Corals are poisoned by seaweed

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In addition to threats from global warming and ocean acidification, corals now face poisoning by seaweed, according to a study by scientists from the Georgia Institute of Technology.

The paper by Douglas Basher and Mark Hay is to be published in an upcoming issue of the Proceedings of the National Academy of Sciences, and experimentally examines the outcomes and mechanisms involved in seaweed-coral competition. 

Seaweeds commonly replace corals in areas where they have suffered decline, but it is unclear whether the seaweeds directly harm corals or merely suppress coral recruitment when they opportunistically colonise areas of coral decline.

The authors carried out a series of field experiments in the Caribbean (Coco Point Reef, Panama) and the tropical Pacific (Votua Reef, Fiji). These experiments involved racks of cement cones to which short branches of Porites corals (P. porites in Panama and P. cylindrica in Fiji) were glued. In some of the cones, a piece of intact seaweed was secured in contact with the coral while other cones were seaweed-free.

The seaweed species used were common-to-abundant on the poritid-dominated reefs studied, observed in contact with corals, and representative of a range of taxonomic and morphological forms. The authors revisited the corals every other day for 20 days, visually noting coral bleaching and replacing seaweeds lost to wave action.

After 20 days, the authors assessed coral bleaching and photosynthetic efficiency using photographic image analysis and in situ pulse-amplitude modulated fluorometry respectively. An additional experiment was carried out in Fiji in which the researchers constructed seaweed mimics of plastic and placed them in contact with the coral to simulate the effects of shading and mechanical damage.

The authors found significant bleaching and death to occur after 20 days when the seaweeds were in contact with the coral, with 40–70% of the seaweed species studied (five of seven species for the Caribbean and three of eight species for the Pacific) capable of causing bleaching and death.  However, the researchers noted that the bleaching was restricted to the areas where the seaweed was in direct contact with the coral.

To confirm the chemical nature of the seaweed-coral interaction, the authors next carried out a series of experiments in which gel matrices containing lipid-soluble extracts of the seaweeds were applied to the corals.

In this set of experiments, the authors found the effects of extracts to parallel the effects of direct seaweed contact. The authors also carried out variations of the experiment in which extracts from the surfaces of the plants or from the whole plants were used.  In these cases, the results confirmed that lipid-soluble allelopathic metabolites occur on algal surfaces and damage adjacent corals following direct contact.

Herbivorous fishes naturally control the populations of seaweeds, but overfishing has led to a significant loss of this control agent. The authors experimentally verified this in Fiji. Using the seaweeds in their previous caged studies, the authors found that the losses of seaweed due to herbivory were 40–100% in Votua Reef (a marine protected area where fishing was not allowed), compared to 0–40% in the heavily fished reef flat adjacent to Votua Reef.

According to coauthor Mark Hay, "Removing the herbivorous fishes really sets up a cascade of effects. The more you fish, the more seaweeds there are. The more seaweeds there are, the more damage is done to the coral. The less coral there is, the fewer fish will be recruited to an area. If there are fewer fish, the seaweeds outgrow the coral. It's a downward death spiral that may be difficult to recover from."

For more information, see the paper: Rasher, DB and ME Hay (2010) Chemically rich seaweeds poison corals when not controlled by herbivores. Proceedings of the National Academy of Sciences doi: 10.1073/pnas.0912095107