Even the first look at this underwater photo reveals that something is wrong here: These coral reefs in the clear waters of the Caribbean seem to be overgrown by a golden-brown crust.
It has long been known that excessive warming of the sea can lead to the death of entire coral reefs. In the Great Barrier Reef off Australia, these coral bleaches have killed up to 90 percent of the corals in places. However, pollution and acidification of the oceans, as well as strong storms, can endanger coral reefs, making them more fragile and more susceptible to disease.
To study these effects, researchers led by Bryan Wilson from the University of Oxford examined the growth of coral reefs in the American Virgin Islands in the Caribbean. In doing so, they discovered the golden-brown crusts on the corals after a strong hurricane, which this photo shows. According to their analyzes, these are red algae of the genus Peyssonnelia.
Algae typically support the formation of new reefs by using microbes to produce biochemical signals and thus attract young corals. The marine animals eventually settle on the rough surface and gradually form the reef structures. But Wilson and his colleagues found that the golden-brown algae in this photo by no means promote the growth of the reefs, but instead overgrown and further destroyed the corals that had been damaged by the storm.
But how did it come about? Upon closer inspection, the research team found that this type of red algae harbors microorganisms that are different from those known to produce the attractants. “These red algae crusts have biochemical and structural defense mechanisms that they use to keep fish and other marine life from grazing,” said Wilson’s colleague Chen-Ming Fan.
This prevents natural processes from curbing algae growth and protecting the reefs from overgrowth. The same mechanisms could also prevent young corals from repopulating these reefs. As a result, the coral gardens infested by these algae are often doomed.