Are corals fighting back?

Corals survive via a mutualistic and obligate symbiosis with dinoflagellate algae of the genus Symbiodinium. These symbionts, called zooxanthellae, provide their coral hosts with the majority of their carbon, and therefore enable the corals to survive, grow and ultimately, reproduce. Acquisition of these symbionts is generally done by one of two methods. Vertical transmission, means a specific zooxanthellae will be passed from the parent coral to their offspring. This saves the progeny of having to find and acquire their symbionts, but it also very limiting. If a coral planulae has acquired the same symbionts that its parent had, then it is likely to run into trouble if its environmental conditions change, as the symbiont is likely to be better suited to the parental environment than the new one. There is much less room for adaptation. The alternative is horizontal transmission, where coral planulae begin life with no zooxanthellae, and soon have to find and acquire their symbionts from their environment, allowing them to take up environmentally relevant symbionts, and therefore are often better able to adapt to the ever changing environment.

Byler et al. (2013), studied the coral species Stylophora pistillata, a coral which has shown it is able to acquire zooxanthellae via both mechanisms, a rare phenomenon. They studied individuals in the planula, juvenile and adult stage, from shallow and deep habitats, over a 3 year course, using PCR, DGGE and Real Time PCR to detect and identify both dominant and low-level symbionts within the coral samples.

Their results showed that not only can this species use both forms of acquisition, but that there is also a difference in its ability to do so based on the type of habitat it was taken from. To begin with, the adult stage and the planulae symbionts from each habitat type were identified. It was stated that while the shallow water corals only hosted A1 type symbionts (both the adult and the planulae), the deep water planulae only hosted C72 type symbionts initially, and later on, in their adult lives, also played host to low-level symbionts, such as those from clade A.

The most likely, and simplest explanation for this is that the corals are able to horizontally acquire other surrounding (free-living) symbionts somewhere between their planula and adult stages, as when adults which hosted more than one type of zooxanthellae created new offspring, their offspring were found to only host the most dominant (original) symbiont in their initial stage of life.

While subsequent vertical transmission of a symbiont which has been acquired horizontally has not been seen over the course of this experiment, that is not to say that it isn’t possible, as it is likely that it could take 100’s or 1000’s of years for this to evolve.

I feel this paper provides an interesting insight into an adaptive mechanism not previously thought to occur in the natural environment. While it is in no means a quick fix for corals under the threat of climate change, and not even an event that is applicable to all corals, it does show that we don’t yet fully understand even some of our most researched organisms. I believe it would be interesting to see exactly what advantages this multiple acquisition method gives the corals, and to what extent it works under changing conditions.

K. A. Byler, M. C.-V. (2013). Multiple Symbiont Acquisition Strategies as an Adaptive Mechanism in the Coral Stylophore pistillata. PLOS.

Available at:

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0059596