Influence of light on microbial metabolism and disease virulence within coral Black Band Disease lesions
Coral Black Band Disease (BBD) is characterised as a cyanobacteria-dominated microbial mat that rapidly kills underlying coral tissue. High solar radiation promotes lesion progression by fuelling the photosynthetic capacity of cyanobacteria, while the activity of sulphate-reducing and sulphide-oxidising bacteria determine sulphide dynamics within the mat. The metabolic capacity of these dominant microbial communities in the mat varied under light and dark condition, though how light influences lesion virulence is poorly characterised. This study recovered 28 near-complete BBD-specific metagenome-assembled genomes (MAGs) using Oxford Nanopore Technologies long-read sequencing, and Illumina generated metatranscriptomic reads from BBD lesions subjected to light and dark conditions were subsequently mapped to the MAGs to compare bacterial gene expression across the different light regimes. Cyanobacterium Roseofilum reptotaenium affiliated genes dominated differential expression patterns, with photosynthesis highly represented during the daytime. Pathways associated with sulphur and nitrogen metabolism, reductive tricarboxylic acid (rTCA) cycle, chemotaxis, and biofilm formation demonstrated increased expression at night, particularly among Campylobacteriales. Enhanced sulphur metabolism, chemotaxis, and biofilm formation support a sulphide-rich and low oxygen micro-environment in the BBD lesion at night, driving lesion progression towards healthy coral tissue. This study provides insights into how light dynamics drive microbial metabolic pathways, thereby promoting BBD virulence.