Coral reefs face unprecedented declines in diversity and cover, a development attributed largely to climate-change-induced bleaching and subsequent disease outbreaks. Coral-associated microbiomes may strongly influence the fitness of their hosts and alter heat tolerance and disease susceptibility of coral colonies. Here, we describe a new coral disease found in Micronesia and present a detailed assessment of infection-driven changes in the coral microbiomes. Combining histological, microscopic and next-generation sequencing assessments, we demonstrate that the outbreak of the disease, named “grey-patch disease”, is associated with the establishment of cyanobacterial mats overgrowing coral tissue. In contrast to the Anna-Karenina principle, which states that disturbances such as disease trigger chaotic dynamics and increase beta-diversity, we found significantly higher community similarity in microbiomes of diseased corals. A possible explanation for this pattern is strong competition between members of the pathobiome and the coral holobiont. Grey-patch disease is characterised by low progression rates, with coral tissue sometimes even growing back over cyanobacterial mats. Further, novel replicated network analysis of microbial communities demonstrated that pathobiomes were dominated by positive connections, and disease-associated microbes formed highly connected “infection-clusters” in microbiomes of apparently healthy corals. Therefore, synergistic effects between multiple pathogens are seemingly required to overcome coral immune defence, creating a structuring mechanism that homogenises pathobiomes.
Oral presentation given by Alfred Burian.