Impact of coral bleaching on symbiotic microbial communities and carbon fixation pathways in the South China Sea

Abstract

Microorganisms play a crucial role in supplying essential nutrients to corals, thereby enhancing their survival under environmental stress. However, the diversity and ecological functions of carbon-fixing microorganisms in coral symbiosis remain poorly understood. In this study, we employed metagenomic analysis to investigate microbial responses to coral bleaching and their impact on carbon fixation pathways in the South China Sea. Principal coordinates Analysis (PCoA) revealed that both bleaching and coral species significantly influenced the symbiotic microbial community composition. Bleached corals exhibited a substantial increase in heterotrophic microorganisms. The findings demonstrated the presence of complete photosynthetic and prokaryotic carbon fixation pathways within coral holobionts, with a clear shift in microbial carbon metabolism following bleaching. Function analysis shows a decrease in key enzymes of photosynthetic carbon fixation and an increase in prokaryotic carbon fixation enzymes in bleached corals, indicating the activation of the prokaryotic carbon fixation pathway in bleached corals. In addition to Symbiodiniaceae, other microbial taxa contributed to carbon fixation, with Suessiales, Cladophorales, Oscillatoriales, Rhodospirillales, and Rhodobacterales identified as the major contributors. Notably, Cladophorales (Boodlea and Botryococcus) emerged as key contributors to RuBP carboxylase activity in bleached corals. These results highlight the significant role of microbial carbon fixation within coral holobionts and provide a foundation for future research on lineage-specific carbon-fixing microorganisms.