Sounds interesting. What sort of experiments will you be running?
I'm critically testing the dominant hypothesis about how heat-stress causes symbiotic cnidarian bleaching. I refer to this hypothesis as the reactive oxygen species (ROS)-induced bleaching hypothesis. The original idea, first popularized by Michael Lesser in the 1990s, was that the photosynthetic machinery in symbiotic algae over-produces high-energy electrons when exposed to elevated temperatures (or high-light and other stressors). As the production rate of these high-energy electrons outpaces the algae's ability to concentrate and reduce/fix CO2, the electrons begin to react with with other chemicals in the chloroplast, including oxygen. This produces chemically-reduced oxygen compounds that are powerful oxidizing agents (ROS) including hydrogen peroxide, superoxide, oxygen radicals, hydroxyl radicals, etc. Eventually, these compounds overwhelm the algae's antioxidant defense mechanisms, make their way into the cnidarian host's cells, and begin damaging the cnidarian. If the ROS leakage from the algae overwhelms the cnidarian's antioxidant capacity, it expels the algae (bleaches) as a last-ditch effort to protect itself from oxidative damage (similar to slowly burning).
Early studies seemed to support this hypothesis, and it was among the earliest hypotheses that provided an internally consistent, plausible molecular mechanism that linked various stressors and coral bleaching. However those early experiments were poorly controlled (we all know how difficult it can be to work with these organisms), and more recent publications have raised questions about the validity of the ROS-induced bleaching hypothesis. For example,
Acropora and aiptasia bleach just as quickly when exposed to heat-stress in total darkness as they do in light (Tolleter et al., 2013), indicating that photosynthesis is not required for bleaching. It also appears that non-symbiotic aiptasia (aiptasia can be kept alive long-term without symbiotic algae if you feed them regularly) experience more oxidative stress than symbiotic aiptasia (Nii & Muscatine, 1997), and that levels of oxidative stress in these animals correlate with rate of respiration, not the rate of photosynthesis. These findings suggest that if ROS play a role bleaching, the ROS are produced by the cnidarian itself rather than by the algae.
In any case, I've been treating aiptasia with various concentrations of H2O2 to test whether elevated ROS levels can induce bleaching without heat. So far, it appears even lethal doses of H2O2 do not cause bleaching. I've also been heat-stressing aiptasia and testing whether incubation with various antioxidants reduces bleaching. So far, antioxidants don't appear to reduce bleaching during heat-stress. I have more experiments to run and need to repeat these tests on corals as well, but I haven't found any direct evidence that ROS play an important role in bleaching. There are other hypotheses that link heat-stress to bleaching, and I plan on testing some of those as well. But the field of coral biology can't move forward if our theory about why and how corals bleach is fundamentally flawed.
