Being in the water educating people about coral reef, there are question or idea that are often difficult to apprehend for non marine biologist peoples, and we often have to find ways to answer these in a simple manner.
One of these question that we are often asked is: Why Symbiosis?
The answer to this is that corals and their mutualistic (benefiting both host and symbiont algae, Zooxanthellae) symbiotic relationship is the foundation of all life on a coral reef.
Corals thrive in nutrient (Nitrogen, Phosphorous, Carbon…) poor water, because in nutrient rich water macro algae (such as Kelp for exemple in temperate waters) are much more efficient at absorbing these and grow much faster. Then they outperform corals, and often kill them.
But in a Nutrient poor water, macro algae don’t have enough food to fuel their rapid growth.
Corals, through their association with micro-algae, are recycling all the waste nutriments derived from prey capture and its digestion through their symbiosis with zooxanthellae micro-algae. Thus the micro-algae receive Nitrogen and Phosphorous otherwise very rare in the surrounding waters. They use these nutrients to produce glucose that they supply a large part of to the coral host, in exchange to shelter and these nutriments.
This mutualistic association allows the coral to survive in nutrient poor environment, and be the foundation of a tremendous amount of life that derive from this association. From fish, to shrimp, though star fish… they all depend on the coral as their life chain foundation to survive.
That is why coral thrive in nutrient poor water such as islands in the middle of the Ocean, Sea next to desert, or next to healthy rain forest, where all nutrients are retained and recycled inside the forest. And nothing leach out to the ocean, or if it does, mangrove and seagrass will filter it out.
The problem comes, now, when we, humans cut down the forest, replace it by plantation, that leach out tremendous amount of nutrients, from fertiliser, that enter the ocean, give a chance to macro-algae to thrive, out compete coral, and kill coral reefs. And this is the major cause of coral reef habitat loss together with climat change…
It’s easy to see that actually in remote area, preserved of pollution, land clearing, plantations, reefs are doing fine. It’s only when too many humans get close to them that they start dying… Corals are just allergic to humans !
Pollution is still the number one Coral killer, before climate change, and it depends on all of us to change this.