In the Absence of Herbivory, Allelochemicals Released by Seaweed cause Coral Bleaching and Death

With an increase in sea-surface temperatures and a flurry of anthropogenic activities, more than 30% of the world’s coral species face extinction. Rasher and Hay (2010) propose that the poisonous allelopathic chemicals released by common seaweed contribute to coral death and bleaching. They studied the effects of the secondary metabolites on coral bleaching, coral death and coral photosynthesis. They also transplanted the coral species onto reefs and studied the impact of herbivory on coral-seaweed interactionS. Their results indicate that when the seaweed comes into direct contact with the coral, it releases a lipid-soluble metabolite which damages the coral and leads to bleaching and a decrease in photosynthetic yield. In the reef community, however, herbivores consume the seaweed and mediate the deleterious effect of the seaweed on the coral reefs. Thus, the reef herbivores control the seaweed population and reduce coral mortality and bleaching.— Sachi Singh
Rasher, D. B. and Hay M., E. 2010. Chemically rich seaweed poison corals when not controlled by herbivores. Proceedings of the National Academy of Sciences of the United States of America 107, 9683–9688.

Previous studies have shown that seaweeds outcompete adult coral in the absence of herbivores so the authors predicted that in the absence of herbivory, the seaweed would chemically outcompete the coral and cause coral death and bleaching. They studied the effects of common seaweed on the bleaching patterns and photosynthetic yield of the Porites porites and Porites cylindrica coral in reefs in the Caribbean and the Tropical Pacific respectively. They used algal mimics to test the physical effects of abrasion and shading by the seaweed on the structure of the coral in a controlled setting and measured the rates of photosynthetic yield using in-situ pulse-amplitude modulated (PAM) fluorometry. The shading and abrasion by the inert algal mimics did not have any discernible effect on the health of the coral. However, when the seaweed comes into contact with the coral, it releases a lipophlic allelochemical which suppresses the growth of new coral and causes coral death and bleaching. The bleaching patterns and photosynthetic rates indicate that the coral was damaged in areas that were in direct contact with the seaweed; this indicates the that the release of lipid-soluble chemicals was triggered by contact with the coral and not proximity alone. The authors suggested that the coral’s interaction with the seaweed may produce a negative feedback that could limit coral recovery.
To study the seaweed-coral interaction in the presence of herbivory, the authors transplanted the P. cylinderica coral into a Marine Protected Area (MPA) and into an area that was subject to fishing. The reef herbivores, fish in this case, suppressed the potentially toxic seaweed by rapidly consuming it, and protected the coral from bleaching. Thus, even modest fishing of these reef herbivores could increase the seaweed-coral chemical interaction and lead to coral death and bleaching. 

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