"Attack of the killer seaweed" may sound like a cheesy horror flick, but for many coral species, murderous multicellular algae have become real-life villains. A new study of reefs in the South Pacific suggests that some algae can poison coral on contact. This chemical warfare may be increasing the pressure on struggling reef communities worldwide, researchers say.
Along the reefs dotting Fiji, overfishing has pitted corals against algae in a battle royale. On swaths of coastline where fishing is restricted, corals such as the tall and branching Acropora millepora rule, says study co-author Mark Hay, a marine ecologist at the Georgia Institute of Technology in Atlanta.
But where Fijians spear lots of herbivores such as bird-beaked parrotfish, few fish remain to prune back the region's seaweeds, a blanket term for many types of big algae. These algae then creep in, extending their tendrils over close to 60% of the ocean bottom, Hay estimates, and turning waters a sludgy green. Such "seaweed-covered parking lots" aren't unique to Fiji, either, he says.
Recent studies have hinted that this ocean greenery may be carrying out a subtle chemical war on sensitive reefs. To investigate this covert struggle, Hay and colleagues strung eight different species of Fijian seaweed across growing corals, including A. millepora colonies. True to the researchers' suspicions, many of these algal species seemed to wield a poison touch. In less than 2 weeks, the test coral often began to discolor and even die where it rubbed against the seaweeds, the team reports today in the Proceedings of the National Academy of Sciences. Faux seaweeds made of plastic had no such effect.
Hay and colleagues then mashed up several of these seaweeds to identify their killer concoction. The key ingredient turned out to be chemicals called terpenes, which some algae use to sicken fish that feed on them. Terpene extracts alone killed off corals, the researchers found. But some algae seemed to be more liberal with their toxins than others, Hay notes. When one particularly nasty specimen called turtle weed (Chlorodesmis fastigiata) rubs against A. millepora, for instance, wide bands of dying tissue girdle the coral.
This seaweed is so nasty, in fact, that most marine herbivores avoid it on sight—except for one species of rabbitfish that quivers with excitement every time it spots this not-so-common algae. That interaction highlights the importance of prudent fishing practices, he adds. If Fijians developed a particular taste for that one rabbitfish, for instance, turtle weed might begin to grow out of control, launching its bid for world, or at least South Pacific, domination. Hay would like to work with Fijians to identify and protect the herbivores most responsible for trimming back deadly seaweeds, giving sensitive corals a fighting chance.
"It's certainly a novel finding," says John Bruno, a marine ecologist at the University of North Carolina, Chapel Hill. But not all seaweeds are poisonous, he adds. Many scientists argue that algae—toxins or no—rarely kill off adult corals en masse. Instead, these opportunistic organisms may simply be capitalizing on the slow death of the invertebrates due to pollution, climate change, or other factors. He adds, however, that the seaweeds Hay and colleagues studied would likely be exceptionally toxic to young, coin-sized corals that have yet to grow big and hale.
Terpenes from seaweed are almost certainly not the only reason for the mysterious global decline of corals, says Jennifer Smith, a marine ecologist at the University of California, San Diego. Most scientists rank overfishing, pollution, and warming oceans among the biggest overall contributors. But corals may suffer from other nasty tricks played by seaweed. In a 2006 study, Smith and colleagues sleuthed out that some tropical algae could take the epidemic route to domination. In the lab, these seaweeds leak huge quantities of dissolved carbon that then fuels the spread of potentially infectious microbes on coral surfaces. "You can imagine that [algae and corals] have evolved over the years different mechanisms for battling each other and fighting these turf wars," Smith says.
Correction: A previous version of this story referred to the algal species studied by Smith and colleagues as "Californian." It has been corrected to read "tropical."