Impact of salinity gradient on sediment microbial communities and the functions of carbon, nitrogen, and sulfur cycling in coastal zone

Coastal sediments, located at the terrestrial–marine interfaces and subject to strong salinity gradients, harbor distinct microbial communities that play indispensable roles in sustaining carbon, nitrogen, and sulfur cycling, yet the responses to salinization remain insufficiently understood. This study aims to characterize microbial community structures within coastal sediments by employing high-throughput sequencing of the 16S rRNA gene. Furthermore, we utilized the FAPROTAX and PICRUSt2 prediction tools to elucidate microbial functions and biogeochemical cycling processes in these sediment ecosystems. The results revealed that increasing salinity significantly increased microbial α-diversity and promoted the expansion of salt-tolerant microbial populations. The salinity-driven increase in microbial network complexity was accompanied by more pronounced competitive interactions. Salinity variations also significantly altered the microbial functions associated with carbon, nitrogen, and sulfur cycling in the sediments. In low-salinity environments, the ability to decompose complex carbon sources was higher, while in high-salinity environments, microbial communities relied more on chitin degradation, starch degradation, and reductive acetogenesis to cope with the high-salinity, low-oxygen conditions. In the nitrogen cycle, mild salinization promoted denitrification, but in the later stages of salinization, high salinity suppressed denitrification, while nitrogen fixation was enhanced in high-salinity environments. The sulfur cycle, both sulfur oxidation and reduction processes were active in medium-salinity sediments, whereas sulfur oxidation was more pronounced and sulfur reduction was diminished in high-salinity sediments, particularly sulfide oxidation and thiosulfate oxidation were enhanced, while dissimilatory sulfate reduction was weakened. This study reveals the effects of salinity gradients on sediment microbial communities and their functions, providing theoretical support for future ecological restoration and environmental management. • Salinity elevation enhances microbial α-diversity and halotolerant taxa dominance. • High-salinity sediments exhibit complex microbial networks with intensified competition. • Low salinity favors complex carbon degradation; high salinity shifts to chitin degradation. • Denitrification peaks at moderate salinity, while nitrogen fixation thrives at high salinity. • Sulfur cycling transitions from dual oxidation-reduction to oxidation dominance with salinity rise.