By Danielle Moloney
Edited by Jasmine Haskell
Coral reefs across the globe face a host of stressors that challenge their success. From warming ocean temperatures to ocean acidification, these already fragile systems have a lot to reckon with in order to ensure their survival. Disease is one of many obstacles for coral reefs. In low frequency, disease can help select for the most resilient corals (as weaker individuals may die off from infection) and can help build a stronger colony. However, disease outbreaks with high frequency can undo generations of growth and impact the biodiversity of the reef. Marine diseases are even more evasive than their terrestrial counterparts- water provides a constantly changing medium in which pathogens can spread easily from place to place and individual to individual, with very little chance for scientists to isolate the spreading disease. Corals provide a host of ecosystem services in addition to supporting many people with food and jobs, making their success essential for humanity as well. In the Caribbean region alone, reefs generate over five billion US dollars per year through tourism and other reef-related activities. A new study assessed how seven species of reef-building corals in the Caribbean respond to disease exposure. Their findings indicate that certain genetic traits are conserved across different species, and that these traits determine whether a coral will be disease resilient or receptive when disease breaks out.
White plague disease
Researchers exposed healthy corals to a disease known as white plague disease. White plague disease causes corals to lose tissue over time via the development of lesions. The lesions are characterized by a lack of pigment and the exposure of bare coral skeleton (no symbiotic zooxanthellae is associated with lesion areas). White plague disease has been established as an extremely lethal disease to many coral species, and historical outbreaks have killed off large swaths of corals on Florida and Caribbean reefs.
Each of the seven species was acclimated to laboratory conditions, then exposed to white plague disease and monitored over the seven day experimental period. Corals were housed in pairs. If one coral had a lesion and its pair was healthy at the end of the experiment, that individual was termed disease-resistant. If one coral had a lesion and its pair developed a lesion too, it was termed disease-susceptible. Rates of disease and disease progression were monitored across all treatments. Coral tissue was processed and assessed for three different disease responses: lesion progression, gene expression among species, and gene expression in various stages of disease expression. The species were ranked on a scale from susceptible to resistant (Figure 1).
Figure 1. A diagram depicting the seven species of coral tested in the experiment along a scale from more susceptible to white plague disease to more resistant. Figure courtesy of MacKnight et. al 2022.
Factors controlling resilience
The results found that disease-susceptible corals had particular immune responses related to their cytoskeletal arrangement (cytoskeleton: a network of proteins inside the cells of many organisms that give them shape and structure) that made them more likely to experience lesions. On the other hand, disease resilient corals exhibited higher levels of intracellular protein trafficking (the transportation of proteins in between cells of the organism) compared to their susceptible counterparts, and that this trait was inherited through the coral’s lineage- making certain lineages more resistant to disease than others. Additionally, they found that whether corals developed lesions or not was partially controlled by the plasticity (plasticity: when one genotype can express different phenotypes based on environmental conditions) of genes that regulate three cellular processes: extracellular matrix maintenance, cellular breakdown of tissue (autophagy), and controlled cell death (apoptosis) (Figure 2).
Figure 2. A list of genes found in five of the seven coral species tested and how those genes correlate with lesion progression in white plague disease. The size of the blue and red circles indicates the p-value of the correlation, with low p-values (closer to the center of the diagram) indicating a strong relationship between that gene and either positive or negative lesion progression. Figure courtesy of MacKnight et. al 2022.
Disease is a powerful tool that has the ability to change how ecosystems look and function. Recent studies indicate that marine diseases are increasing in both scale and severity, making it even more imperative to understand what that will mean for corals in the future. Few scientific studies have drawn comparisons between the immune responses of different coral species to disease, especially when those species differ in disease resistance. Results from this study may provide scientists with a better understanding of the future of certain coral species in the event of disease outbreaks. Further research may assess a similar framework as this study, but expand the scope to include a wider array of species or examine immune responses to a different disease altogether (such as stony coral tissue loss disease, a recent disease that has plagued many corals in Florida and Caribbean reefs). Findings showing that certain traits that promote disease resilience can be passed down along genetic lineages of corals shows hope for healthy corals in the future when disease outbreaks occur.
Read the full study from MacKnight et.al here.
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Featured Photo courtesy of The Nature Conservancy.