Reef-building corals are obligate associations of heterotrophic animals and phototrophic dinoflagellate endosymbionts (zooxanthellae). Scientists have become interested in a conspicuous stress response -- coral bleaching -- that involves the `breakdown` of this symbiosis, occasionally followed by ecologically devastating coral mortality. Interest in coral bleaching is motivated by the belief that these events are increasing world-wide, and by their potential value as a sensitive bioindicator of environmental change. However, large gaps in our understanding of both the ecology and causes of coral bleaching remain. These gaps frustrate attempts to relate coral bleaching to environmental issues. Recently completed studies on the Caribbean's dominant and most studied corals, Montastraea annularis and M. faveolata, revealed a fundamental misunderstanding that relates directly to coral bleaching. Contrary to the earlier, widely accepted belief that corals harbor only one symbiont, these corals host dynamic communities of three taxa of zooxanthellae that are primarily organized by gradients of ambient irradiance. This phenomenon explained a longstanding issue in coral bleaching ecology -- an otherwise bewildering variation, both among and within coral colonies -- in simple terms. Because it upsets a basic premise on which corals have been studied for decades, this discovery also opens up new and unexpected opportunities for experimental research. This project will analyze the environmental biology of Montastraea spp. in the explicit contexts of multi-species zooxanthella communities and environmental stress. Previous studies of these communities imply parallels with much better studied communities of terrestrial plants.
