Microbial metabolism and disease virulence changes across day and night in coral Black Band Disease lesions. Black Band disease mat from diseased coral Acropora cf. hyacinthus

Coral Black Band Disease (BBD) is characterised as a cyanobacteria-dominated microbial mat that rapidly kills underlying coral tissue. Solar radiation promotes lesion progression by fuelling the cyanobacterial photosynthesis, while sulphate-reducing bacteria and sulphide-oxidising bacteria are implicated in sulphide dynamics within the mat. However, how metabolisms of these dominant microbial communities in the mat vary under light and dark conditions and impact lesion virulence is poorly characterised. To compare microbial gene expression under different light regimes, we recovered 28 near-complete BBD-derived metagenome-assembled genomes (MAGs) using Oxford Nanopore Technologies long-read sequencing, and profiled Illumina metatranscriptomic reads from BBD lesions collected at day and night by mapping to these MAGs. Genes from cyanobacterium Roseofilum reptotaenium dominated the differentially expressed genes, with photosynthesis highly represented during the daytime. Relative expression of sulphur and nitrogen metabolism, reductive tricarboxylic acid (rTCA) cycle, chemotaxis, and biofilm formation increased at night, particularly those among Campylobacteriales. Enhanced sulphur metabolism, chemotaxis, and biofilm formation in the overall community at night likely supporting a sulphide-rich and low oxygen micro-environment in the lesion, driving lesion progression towards healthy coral tissue. This study provides insights into how diurnal light dynamics drive microbial metabolic pathways changes, thereby promoting the BBD virulence.