"Algae kill coral by enhancing microbial activity "

An edited version of this article appears in the 7 October edition of New Scientist:

Algae kill coral by enhancing microbial activity
Caspar Henderson

Evidence for a previously unknown mechanism by which algae can harm coral reefs was presented at the meeting International Society of Reef Studies in Bremen, Germany last month.

“Our work shows that algae can indirectly cause coral mortality by enhancing microbial activity. The evidence indicates that it does this via the release of compounds likely to be dissolved organic carbon.” says Jennifer Smith a researcher at the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara, and a lead author on the paper (Indirect effects of algae on coral: algae-mediated, microbe-induced coral mortality. Ecology Letters [2006] 9: 835-845).

The last several decades have seen an alarming decline in coral reefs around the world as a result of numerous factors including climate change, overfishing, eutrophication, coral disease. As corals die or become stressed they are generally replaced or out-competed by faster growing fleshy algae. These "phase-shifts" from coral to algal dominance result in a communities that are completely differnent in structure and function than the healthy coral-dominated state.. Perhaps the most poorly understood component of coral decline, say many scientists, is how coral disease, increasing algal cover, and human impacts such as pollution and overfishing, interact. Understanding these relationships is crucial for the development of conservation, restoration and management plans for coral reefs around the world, they say.

Smith and her colleagues performed a series of ship board experiments while on an expedition last summer to the Line Islands in the Central Pacific, some of the most remote and most pristine coral reefs left in the world. When coral and algae were placed in chambers together but separated by 0.02 micron filter that prevented the passage of microbes and viruses, corals suffered 100% mortality. With the addition of the broad-spectrum antibiotic ampicillin, mortality was completely prevented. Physiological measurements showed complementary patters of increasing coral stress with proximity to algae.

“You could make an analogy with gum disease in humans” says Stuart Sandin, a marine ecologist at University of California, San Diego and one of Smith’s co-authors, “with sugars fertilising bacteria already present on the coral’s surface, like those present in your mouth, so that these resident microbes multiply and become pathogens”.

Leading marine researchers have hailed the findings. “The [work] … is important because it highlights another potential mechanism by which macroalgal-dominated reefs could persist and reduce the likelihood of switching back to a coral-dominated state”, says Peter Mumby of the Marine Spatial Ecology Laboratory University. But he adds a caution. “The mechanism still needs to be examined under natural field conditions because laboratory-based studies may not adequately represent the effects of algae in nature”.

Smith and Sandin agree. “Our main goal was simply to show that there is a mechanism at work. Most ecologists study what you can see. But micro-organisms could be playing a major role at the foundations of reef ecology. The study opens up so many new questions that need to be addressed in the future”.