The Biology and Threats to Coral Reefs
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Corals are animals, and they eat like any animal would. A coral is, simply put, a small sea anemone with a skeleton. Each coral takes the form of an individual polyp and lives in a colony with many other tiny polyps. Each individual coral is a tiny filter feeder that catches plankton - microscopic plants and organisms - for nutrition with its tentacles. But corals have another means of nutrition as well: they harbour microscopic green algae within their tissue. Algae are aquatic plants, and through the process of photosynthesis, convert sunlight into biological energy. In this way, these symbiotic algae power the coral and themselves. Because of the coral’s dual ability to make a living, it’s called a holobiont, a ‘total organism’, incorporating the best of the animal and plant worlds. Especially for many hard corals, which build the most massive calcium carbonate skeletons, the energy derived from photosynthesis makes up the majority of the organism’s energy budget. Hence, clear water, transparent to sunlight, is of paramount importance for the flourishing of a classic coral reef.
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…One [human] activity detrimental to the health of coral reefs is ‘coastal development’, a term that sounds Orwellian to the conservation biologist. Coastal reclamation directly destroys reefs by constructing artificial islands on top of them. And even construction activity on or near the seashore can damage coral reefs: a concrete seawall will filter rainwater flowing into the ocean much less efficiently than an intact mangrove forest. Dense coastal human populations with poor liquid-waste treatment in their communities will make the ocean’s water more turbid and rich in nutrients. These ‘developments’ knock out corals that need clear water for sufficient light to reach their symbiotic algae. In the absence of hard corals, other animals like the soft leather coral Sarcophyton can come to dominate stressed coastlines. Not only does the reduced sunlight harm the hard-coral holobionts, the excessive nutrients serve as fertiliser for other marine algae, often arch-enemies of corals competing with them for space on the reef. Reefs dominated by these algae are a hallmark of many stressed ecosystems in the Caribbean.
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The greatest threat to the stability of the planet’s biosphere is human-caused climate change. The massive amounts of carbon dioxide released into the atmosphere as we burn fossil fuels leads to a rise in temperatures. The physiology [of corals] is very finely tuned to work at the temperatures of sun-drenched shallow tropical reefs, with an optimum just a bit below 30 degrees Celsius. Any temperature above that will cause the algae living symbiotically within the coral to produce so much reactive oxygen as a byproduct of their photosynthesis that their coral hosts suffer damage. Responding to that emergency, corals repulse the algae from their bodies. This is the right move in the short term: the corals remain unharmed by the reactive oxygen. But in the long run they will starve without the energy generated by their algal symbionts, and may eventually die if they do not recover symbiont numbers in time. Since the loss of the algae also leads to the loss of the beautiful colours of the corals, this creates the tragedy on the reef known as ‘coral bleaching’. Hard corals are among the most severely affected by this stress…
