[A-CG41] Coastal Ecosystems -2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG41-08] Epizootiology of a temperate coral disease driven by thermal stress and macroalgal interactions

*Joshua Masataro Heitzman¹, Nicole Caputo², Sung-Yin Yang¹, Ben P Harvey¹, Sylvain Agostini¹ (1.Shimoda Marine Research Center, University of Tsukuba, 2.CIRI-FRAME, Alma Mater Studiorum, Università di Bologna)

Keywords:Porites heronensis, coral disease, warm temperate, marginal coral communities, Gelidium elegans, Thiothrix spp.

Coral reefs are important marine ecosystems as they represent a large portion of global marine biodiversity. Increasing threats, including coral diseases, are resulting in their widespread loss. While coral diseases are well described in tropical and subtropical areas, none have been described in warm temperate areas. Here in Shimoda city, Japan, we report for the first time a coral disease outbreak in a warm temperate marginal coral community. The disease gross lesion is characterized by a white mat formation on the temperate coral species Porites heronensis, with infected coral tissue resulting in mortality. Based on its visual characteristics, we have tentatively named this disease “White Mat Syndrome” (WMS). Field surveys were conducted to determine disease incidence, the number of unique new disease occurrences weekly, and prevalence, the population of coral affected by WMS. Disease outbreaks were observed to only occur during the warm summer months from August to November, suggesting an association with high temperatures. The WMS microbial mat was analyzed, revealing a bacterial consortium mainly comprised of Thiothrix spp., a sulfide-oxidizing bacterium. During yearly disease outbreaks, the common macroalgae Gelidium elegans was associated with 75% (30/40) of WMS occurrences observed suggesting its role as a potential contributory factor. This macroalgae often overgrows corals and was observed to senesce and trap sediment during the warm summer months. Furthermore, we showed the presence of Thiothrix spp. within the microbial community of sediment-trapping G. elegans. Incubation experiments to test both the effect of seasonal temperature (winter and summer) and the pathogenicity of potential contributory factors (WMS microbial mat, sediment-trapping G. elegans and sediment) were conducted. These experiments confirmed that high temperature is required for disease proliferation, and that sediment-trapping G. elegans acts as a pathogen reservoir for WMS. As warming waters and increased macroalgal interactions contribute to WMS pathogenicity, future outbreaks may increase in duration and virulence. With tropical and subtropical coral reefs continuing to degrade into algal-dominant ecosystems, algal-mediated diseases like WMS may increase in number and spread. In this context, warm temperate marginal coral community may provide clues for future management strategies of both future algal dominated coral reefs and marginal coral communities.