Increased sulfate-reducing bacteria can drive microbial dysbiosis in bleached corals.

Aims: Coral bleaching occurs when coral colonies lose their Symbiodiniaceae partner and turn pale or white. Although this event is generally temperature-induced, there is also the possibility of holobiont microbial infection and dysbiosis. To address this issue, this study was conducted to investigate the diversity and composition of Symbiodiniaceae and bacteria in healthy and bleached colonies of Porites lutea collected from eastern Shenzhen.

Methods And Results: Internal transcribed spacer 2 and 16S amplicon sequencing analysis were used to explore the diversity and composition of Symbiodiniaceae and bacteria in healthy and bleached colonies of P. lutea. Bacterial diversity and richness were significantly higher in bleached colonies than in healthy colonies (P < 0.05), whereas the diversity and richness of Symbiodiniaceae showed no significant changes. The bleaching event exerted a more significant impact on Symbiodiniaceae composition, which differed between healthy and bleached colonies (PERMANOVA, F = 8.246, P < 0.05). In terms of composition, Clade C (Cladocopium) was the predominant Symbiodiniaceae, whereas subclade C116 and C2r were significantly less abundant in bleached colonies than in healthy colonies (P < 0.05). The phyla Bacteroidetes, Acidobacteria, and Actinobacteria were significantly more abundant in bleached colonies than in healthy colonies (P < 0.05). The sulfate-reducing bacteria (SRB) Desulfobulbus and Desulfobacter at the genus level and Desulfobacterales and Desulfuromonadales at the order level were significantly more abundant in bleached colonies than in healthy colonies (P < 0.05). The co-occurrence patterns of Symbiodiniaceae and bacteria revealed a negative correlation of Desulfofaba, Desulfovibrio, Desulfarculus, and Desulfobulbus with Endozoicomonas, a very common symbiotic bacterial genus found in corals.

Conclusion: Coral bleaching may be associated with significant shifts in microbial communities, including increased SRB abundance, which may disrupt microbial balance and contribute to bleaching.