Development of novel genetic tools for studying dinoflagellates and coral bleaching

Coral bleaching is one of the most catastrophic environmental consequences of global warming. Even if the corals affected recover, the recovery is very slow and there is a massive loss of biodiversity as a result. Corals are a remarkable symbiosis between an animal and an alga, usually belonging to the group known as dinoflagellate algae, and a sub-group called Symbiodinium. The dinoflagellates live inside the coral cells and provide them with nutrients that they make by photosynthesis. In return they get physical protection. In coral bleaching the symbiosis breaks down and the corals expel the dinoflagellate symbionts. This seems to happen as a result of increased water temperature, but exactly why this should cause the symbiosis to break down is not clear. Scientists generally think it is due to some disturbance of photosynthesis in the photosynthetic compartment of dinoflagellates, the chloroplast. For some reason the reactions of photosynthesis become out of balance as temperature rises. Probably this results in the production of toxic chemicals called reactive oxygen species by the dinoflagellates. The coral recognises this danger and expels the dinoflagellates.

We know very little about why raised temperature causes the disturbance in the chloroplasts, and one of the reasons we know so little is because of a lack of genetic tools to modify the chloroplast. The chloroplast has some of its own DNA and this contains the information to make proteins that are important in photosynthesis. If we could modify the dinoflagellate chloroplast, we could put in genes for proteins that would help to tell us what is going wrong and how we might make it better. The problem is that dinoflagellates are very difficult to modify genetically.

Recently we managed to modify the chloroplast of another dinoflagellate, called Amphidinium. This strain does not form coral symbioses, but importantly our work shows that genetic modification of dinoflagellate chloroplasts is possible. For technical reasons the method does not work with the coral symbiont Symbiodinium, but we think we have found a way round these problems, based on research into Symbiodinium chloroplasts. We have made strains of Symbiodinium that are not able to grow well by photosynthesis because they have lost one of their chloroplast genes. Putting the gene back will allow the strains to grow well again.

Our project will test if we can successfully get the missing photosynthesis gene back into dinoflagellate chloroplasts that have lost it. This will make our non-photosynthetic strains able to carry out photosynthesis once again. This technique will open the way to making controlled changes to the dinoflagellates in the future. As we can reconstitute Symbiodinium into their coral hosts, our ability to modify dinoflagellate chloroplasts genetically will open up a whole new set of tools for studying biochemically what goes wrong in coral bleaching and how we might be able to avoid it happening.