Corals are animals that live in tropical oceans around the planet. The tropical oceans don’t have a lot of food floating in the water so for 100’s of millions of years corals have learned to work with plants (some marine plants are called algae. You can see these algae as little brown spots in the coral polyp photo to the left) and bacteria to better survive in tropical waters. This collaboration called a symbiosis works because the coral provides a home for the algae and the algae pay rent in the form of sugar. This is easy for the algae, it captures sunlight and converts solar energy into sugar (the same photosynthesis that goes on in terrestrial plants). The sugar is a critical source of carbon for the coral and provides a large percentage of corals energy. Food for shelter has been a successful partnership for millions of years.
But…of course there is a but. The algae can’t function in high seawater temperatures. The process of photosynthesis breaks down and they can no longer create the sugar they need to pay rent. In fact high temperatures cause so much stress to the algae they produce oxygen radicals. These toxins are bad, they are bad for people, zebras, and even corals. These oxygen radicals damage cells, proteins, fats, all the basic building blocks of living creatures. Needless to say they are the source of much of the world’s stress! There are many ways to combat these toxins, a current diet fad for humans is to eat antioxidants…those are the chemicals that break down these toxins.
Unfortunately blueberries can’t protect corals from stress. In some cases corals produce enzymes that help to detoxify oxygen radicles. But often this isn’t enough and the symbiosis collapses. The algae leave the corals just as you would evict a tenant that doesn’t pay rent. This turns the coral from its normal yellow-brown color to a bright white (you can see this in the photos on the right, some coral colonies are still brown and some have bleached and are pale white). This is called coral bleaching because the coral has changed color to a bright white, just like bleaching your cloths. Ok, but if there are no algae then what do the corals get to eat? Very little…thus the big deal. Coral bleaching is a big deal, it is the break down of a critical symbiosis in a creature that builds habitat and reefs all over the world. This breakdown does not kill the coral immediately, but the corals do go hungry. If bleaching lasts for a few days to a few weeks, most corals can survive and regain their algae. But if high seawater temperatures persist for a long time corals will die. Talk about stress!
So bleaching is bad and corals are threatened by high seawater temperatures. Of course it is complicated…it's biology. Some species of corals bleach some don’t. Some corals live in really high seawater temperatures (like those found in the Red Sea) and don’t bleach. Some corals bleach every year even with little change in seawater temperatures. Some times every coral on a reef bleaches and other times two individuals of the same species that are right next to each other are bleached and unbleached (illustrated in the photos on the right). These are the kinds of questions that scientists are currently pursuing. We still don’t really understand why bleaching happens to some corals but not others and what we can do about it.
Well coral bleaching is a threat and it is important for everybody to be aware of, but what can we do, this is a big problem. Well for one you can act locally. Recent research is showing that healthy corals are more resistant to higher seawater temperatures. This boils down to good local conditions ensure persistence of corals during extreme temperature events. Local habitat quality is an important issue for corals because they live in shallow waters next to tropical islands. This is right next to where all the people live. So how people take care of the land directly impacts the water quality of near shore habitats like coral reefs. If we cut down trees we create sediment run-off that goes into streams and is delivered right on top of corals often smothering them. If we dump oil down drains, this flows into near shore habitats contaminating corals. If we overfish a reef we change the dynamics of the ecosystem shifting the habitat away from corals towards fast growing algae. This means less space for corals and competition for the little bit of clear space that still exists. What we need to learn is even though we don’t see all the underwater creatures every day, they are directly impacted by the daily choices that we make. Our choices influence the amount of “stress” we create for many different organisms.