As the term suggests, coral bleaching is the whitening of living coral colonies, a phenomenon associated with anthropogenic changes in the animal’s habitat which lead to the weakening of the colony’s health and its eventual death. Let’s take a look at what exactly is being bleached, what’s causing the bleaching and why bleaching is so devastating to these animals and their ecosystem.
On the left is a healthy piece of living coral. On the right is the same colony only one month after conditions in the surrounding water changed. These photos were taken in the Florida Keys by M. Brandt, of the Rosenstiel School of Marine and Atmospheric Science in 2005, a year that was a particularly bad one for corals around the globe.
What is Being Bleached?
Corals, and many other tropical and sub-tropical species of marine invertebrates, depend on a symbiotic relationship with a microscopic algae known as zooxanthellae that live within the tissue cells of the animal. In addition to coral, these other animals include some species of jellyfish, such as cassiopeia, sea anemones and giant tridacna clams, like the one pictured below.
Pretty much all of the animals that take part in this symbiosis are filter feeders that would normally subsist on floating plankton in the surrounding water. As I’ve mentioned in other diaries, tropical seas are notoriously deficient in plankton, so tropical filter feeders need to come up with additional food sources.
Symbiosis, of course, is an intimate relationship between two different species. There are several types of symbiotic marriages ranging from parasitism, where one benefits and the other is harmed, commensalism, where one benefits and the other is unaffected, to mutualism, where both species benefit. The coral/zooxanthellae relationship is a mutualistic one. The algae has a safe substrate to live on which gives it access to sunlight for photosynthesis, and the coral polyp is nourished by the metabolic end products produced by the algae.
I want to focus on mutualism for a bit because it’s really important to understand this concept in order to understand the seriousness of bleaching. We can further divide mutualism in a couple of ways. One is dependent vs independent mutualism. In the former the two species cannot live apart. A good example of this is the termite and the protist which lives in its gut. Independent forms can survive without each other, but basically life would be much easier for both if they worked together. A well known example of this would be the clownfish and sea anemone. In the world of zooxanthellae and their hosts the relationship is mutualistic dependence. The two cannot survive alone. In some coral species up to 90% of their nourishment comes from the algae, and many zooxanthellae species are host-specific; they can’t survive in the tissues of any other organism.
Another mutualism division is based on the ideas of service and resource. In other words each beneficiary receives either a service (access to sun) or a resource (provided with food). Relationships can be service/service (clownfish/anemone), resource/resource (termite/protist) or service/resource (honey bee/flower). The algae/coral relationship is a complicated form of all the above, what I like to call service-resource/resource-service. The algae provides the coral with oxygen and nutrients from photosynthetic wastes (resource) and metabolizes the coral’s waste, which is vital to limestone production (service). The coral provides the algae with a sunlit surface (service) and waste products that the algae can feed on (resource). Let’s move on, but first a pretty picture of a coral polyp infused with orange zooxanthellae:
So now that we’ve established the relationship that exists between these two species, you can probably see what happens when one of them starts to die off. And to make matters more complicated, corals are colonial animals. What we see as a coral head is really an interconnected group of dozens, or hundreds, of individual animals. Each animal (called a polyp) has it’s own mouth and food-collecting tentacles but all of them share a common digestive system. What happens to one individual can impact the health of the rest. The positive side to this is that polyps on a shaded area of the colony can be nourished by the more efficient feeders on the sunlit side. The flip side of this is that when members start dying off the entire colony is weakened.
When coral bleaching occurs the entire coral colony turns white, like the dry coral skeletons they sell in gift shops. Coral polyps are transparent, the color of the colony comes from the zooxanthellae living in the tissues. Red coral have red algae, yellow coral has yellow algae, etc. Environmental stress can cause the zooxanthellae to be expelled from the tissues by the host, leaving only the white coral skeleton visible beneath the transparent polyps. Generally the zooxanthellae don’t actually die off as is often assumed, although why they are expelled by the coral isn’t clear.
What’s Causing Bleaching?
The number one reason a coral colony will expel its symbiotic algae is from stress due to increased temperature of the surrounding water. Other factors that have been shown to cause bleaching include ocean acidification, an increase in solar ultraviolet radiation, sedimentation from land runoff due to coastal construction or increased erosion and changes in salinity.
All of these factors can have direct or indirect anthropogenic causes. Some can also be due to natural conditions such as El Nino events, however it should be kept in mind that a reef system that is surviving, but weakened by man-made stress conditions, will be less likely to survive or bounce back from natural events.
The ability to recover from severe bleaching requires that the colony return to a pre-stress condition so the symbiotic algae can repopulate the polyps. Bleaching that lasts too long will kill off the colony before this can happen, but recovery varies greatly among coral species. In general the massive corals bounce back quicker and have higher survival rates than the more delicate branching species.
As mentioned, elevated seawater temperature constitutes the greatest threat to reefs, but ocean acidification should be examined as well. As levels of carbon dioxide increase in the atmosphere, more and more of it is absorbed by the oceans. CO2 forms carbonic acid (it’s what stings your nose when you drink a coke) which inhibits the formation of carbonate minerals such as limestone and aragonite (the main component of coral skeletons). Climate change will continue to increase both of these stresses as time goes on.
Corals will survive in one form or another. They’ve made it through mass extinction events before, and they will again. Perhaps with the survival of deep water species eventually evolving and colonizing the shallows in a few million years. We will, however, lose the coral reefs as we presently know them.
Other diaries in this series can be found here.
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