Cyanobacteria are the smallest and simplest producers on the coral reef. Producers, or autotrophs, are organisms that are capable of making food molecules. Cyanobacteria are members of the kingdom Monera and have been on Earth longer than any other living thing. Cells just like them are believed to have formed the reefs of ancient seas on early Earth. Cyanobacteria are still abundant on present-day coral reefs, although they play different roles than those of their ancestors. Some types of cyanobacteria provide life-sustaining food and oxygen for the coral reef system, but others are responsible for disease and death.
A few species of cyanobacteria are capable of nitrogen fixation, a job that falls to heterotrophs, organisms that cannot produce food but must consume it, in many other ecosystems. In all cells nitrogen is an essential element that is used to make proteins and DNA, the genetic material that carries each cell’s blueprint. There is plenty of nitrogen gas in the atmosphere and dissolved in ocean water, but the majority of cells cannot capture and use it. Cyanobacteria are one of the few organisms that can take in atmospheric nitrogen and change it into life-supporting nitrogen compounds.
In the reef cyanobacteria also supply nutrition for animals such as sponges that filter their food out of the water. In addition these monerans are captured and consumed by heterotrophic protists (protozoans) and small animals.
Under optimal conditions, cyanobacteria can reproduce rapidly, doubling their numbers within hours. Periods of fast growth can lead to population explosions, events that are also called algal blooms. In a bloom, the numbers of cyanobacteria increase so rapidly that the organisms form a dense blanket in the upper layers of the water. On a reef an algal bloom could block sunlight needed by photosynthetic organisms living deeper in the water. Low levels of sunlight slow reef productivity.
A few species of cyanobacteria also cause diseases in corals. Coral diseases are one of the primary reasons that living corals are lost to reefs. White band and red band are among several “band diseases” attributed to cyanobacteria. Black band disease, one of the worst and most extensive, is caused by a species of cyanobacteria named Phormidium corallyticum, although other microbes may also be involved. This particular disease can move across a reef quickly, killing it at a rate of a half-inch a day. In cases of severe infection, all of the coral animals may be destroyed.