From colonization to stability: Shifting roles of cyanobacteria and aerobic anoxygenic phototrophic bacteria in coral island biocrust formation and development

Biological soil crusts (biocrusts) play pivotal roles in global biogeochemical cycles，stabilizing soil, preventing erosion, and enhancing nutrient availability. On coral islands, phototrophic bacteria, especially cyanobacteria and aerobic anoxygenic phototrophic bacteria (AAPB), are vital to biocrust formation, yet their community dynamics remain poorly understood. This study investigated the abundance, diversity, and interactions of these two groups at different developmental stages (2-, 4-, and 6-years) in the early stages of coral island biocrust succession. Using qPCR, high-throughput sequencing, and physicochemical analysis, we tracked the biocrust development stages. The results showed that the coral island biocrust development is accompanied by reduced soil pH and increased levels of total nitrogen, total organic carbon, total phosphorus, and chlorophyll-a content, which indicated that biocrust formation reshaped the soil properties. Cyanobacteria exhibited consistent abundance across developmental stages, while AAPB abundance peaked during intermediate stages, reflecting their pivotal role in early colonization. The community structures of both AAPB and cyanobacteria showed significantly variation during the biocrust development. Co-occurrence network analysis revealed intensified cyanobacteria-AAPB interactions in early stages, followed by reductions in the terminal stage (6-years). Environmental factors, including pH, nutrient content, and chlorophyll-a, were found to be strongly correlated with the microbial community composition. This study provides new insights into the dynamic interactions between cyanobacteria and AAPB during coral island biocrust development, highlighting their complementary roles in soil stabilization, nutrient cycling, and ecological succession.