Thiocapsa, Lutimaribacter, and Delftia Are Major Bacterial Taxa Facilitating the Coupling of Sulfur Oxidation and Nutrient Recycling in the Sulfide-Rich Isinuka Spring in South Africa

Sulfur cycling is a fundamental biogeochemical process, yet its microbial underpinnings in environments like the Isinuka sulfur pool remain poorly understood. Using high-throughput Illumina 16S rRNA sequencing and PICRUSt-based functional inference, we analyzed bacterial diversity and metabolic potential in sediment and water samples. Sediments, characterized by high sulfide/sulfate/thiosulfate, salinity, alkalinity, and organic matter content under anoxic conditions, supported diverse sulfur-reducing and organic-degrading bacteria, primarily from the Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria phyla. In contrast, the anoxic water column harbored a less diverse community dominated by α-, γ-, and β-Proteobacteria, including <i>Thiocapsa</i> and <i>Lutimaribacter</i>. Sulfur oxidation genes (<i>soxABCXYZ</i>, <i>sqr</i>) were abundant in water, while sulfate reduction genes (<i>dsrAB</i>, <i>aprAB</i>, and <i>sat/met3</i>) were concentrated in sediments. Core microbiome analysis identified <i>Thiocapsa</i>, <i>Lutimaribacter</i>, and <i>Delftia</i> as functional keystones, integrating sulfur oxidation and nutrient recycling. Sediments supported dissimilatory sulfate-reducing bacteria (unclassified Desulfobacteraceae, <i>Desulfosarcina</i>, <i>Desulfococcus</i>, <i>Desulfotignum</i>, and <i>Desulfobacter</i>), while water samples were enriched in sulfur-oxidizing bacteria like <i>Thiocapsa</i>. Metabolic profiling revealed extensive sulfur, nitrogen, and carbon cycling pathways, with sulfur autotrophic denitrification and anoxygenic photosynthesis coupling sulfur-nitrogen and sulfur-carbon cycles. This study provides key theoretical insights into the microbial dynamics in sulfur-rich environments, highlighting their roles in biogeochemical cycling and potential applications in environmental management.