidation, distinct Sox gene cluster for sulfur oxidation, as well as group I, II, and IV hydrogenases for hydrogen consumption and production. Notably, these strains lacked the complete denitrification pathway, instead having all genes for nitrogen fixation, which might facilitate their survival in the nitrogen-limited mangrove sediments. Moreover, they also demonstrated the ability to adapt to low O ₂ conditions, such as a more efficient 2-oxoglutarate:ferredoxin oxidoreductase complex for CO ₂ fixation and diverse terminal oxidases including Cco, Cox, and Cyd. Metatranscriptomic analysis further confirmed their activity and different adaptation mechanisms in in situ mangrove sediments. Assessing their occurrences indicated that these lineages were globally distributed in hypoxic and anoxic environments and dominant members of marine and mangrove sediments. Overall, these results indicate that these new Campylobacteria members are metabolically versatile and play an underappreciated role in the biogeochemical cycling of carbon-rich mangrove sediments. IMPORTANCE: Chemolithoautotrophic Campylobacteria spp. are generally associated with sulfide-rich environments, where they play a key role in the cycling of carbon, nitrogen, and sulfur. Yet, only a limited number of cultured isolates are currently available. In this study, we isolated seven potentially new species belonging to three new genera from mangrove sediments, which significantly expanded our understanding of the species diversity within the class Campylobacteria . These isolates demonstrated diverse and unique metabolic potentials for CO ₂ fixation, sulfur oxidation, hydrogen oxidation, nitrogen metabolism, and oxygen respiration, making them well adapted to the sulfur-rich, nitrogen-limited, and low-oxygen habitats they inhabit. The frequent detection of these novel species in marine and mangrove sediments, as revealed by 16S rRNA gene sequences in public databases, indicates a potential preference for oxygen-limited environments. Overall, this study promotes our understanding of the in situ function and ecological role of Campylobacteria , especially in previously overlooked carbon-rich sediment ecosystems.