Can coral change their zooxanthellae to overcome thermal stress?

Zooxanthellae provide an extremely efficient source of nutrients for their coral symbionts, meeting up to 90% of their requirements. This symbiosis is under threat due to rising sea temperatures caused my global warming, and coral, along with their zooxanthellae will need to be able to adapt well to prevent widespread dieback. It seems that some coral types can associate with a variety of zooxanthellae symbionts as oppose to just one, possibly making these species more able to cope with the selective pressures that rising sea temperatures will cause. Evidence shows that coral harbouring a larger proportion of D-type symbionts are likely to be more heat resistant than those with other types and those coral reefs that have been subject to repeated bleaching periods are more likely to be harbouring those heat resistant strains.

This paper looks into the possible ability of some coral species to change the zooxanthellae that they form a symbiosis with in order to overcome the stresses caused increased sea temperatures brought about global warming. Colonies of an indo-pacific coral species were transplanted from two areas of cool water into an area of warmer water, already containing a colony of the same species (A. Millepora). Colonies were also tested in their original cooler areas as a control. The zooxanthellae genotypes were tested before transplantation to ascertain which types were present within the coral (all contained less thermally efficient type C2 zooxanthellae while some from one colony were found to also contain more thermally efficient type D zooxanthellae). After a period of 14 months, 12 nubbins of 3-5cm were taken from the transplanted and control samples. These were then placed in tanks and tested for thermal resistance at 27.5 (control), 30, 31 and 32 ^oC.

The authors found that in one of the transplanted colonies (with D type already present), the zooxanthellae type had switched from type C2 and type D to type D only after thermally induced bleaching. Corals from the other transplanted colony however, were found to recover from bleaching with only the original type C2 zooxanthellae. It was said to be unclear whether the switching of zooxanthellae recorded was due to the presence of the D type before treatment, or whether more of the thermally efficient type was taken in by the coral from the environment.

Type D zooxanthellae were found to have a thermal tolerance of only 1-1.5^oC higher than that of type C2. The authors infer that this difference could have huge ecological benefits; however, due to the predicted average rise in tropical sea temperatures (1-3^oC) over the next 100 years, it is not thought that this will be enough to save many coral species.

I think this paper provides a good insight into ways coral zooxanthellae can possibly change to cope with rises in sea temperatures. If used effectively, some of the data from this investigation could buy us some time so that we can continue to combat global warming. Naturally, further investigation would need to be conducted, especially because the results from this paper were found to be inconclusive. Further study could possibly include a wider range of coral species to see whether results vary between them, and whether there are many that could make use of more thermally resistant symbionts.

Reference:

Berkelmans, R., van Oppen, M.J.H. (2006) The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change, proceedings of the royal society, 273, 2305-2312