Novel insights into the coupling mechanisms underlying carbon nitrogen sulfur cycling at an estuarine scale

As hot spots of biogeochemical cycling, yet the coupling mechanism between biogeochemical processes remains poorly understood in estuarine ecosystems. Here, we investigated the microbial-mediated carbon (C), nitrogen (N), and sulfur (S) cycling genes/ pathways and their potential coupling mechanisms in the sediments of a subtropical estuary through metagenomic sequencing technology. Results revealed that the metabolic potential for anaerobic and aerobic carbon fixation was higher in the wet season, while nitrate reduction + nitrite oxidation exhibited lower levels. Notably, assimilatory sulfate reduction surpassed sulfur mineralization in the dry season, with the reverse observed in the wet season. And it manifested that OM, T and N-NH4+ were driving factors for microbiomes related to C, N, S cycling. The coupling between nitrate reduction with S oxidation was identified, offering potential applications for mitigating nitrogen pollution and greenhouse gas emissions in the PRE. Furthermore, the cooperative interactions between carbon decomposition and N cycling processes {nitrate reduction (driven by Herbaspirillum) and denitrification (Sphingomonas)} were evident, with the coupling between denitrification, sulfur oxidation, and carbon fixation (Sulfuricauli) observed in the dry and/ or wet seasons. Remarkably, denitrification and S oxidation were tightly coupled by S-driven denitrifiers (Sulfuricaulis) in the dry season. Our study presents novel evidence for the coupling of biogeochemical processes within estuarine ecosystems.