While even astronauts can see the structures made by the coral animals from space, not everyone is aware of the real marvel of the reef, algae. Inside the gut of many coral polyps are microscopic algae commonly called zooxanthellae (Symbiodinium spp.). These algae are photosynthetic, making food from sunlight which is also used to feed the coral. In fact, for some species of coral up to 95% of the corals daily requirement can come from the algae. Coral bleaching occurs when the algae is expelled from the coral host, which leaves it with a ghostly white color as if someone had poured Clorox all over it, hence the term bleached.
|Bleached coral on a wreck, 2011. Photo credit: Me|
The exact reason for the zooxanthellae to leave the coral is still being studied. One of the things that was clear in my previous life as a marine biology graduate student, was that the photosynthetic machinery of the algae was damaged during bleaching. There's evidence of damage to both the oxygen evolving side ("light" reactions, mainly photosystem II) and the carbon fixation side ("dark" reactions, aka Calvin Cycle) of photosynthesis in heat stressed zooxanthellae.
Recently, a paper came out called "Thermal bleaching induced changes in photosystem II function not reflected by changes in photosystem II protein content of Stylophora pistillata" by Jeans, et al. and published in Coral Reefs. It is unfortunately behind a paywall and was published earlier this year. The title intrigued me because in my work there were clear changes in photosystem II protein content during bleaching. However, my work did not contain any Clade C, the type of zooxanthellae present in the coral Stylophora.
The setting: Heron Island Research Station, Heron Island Australia.
|Me, Heron Island Research Station 2001.. seriously this is what the view looks like|
Photosynthetic efficiency is known to decrease in corals that are bleaching. In this study, they showed that expelled zooxanthellae had extremely decreased Fv/Fm in the first few hours at increased temperature. As the time increased, expelled zooxanthellae Fv/Fm increased while that of the in situ zooxanthellae decreased until they were almost the same.
For the 3 photosynthetic proteins, 2 formed the core of photosystem 2 and 1 is rubisco, aka the main enzyme in the carbon fixation cycle. The expelled zooxanthellae had higher concentrations of the photosynthetic proteins per cell than those that remained in the coral at the early time points only, after 10 hours though, the levels were the same. The researchers used Western blots which relies on antibody-protein interaction. This approach cannot tell the difference between active and inactive proteins. Thus combining the higher concentrations of the photosystem II core proteins with the extremely low Fv/Fm readings of the expelled zooxanthellae suggests that the majority of these proteins must be damaged.The increased rubisco content could also be due to damage but they had not other data to support this.
When you stop to think about it, these results makes perfect sense. The coral expels the "sickly" algae first, the strong zooxanthellae remain. But as the stress continues, even the stronger algae are expelled.This could be due to either damage occurring in the stronger algae, or perhaps the corals own immune system has labeled them all a threat that must be expelled.