Detection of dominant viruses from Mediterranean endemic scleractinian coral Cladocora caespitosa when exposed to heat stress and a coral pathogen, Vibrio coralliilyticus

Global warming leads to increased coral bleaching due to higher sea temperatures and increased pathogenic activity affecting coral communities. Together with rising temperatures, the pathogenic activity affecting coral communities also increases. However, the role of viruses in coral bleaching has been underestimated. Recent studies demonstrated the presence of virus-like particles (VLP) on the surface of bleached corals, indicating the need for further analysis to determine the impact of viruses on the bleaching phenomena. In this context, our objective was to detect the dominant viruses on Cladocora caespitosa, a coral species endemic to the Mediterranean basin, when exposed to a bacterial pathogen, namely Vibrio coralliilyticus, and heat stress. Thermally stressed and both thermally and pathogenically stressed colonies were bleached during the experiment, representing the detrimental impact of the mentioned stressors on coral health. The viral load of the control groups significantly differed from each other prior to the introduction of any stress factors, indicating the complex interactions within coral microbiota. The increased abundance of viruses in thermally and both thermally and pathogenically stressed corals demonstrates a positive correlation between the viral activity and stress. Interestingly, while the increased abundance of viruses was attributed to bacteriophages in thermally stressed corals, the abundance of eukaryotic viruses increased in both thermally and pathogenically stressed corals, presumably indicating a difference in the bleaching mechanisms observed in those tanks.