Decoding microbial-mediated sulfur transformation pathways in mangrove wetland: Metagenomic and hydrogeochemical...

Sulfur (S) cycling is essential to the ecological function of mangrove wetlands, but how microbial processes and gene-level patterns respond to environmental gradients remains poorly understood. Here, we integrated high-resolution hydrogeochemical profiling with metagenomic sequencing to characterize depth-resolved microbial communities and S-cycling genes in the mangrove wetlands of Dongzhai Harbor, Hainan, China. The results revealed pronounced differences in microbial community composition between zones, with Escherichia dominating mangrove sediments (4.22-20.07 %) and Salmonella prevailing in mudflat sediments (23.87-60.98 %). The abundance of S-cycling genes (e.g., tusA, soeA, aprA, dsrAB, sat) declined markedly with depth. Spatial variation in biogeochemical conditions shaped functional gene distributions: oxidative genes (aprA, soeA) were more abundant in mudflat profiles, whereas sat dominated reductive pathways in mangrove sediments. Environmental gradients structured microbial communities, with salinity, pH, total nitrogen (TN), and total organic carbon (TOC) showing negative correlations, and total sulfur (TS), total phosphorus (TP), SO₄^2- acting as positive drivers. Co-occurrence network analysis indicated tighter microbial associations in surface layers compared to deeper strata. The thiosulfate oxidation pathway was confined to the 5-10 cm interval in mudflat sediments and appeared at both 5-10 cm and 15-20 cm in mangrove sediments, while direct sulfite oxidation occurred in both zones. Moreover, methanogenesis, nitrification, and denitrification were more prominent in mudflat sediments, whereas methane oxidation prevailed in mangrove profiles. These findings advance our understanding of how microbial functional stratification and S metabolic pathways respond to environmental gradients, with implications for biogeochemical coupling in coastal wetland ecosystems.