Abstract: The objective of this study was to investigate the activity of denitrifying microorganisms and the factors influencing their community composition in lagoons along the coast of South China, with the aim of elucidating the key mechanisms underlying the nitrogen cycle in lagoon ecosystems. Three representative lagoons in South China were selected for this research. By measuring the physicochemical parameters of water and sediment, combined with denitrification rates and metagenomic sequencing data, we analyzed the community composition of three denitrifying functional genes (nirK, nirS, nosZ) and their correlations with environmental factors. The results indicated that the denitrification rate in South China lagoons was significantly below the average for marine sediments, primarily due to low nutrient concentration, high water transparency, and limited sediment water exchange capacity. Notably, the abundance and diversity of nirK genes within the three denitrification functional gene communities were significantly lower than those of nirS and nosZ, suggesting that nirK is more susceptible to environmental variations. Significant differences were observed in the community composition of nirK, nirS, and nosZ among the different lagoons. At the order level, the dominant species were Nitrosopumilales, Oceanospirillales, and Flavobacteriales, respectively, with the main community composition closely resembling that of denitrifying microorganisms in the marine environment. Water transparency, pH, and NH4+ concentration emerged as the key environmental factors regulating the composition of the three denitrification functional gene communities. Specifically, the nirS community was more influenced by sediment physicochemical parameters, while the nirK community was more affected by water physicochemical parameters. Additionally, the nirS and nosZ communities were similarly impacted by the same environmental factors, indicating a stronger synergistic effect during the denitrification process. This study provides valuable data to enhance our understanding of the denitrification function and microbial community composition in lagoon ecosystems, thereby laying a foundation for the protection and management of these ecosystems.

Key words: Lagoon, Denitrification, Community characteristics, Water transparency, Nutrient concentration.