Impacts and mechanisms of oyster farming on denitrification and anammox in coastal sediments from Tieshan Bay, China.

Oyster aquaculture is widely recognized as an effective way to mitigate coastal eutrophication, but critical knowledge gaps remain regarding how it modulates nitrogen removal processes, which requires further investigation. In this study, sediments from oyster farming (OY) and non-oyster farming (NOY) areas in Tieshan Bay, China, were analyzed to explore the impacts of oyster farming on denitrification and anammox in coastal sediments. OY sediments exhibited significantly higher sediment oxygen demand (SOD), total organic carbon (TOC), ammonium (NH4+), ferrous iron (Fe2+), and sulfide than NOY sediments (P < 0.05), collectively creating hypoxic conditions favorable for nitrogen removal. Denitrification dominated nitrogen removal in both areas (OY: 91.84%; NOY: 90.19%). Potential denitrification and anammox rates in OY areas were 2.25 and 1.80 times higher than those in NOY areas, respectively (P < 0.01). Redundancy analysis (RDA) identified TOC, SOD, sulfide, Fe2+, Fe3+, and NH4+ as key drivers of these processes. Metagenomic analysis revealed that OY sediments were enriched in Bacillota and Bacteroidota at the phylum level, and key functional genes (nirS, nosZ, hzsB) were significantly more abundant in OY areas (P < 0.05), suggesting enhanced genetic potential for nitrogen removal. The stronger promotion of denitrification over anammox was attributed to the combined effects of TOC (electron donors), Fe2+ (electron carrier), and sulfide (alternative electron donor for denitrification but inhibitor for anammox). These findings demonstrate that oyster aquaculture enhances the functional capacity for sedimentary nitrogen removal through physicochemical modification, microbial community selection, and enrichment of key functional genes.