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."
The researchers studied coral of different species in the Pacific and Caribbean, matching them up against different species of seaweed common to their geographic areas. The coral species chosen Porites porites in Panama and Porites cylindrica in Fiji are among the hardies
|Contact: John Toon|
Georgia Institute of Technology Research News