Integrated approach to understanding the onset and pathogenesis of black band disease in corals

Emerging infectious diseases are contributing to global declines in coral reef ecosystems, highlighting a growing need for aetiological knowledge to develop effective management strategies. In this review, we focus on black band disease (BBD), one of the most virulent diseases and the only polymicrobial disease so far known to affect corals. A multipartite microbial consortium dominated by Cyanobacteria, but also including sulfur-cycling bacteria, other bacterial groups and members of the Archaea and Eukarya, forms a sulfide-rich anaerobic mat that migrates across the surface of coral colonies, killing the underlying tissues. The polymicrobial nature of the disease challenges classic aetiological approaches to unravelling disease causation. Here, we synthesize current knowledge on the range of pathogens forming the microbial consortium with recent studies on the transmission, biogeochemistry and environmental drivers of BBD to develop a conceptual model of BBD pathogenesis. The model illustrates how the development of BBD virulence factors is linked to a cascade of microbial community shifts and associated functional roles that progressively develop the microbial consortium from comparatively benign cyanobacterial patches to virulent BBD lesions. This review showcases how an approach that integrates multiple key aspects of the disease provides insights essential to elucidating the aetiology of BBD.