Seasonal coral-algae interactions drive White Mat Syndrome coral disease outbreaks.

Ocean warming drives not only the increase of known coral disease prevalence but facilitates the emergence of new undescribed ones too. As climate change is restructuring coral ecosystems, novel biological interactions could lead to an increase in coral disease in both tropical and marginal coral communities. White Mat Syndrome (WMS) represents one such emerging coral disease, with outbreaks associated with high algal interactions and seasonal summer temperatures. However, the mechanisms behind its pathogenesis, modes of transmission and causative pathogens remain to be identified. Ex situ infection experiments pairing the coral Porites heronensis together with local potential contributory factors show that the macroalga Gelidium elegans hosts and proliferates the WMS microbial mat. This pathogenic consortium then infects adjacent corals, leading to their mortality. WMS was also observed to transmit following the fragmentation of the microbial mat, which was able to infect healthy corals. Sulfur-cycling bacteria (i.e., Beggiatoa, Desulfobacter sp., Arcobacteraceae species) and the free-living spirochete Oceanospirochaeta sediminicola were found consistently in both WMS and G. elegans consortia, suggesting they are putative pathogens of WMS. The predicted functional roles of these pathogenic consortia showed degradative processes, hinting that tissue lyses could drive mat formation and spread. Coral-algae interactions will rise due to ongoing ocean warming and coral ecosystem degradation, likely promoting the virulence and prevalence of algal-driven coral diseases.