You've likely heard of the many different colors corals can have, and that they lose their colors in a process called coral bleaching. You may also have heard that most corals need sunlight to live. Something less known about the oceans is that in certain times and places, they glow! And sometimes, oceanic coastlines are covered in mats of red--entire areas where fish die and float to the surface, causing a terrible stench and a sharp decrease in human visitors. How do all of these phenomena relate? Each is caused by a single kind of algae known as dinoflagellates. They are all unicellular, meaning their entire body is made up of one single cell (the human body is estimated to be made of 30-40 trillion cells by comparison)--but these tiny creatures have a big impact on the world and life around them!
Various dinoflagellates are depicted below.
Photo by fickleandfreckled on Flickr https://flickr.com/photos/fickleandfreckled, licensed use via https://creativecommons.org/licenses/by/2.0
The dinoflagellates that give corals their color are all part of one genus, Symbiodinium, and are also known as zooxanthellae.
Giant clams also have zooxanthellae in their mantle (a specialized layer of tissue that creates their shells). Some species of giant clam even have lenses that focus light on their zooxanthellae!
Some other groups of animals that can house dinoflagellates include sponges, flatworms, anemones, and jellyfish (look for the "upside-down jellyfish" talked about in my post on mangroves)!
The above examples of cooperation are known as symbiotic feeding. Zooxanthellae make their home under the "skin" of certain ocean creatures and photosynthesize, harvesting energy from sunlight and giving leftover energy to the organism they live in. In return, the host organism provides the zooxanthellae shelter and gives them leftover nutrients from whatever food the host organism is able to capture. This symbiotic relationship ("symbiotic" or "symbiosis" mean "living together") is known as mutualism, an occasion where both organisms benefit.
When corals become stressed by heat, chemicals, or other factors, they may forcefully eject their zooxanthellae or the zooxanthellae may leave on their own initiative. Because the zooxanthellae contain colorful pigments to capture sunlight just like plants do, when the zooxanthellae leave a coral, the coral loses its color--hence the term "bleaching". Roughly 90% of a reef-building coral's energy comes from its symbiotic zooxanthellae. When these dinoflagellates leave, the bleached corals also rarely survive long before dying. When a coral bleaches, however, it's not necessarily the end of that coral. When stress factors such as excess heat or harmful industrial chemicals leave the coral's environment, the zooxanthellae may return and re-enter the coral, eventually restoring its color and ensuring the coral's survival.
Red tide is one of the most unpopular events in the southern, coastal United States. The water turns red with a floating mat of algae, the water and very air near those areas becomes filled with toxins that will make humans ill and kill marine life, resulting in previously clean beaches that are now vacant--other than the hundreds of killed fish that wash on shore. With this description, you can see that red tide can be an dampener on coastal life even without having witnessed red tide firsthand. At least three species of dinoflagellate contribute to much of the red tide events of the United States, which are most common in the Gulf of Mexico, off of California, and in the Gulf of Maine. The dinoflagellates of these regions normally are always present in low, unnoticeable numbers--but in the right conditions once or twice a year, a population explodes in an algal bloom and has a terrible impact on the local area.
The last big topic for dinoflagellates is glowing oceans. In some regions and environmental conditions, dinoflagellate populations will become large enough for one of their most loved adaptations to show. When certain dinoflagellates--such as those that cause red tide--are disturbed by something in their environment, they let off a flash of light. The ability of an organism to produce light is known as bioluminescence. One flash is imperceptible to the eye, but when the dinoflagellates number to millions and more in a small area of water, the effect is astounding. Below is an image of such dinoflagellates getting washed up against a coastline.
Image Credit Johanna Brehmer via WIkimedia, Licensed use under Creative Commons https://creativecommons.org/licenses/by-sa/4.0
I first became interested in dinoflagellates when I learned about symbiosis and glowing oceans. I plan to write more about other kinds of symbiosis and bioluminescence in the future!
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