Researchers have offered first proof that several common species of seaweeds in both the Pacific and Caribbean Oceans can kill corals upon contact using chemical means.
While competition between seaweed and coral is just one of many factors affecting the decline of coral reefs worldwide, this chemical threat may provide a serious setback to efforts aimed at repopulating damaged reefs. Seaweeds are normally kept in check by herbivorous fish, but in many areas overfishing has reduced the populations of these plant-consumers, allowing seaweeds to overpopulate coral reefs.
A study documenting the chemical effects of seaweeds on corals was scheduled to be published May 10, 2010 in the early edition of the journal Proceedings of the National Academy of Sciences (PNAS).
“Between 40 and 70 percent of the seaweeds we studied killed corals,” said Mark Hay, a professor in the School of Biology at Georgia Tech. “We don’t know how significant this is compared to other problems affecting coral, but we know this is a growing problem. For reefs that have been battered by human use or overfishing, the presence of seaweeds may prevent natural recovery from happening at all.”
Coral reefs are declining worldwide, and scientists studying the problem had suspected that proliferation of seaweed was part of the cause – perhaps by crowding out the coral or by damaging it physically.
Using racks of coral being transplanted as part of repopulation efforts, Hay and graduate student Douglas Rasher compared the fate of corals from two different species when they were placed next to different types of seaweed common around Fijian reefs in the Pacific – and Panamanian reefs in Caribbean. They planted the seaweeds next to coral being transplanted – and also placed plastic plants next to some of the coral to simulate the effects of shading and mechanical damage. Other coral in the racks had neither seaweeds nor plastic plants near them.
The researchers revisited the coral two days, 10 days and 20 days later. In as little as two days, corals in contact with some seaweed species bleached and died in areas of direct contact. In other cases, the effects took a full 20 days to appear – or for some seaweed species, no damaging effects were noted during the 20-day period. Ultimately, as much as 70 percent of the seaweed species studied turned out to have harmful effects – but only when they were in direct contact with the coral.
To confirm that chemical factors were responsible, Hay and Rasher extracted chemicals from the seaweeds – and from only the surfaces of the seaweeds. They then applied both types of chemicals to corals by placing the chemicals into gel matrix bound to a strip of window screen, forming something similar to a gauze bandage and applying that directly to the corals. To a control group of corals, they applied the gel and screen without the seaweed chemicals.
The effects confirmed that chemicals from both the surface of certain seaweeds and extracts from those entire plants killed corals.
“In all cases where the coral had been harmed, the chemistry appeared to be responsible for it,” said Hay. “The evolutionary reasons why the seaweeds have these compounds are not known. It may be that these compounds protect the seaweeds against microbial infection, or that they help compete with other seaweeds. But it’s clear now that they also harm the corals, either by killing them or suppressing their growth.”