The phylumCampylobacterota, notably represented by the generaSulfurimonasandSulfurovum, is ubiquitous and predominant in deep-sea hydrothermal systems. It is well-known to be the major chemolithoautotrophic sulfur-oxidizing group in these habitats.

Background: Mangroves are hotspots of carbon sequestration in transitional zones between marine and terrestrial ecosystems. Microbially driven dark carbon fixation (DCF) is prominent in sediments, yet our understanding of the DCF process across this continuum remains limited. In this study, we explored DCF activities and associated chemoautotrophs along the sediment depth of different mangrove sites in Fujian Province, China, using radiocarbon labeling and molecular techniques. Results: Our results showed that the DCF rates ranged from 0.02 to 3.27 mmol C m⁻² day⁻¹ in all samples, showing a depth-dependent spatial variation. These rates of DCF were closely related to the environmental factors such as DIC, TS, AVS, NH₄ ⁺, NO₃ ⁻, and NO₂ ⁻. Metagenomic analysis revealed six carbon-fixing pathways, with the Calvin-Benson-Bassham (CBB) cycle and Wood-Ljungdahl (WL) pathway being predominant. Further analysis of MAGs revealed that Gammaproteobacteria, Desulfobacteria, and Campylobacteria were the most abundant carbon-fixing groups. Intriguingly, some new lineages were found to have carbon-fixing potential, including two candidatus taxa JAJVIF01 and BMS3Abin14. Metatranscriptomic analyses confirmed that these carbon-fixing microbes were active in situ and occupied different niches. In the surface layers, Gammaproteobacteria with the CBB cycle played an important role in DCF, mainly driven by sulfur and hydrogen oxidation with oxygen reduction; in the deeper layers, Campylobacteria with the reductive tricarboxylic acid (rTCA) cycle and Desulfobacteria with the WL pathway were active members for DCF, mainly through sulfur, hydrogen, and CO oxidation. While in the deepest layers of 18–20 cm, methane-producing archaea Methanosarcinia was the essential member driving DCF. In addition, most taxa containing the WL pathway displayed a mixotrophic lifestyle corresponding to flexible carbon acquisition strategies. Conclusions: Overall, this study provides new insights into the understanding of biological carbon fixation and its ecological functions in mangrove sediments.

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.

Bacteria of the genus Sulfurovum within the class Campylobacteria are widespread in global oceans and are predominant in sulfide-rich environments. However, little is known about their adaptation to such harsh environments owing to their resistance to cultivation. In this study, we obtained three pure cultures of this genus from marine coastal environments and compared them with those obtained from the deep sea. Phylogenetic analysis of 16S rRNA gene sequences indicated that they represent two novel species of the genus, sharing 95.9%–96.1% sequence similarities to Sulfurovum aggregans Monchim33^T. Based on the polyphasic classification results, the type strains XTW-4^T and zt1-1^T were proposed to represent two new species: Sulfurovum xiamenensis sp. nov. and Sulfurovum zhangzhouensis sp. nov., respectively. These coastal isolates were also obligate chemoautotrophs featuring molecular hydrogen as an electron donor and molecular oxygen, thiosulfate, or elemental sulfur as the sole electron acceptor. Comparative genomic analyses based on 11 Sulfurovum species further revealed a clear differentiation between hydrothermal vent and non-vent origins. The non-vent Sulfurovum can use thiosulfate as an electron acceptor but lacks denitrification pathways, whereas the vent bacteria can respire nitrate through complete denitrification pathways. Moreover, the non-vent Sulfurovum contained a nitrogen fixation pathway, implying their adaptation to nitrogen source-deficit niches. In addition, non-vent Sulfurovum species adapted to a higher oxygen concentration via multiple antioxidative defense mechanisms. These phenotypic and genotypic features help us to understand the ecological role of Sulfurovum bacteria in marine ecosystems.

Two novel Gram-strain-negative and rod-shaped bacteria, designated strain G1T and G2T, were isolated from sediment samples collected from the coast of Xiamen, PR China. The cells were motile by a single polar flagellum. Growth of strain G1T occurred at 10–40 °C (optimum, 30 °C), at pH 6.0–9.0 (optimum, pH 7.5) and with 5–1530 mM NaCl (optimum, 510 mM), while the temperature, pH and NaCl concentration ranges for G2T were 4–45 °C (optimum, 28 °C), pH 5.5–8.0 (optimum, pH 6.5) and 85–1530 mM NaCl (optimum, 340 mM). The two isolates were obligate chemolithoautotrophs capable of using thiosulfate, sulfide, elemental sulphur or tetrathionate as an energy source. Strain G1T used molecular oxygen or nitrite as an electron acceptor, while strain G2T used molecular oxygen as the sole electron acceptor. The dominant fatty acids of G1T and G2T were summed feature 3 (C16:1 ω7c and/or C16:1 ω6c), C16 : 0 and summed feature 8 (C18:1 ω7c and/or C18:1 ω6c). The DNA G+C content of G1T and G2T were 45.1 and 48.3 mol%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain G1T and G2T were members of the genus Thiomicrorhabdus , and most closely related to Thiomicrorhabdus hydrogeniphila MAS2T (96.0 %) and Thiomicrorhabdus indica 13-15AT (95.4 %), respectively. The 16S rRNA gene sequence similarity between strains G1T and G2T was 95.8 %. Based on the phylogenetic, genomic and phenotypic data presented here, the isolate strains represent novel species of the genus Thiomicrorhabdus , for which the names Thiomicrorhabdus sediminis sp. nov. (type strain G1T=MCCC 1A14511T=KCTC 15841T) and Thiomicrorhabdus xiamenensis sp. nov. (type strain G2T=MCCC 1A14512T=KCTC 15842T) are proposed.