Composition of bacterial communities in mangrove sediments at different depths

Mangroves are one of the most productive ecosystems in the intertidal zone and hold vital ecological functions. Using phospholipid fatty acid (PLFA) analysis, we investigated the composition of bacterial communities in mangrove sediments at different depths and their correlations with environmental factors in Mai Po Nature Reserve of Hong Kong. The results showed significant variations in PLFA content across sediment depths, with total PLFA decreasing from 37.25 μg·g^-1 in surface sediments (0-5 cm) to 19.52 μg·g^-1 in deeper layers (25-30 cm). Anaerobic bacteria (12.23-24.15 μg·g^-1) were the dominant microbial group, followed by Gram-negative bacteria (3.19-6.45 μg·g^-1), aerobic bacteria (1.99-4.34 μg·g^-1), and Gram-positive bacteria (1.62-4.47 μg·g^-1), all displaying decreasing trends with increasing sediment depth. Redundancy analysis (RDA) revealed that PLFA composition in mangrove sediments was significantly influenced by polybrominated diphenyl ethers (PBDEs) and basic physicochemical parameters, including total organic matter (TOM), total nitrogen (TN), redox potential (Eh), ferric iron (Fe³⁺), and pH (adjusted R²=0.75). Variance partitioning revealed that basic sediment physicochemical factors explained 26.0% of the variation in PLFA composition, while PBDEs independently explained 6.0%. Additionally, TOM, TN, sulfate (SO₄^2-), Fe³⁺, pH, and PBDEs all showed significant correlations with the abundances of Gram-positive bacteria, Gram-negative bacteria, aerobic bacteria, anaerobic bacteria, and Hydrogenobacter. This study systematically analyzed the vertical distribution patterns of bacteria in mangrove sediments and their influencing factors, providing critical insights for understanding material cycling and energy flow in mangrove ecosystem.