Macroalgae frequently interact with corals, usually by overgrowing them from their edges. In the October issue of Ecology Letters, Nugues, Smith, van Hooidonk, Seabra and Bak demonstrate a sinister aspect of this competition. One of the most common macroalgae, Halimeda opuntia, is shown to trigger a particularly virulent disease known as white plague type II.
In the Caribbean, a large number of coral colonies on which this alga was transplanted developed white plague whereas unexposed colonies did not. In addition, the plant was found to be a reservoir for the marine bacterium Aurantimonas coralicida, causative agent of the disease. The spread of macroalgae on coral reefs may account for the elevated incidence of coral diseases over past decades. Moreover, measures to reduce seaweed abundance may be essential if significant coral populations are to survive on coral reefs.