Is coral bleaching due to the instability of the zooxanthellae dark reactions?

Coral reefs are dominated by symbioses between marine invertebrates (e.g. corals, clams, sea anemones, sponges) and the dinoflagellate Symbiodinium. Increases in seawater temperature linked to global warming have lead to the phenomenon known as "bleaching", involving the disassociation of these symbioses and mass mortalities of the invertebrate host. Mass bleaching events have been linked to a thermal lesion in the photosynthesis of Symbiodinium, either at photosystem II or in the dark reactions of photosynthesis. Examination of two of the proposed lesion points, the carbon concentrating mechanism (CCM) and form II Rubisco, indicate that, over the temperature range examined, the CCM is not disrupted in Symbiodinium isolated from giant clams, however it was not possible to determine if Rubisco is the point of thermal lesion resulting in bleaching. Maximum photosynthetic rates were measured between 28 and 31°C and declined as temperature was increased. Despite a low correlation between Rubisco activity, oxygen evolution and temperature, Rubisco activity declined with increasing temperature.