Some of the most important evolutionary adaptations to
result from coevolution are the mutualistic symbiotic relationships that allow
two or more species to thrive through mutually beneficial interactions. An
example occurring in one of the most ecologically, economically, and
aesthetically important ecosystems on Earth is the coevolution of hermatypic
(reef-building) corals and their algal symbionts of the genus Symbiodinium. The algae are incorporated
into the coral cells where they produce carbohydrates through photosynthesis,
passing this on to the animal tissue and receiving nitrogen and phosphate
compounds(1).
For many years it was
thought that individual coral species were very specific as to which clade or
type of symbiont is hosted*. Recently, with the aid of larger sample sizes more
sensitive molecular techniques, it has been shown that corals often host
secondary types of symbionts and are able to change which clade is dominant
within its cells, especially after periods of stress or coral bleaching(2). In "The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change (2006)," Ray Berkelmans and Madeleine van Oppen tested the hypothesis that this sort of adaptation could help corals survive in a future sure to me marred by global warming. |
| Acropora millepora(3) |
In this study, Berkelmans and van Oppen transplanted colonies of the common, bleaching sensitive coral species Acropora millepora from a cool offshore reef to much warmer inshore sites on the Australian Great Barrier Reef. The result was that many colonies pale, bleached, and subsequently died from the unfamiliar environment. However, a few colonies were able to bounce back from paling and bleaching when the abundance of clade D symbionts, a group much more thermally resistant than the previously dominant clade C, began to increase. Together, A. millepora and clade D Symbiodinium were able to adapt to the changing circumstances and begin to reclaim the dead carbonate skeleton of the bleached part of colonies. This provides some hope that the beautiful coral reefs of the world will be able to survive when our world inevitably continues to increase in temperature.
 |
| Results of Berkelmans and van Oppen's 2006 study. |
*Type is a transient term, used because some types of Symbiodinium have such genetic
similarity they may at some point be proven to be the same species. Clade is an
intermediate classification of the Symbiodinium
genus into groups A-H. For example one type of algae is B3.
Sources:
1. Pearse, Vicki & Muscatine, Leonard. Role of
Symbiotic Algae (Zooxanthellae) in Coral Calcification, 1971. http://www.biolbull.org/content/141/2/350.full.pdf+html
2. Berkelmans, Ray & van Oppen, Madeleine. The role of zooxanthellae in
the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era
of climate change, (2006). http://rspb.royalsocietypublishing.org/content/273/1599/2305.full
In class we have discussed how natural selection is not forward-looking. It's really interesting to see a case in which an adaption suited for today has the potential to be very useful in the future.
ReplyDelete-Mary Morales
I think the adaptive bleaching hypothesis is very interesting and I am curiouss to see what evidence future research will provide on this topic. The fact that such a catastrophic event can cause a shift in the dominant type of Symbiodenium seems really promising with the current threat of global warming! However, in order for this to happen clade D must be present in some rare abundance within the colony prior to the bleaching event. It would be interesting to see which coral species host this clade and test whether this hypothesis holds true for them as well!
ReplyDeleteIt is interesting that the different clade of algae need not already be present in the coral polyp when bleaching occurs, though this is very possible. New Symbiodinium can also be absorbed from the water column and incorporated into the coral cells through a process called horizontal transmission.
DeleteIt's interesting to compare our predictions with what will actually occur in the future for coral survival. As Mary mentioned, natural selection is not forward-looking itself, but I wonder if this knowledge about symbionts could lead us to influence the natural selection of corals in any way due to our conservation efforts.
ReplyDeleteThis is a very interesting blog post. Its nice to see you highlight the connections between the material that we learnt from two different classes - coral reef and evolution.
ReplyDeleteWhile this adaptive bleaching hypothesis can help save some corals species during a warming event, I worry that majority of the coral reefs and their symbionts might not be able to adapt fast enough to cope with the rapidly changing global conditions, most of which are cause by mankind.
On a separate note, I wonder if the rising sea temperature will also lead to a evolution of the different Symbiodinium clades towards being more thermal-resistant. By being more thermal-resistant, these Symbiodinium are then able to remain with their coral host to tap on the mutualistic benefits.
Thanks for the comment, Ben. You are quite right that while this does provide a sliver of hope, the future is still looking quite rough for the coral reefs of the world.
ReplyDelete