While corals reefs are often the posterchild for the drastic effects of global climate change, the diseases that infect them can be a more localized threat to the health of reefs already struggling to keep up. The Florida Reef, a barrier reef stretching from the southeastern peninsula to the lower Florida Keys, has suffered for years from the increasing spread of Stony Coral Tissue Loss Disease (SCTLD). New research published in Frontiers in Marine Science suggests the health of these corals are threatened by nutrient pollution from land and predation.
To investigate potential mechanisms, researchers tested how the immune system of a species of hump coral, Porites porites, reacted when stressed by increased nutrients and predation from parrotfish. Key enzymes needed to repair coral tissue were used as proxies for the coral's immune response to these conditions when fragments were placed in enclosures that allowed for controlled nutrient additions and exposure to predation.
To investigate potential mechanisms, researchers tested how the immune system of a species of hump coral, Porites porites, reacted when stressed by increased nutrients and predation from parrotfish. Key enzymes needed to repair coral tissue were used as proxies for the coral's immune response to these conditions when fragments were placed in enclosures that allowed for controlled nutrient additions and exposure to predation.
The authors observed that predation impacted an enzyme, laccase phenoloxidase (PO), used in pathways for tissue repair and pathogen defense. Importantly, nutrient enrichment also had caused response in tyrosinase-PO a different enzyme involved in coral immune pathways. This suggests that not only can nutrient pollution assist pathogens abundance and virulence, but may also impact the coral host's immune response to infection. An important caveat to their results is that it is not entirely certain that the corals, rather than another organism in their microbiome, are producing these enzymes.
Regardless of the source of the immune enzymes, what was the impact of the combined stressors on the corals themselves? Their experiments also showed that while predation from parrotfish wasn't statistically significant between nutrient treatments, wounds were larger and more abundant in higher nutrient treatments. The also authors believe future work should examine whether parrotfish can act as vectors of disease to corals.
There is still a lot of work to be done determining how corals survive in degraded environments. For example, a recent discovery of healthy brain coral populations in the warm shallows of Biscayne Bay, which was plagued by fish kills this summer, suggests more extreme environments may harden corals to survive multiple stressors and could help scientists decide how to restore more resilient reefs. Even though certain populations of corals may be more resilient than they look under the surface, it is clear from the results of this study that we mitigate nutrient pollution from land to keep corals as healthy as possible.
Regardless of the source of the immune enzymes, what was the impact of the combined stressors on the corals themselves? Their experiments also showed that while predation from parrotfish wasn't statistically significant between nutrient treatments, wounds were larger and more abundant in higher nutrient treatments. The also authors believe future work should examine whether parrotfish can act as vectors of disease to corals.
There is still a lot of work to be done determining how corals survive in degraded environments. For example, a recent discovery of healthy brain coral populations in the warm shallows of Biscayne Bay, which was plagued by fish kills this summer, suggests more extreme environments may harden corals to survive multiple stressors and could help scientists decide how to restore more resilient reefs. Even though certain populations of corals may be more resilient than they look under the surface, it is clear from the results of this study that we mitigate nutrient pollution from land to keep corals as healthy as possible.