Coral reefs, as most of us think of them, occur primarily in relatively shallow, warm, crystal clear waters. The clarity of the water is a good sign that there are few nutrients in the water column. Yet, the richness of life in these ecosystems is unparalleled. How is that possible?
In terrestrial ecosystem, energy is captured through photosynthesis by plants, and incorporated in sugars by combining the carbon available in carbon dioxide with the hydrogen in water, producing oxygen in the process. But on a healthy coral reef, there are no plants, and generally only a few macro-algae that can do the same thing. There are, however, small single-celled organisms that take the place of plants. These have variously been placed among the plants, because they can photosynthesize, but are now treated as protozoans (≈first animals). They do not live in the water column, but in the tissues of the corals. In fact, they are the organisms that give corals their colour – generally green or brown. They are special mutualistic dinoflagellates (Phylum Dinozoa) called zooxanthellae. These tiny organisms capture light, and provide the anemone or coral with carbohydrates (sugars) that can be used for growth and reproduction. In fact, most corals kept in a reef tank do not require feeding, but can survive solely on the photosynthetic products provided by their tiny helpers. Some corals do require feeding, but for the most part they survive on photosynthesis alone.
Which corals are “solar powered”, and which ones are not? In general, corals that are green or brown tend to be dependent on their symbiotic zooxanthellae, whereas yellow and red corals may depend more heavily on capturing organic food. In reality, most corals can subsidize their nutrient requirements by capturing food particles and plankton. But without a strong light source, it is very difficult to maintain healthy corals in a reef tank. Depending on the type of coral kept, sufficient light may be provided by fluorescent lights, but a stronger light source is required for most of the reef-building species. Until recently, the main option to fluorescent lighting was metal halide lights, which provide excellent light, but are expensive and produce a lot of heat. Recently, LED technology has emerged which provides good lighting, produce little heat, are long lasting, and can be programmed to provide more natural transitions between night and day (http://www.pacificeastaquaculture.com/lighting.asp).
One of the main threats to coral reefs is increasing water temperature. When water temperatures increase by even a few degrees, the result may be “coral bleaching”. Thermal coral bleaching is the result of the corals expelling their symbionts, thereby losing their colour and appearing white. A recent study indicated that this may be due to changes in the interactions of the coral and the zooxanthellae (http://www.biomedcentral.com/content/pdf/1472-6793-9-14.pdf). In fact, a coral bleaching event occurred in the UNBC reef tank a few years ago, when the installation of a new and more powerful sump return pump caused a sudden increase in water temperature. In addition, bacterial infections and other external factors may cause coral bleaching.
In summary, even at the most basic level of coral reef functioning, the symbiosis between two different types of organisms. Many more obvious symbiotic interactions occur as well, and some of those will be the subject of future blog posts.