For a basic introduction to the plants of the coral reefs, the categories are simple: most of them are either algae or phytoplankton! Corals are classified as animal, even if they are microscopic animals which group in massive numbers to form colonies.
At the bottom of the coral reef food chain are the plankton. There is some confusion about plankton as the name includes both animal (zooplankton) and plant (phytoplankton) species. The word comes from the Greek "Planktos", or drifter, or wanderer, and phytoplankton do wander as the tides and currents carry them. Some phytoplankton are able to move on their own. Phytoplankton range from the microscopic in size to the huge kelp fields.
Phytoplankton are predominantly diatoms and dinoflagellates, where diatoms are microscopic, single celled plants that have two shells, giving them a glossy appearance. By the billions, the shells of dead diatoms build up, making a substance that is useful in filtering water and cleaning up other problems. The mass of these shells is called "diotomaceous earth". Dinoflagellates have white shells that allow them to move around with a whipping motion.
Of all of the plants in the ocean and in the coral reefs, phytoplankton make up 90 percent of the population of plants in the ocean and in the coral reefs. They reside in the upper regions of the water because, as with all plants, they need sunlight for survival. When the surface of the ocean is colder, the additional nutrients needed by phytoplankton, such as iron, silicic acid, nitrate and phosphate come up to the surface in the upwelling of biological nutrients in the ocean.
In warmer El Nino oceans, there are problems with the nutrition and the phytoplankton can die. These plants take in C02 and produce oxygen. In taking in the C02, they are helping to reduce the temperature of the water and completing a cycle of balance and life. Phytoplankton account for about half of the photosynthesis on the planet, and thus about half of the oxygen that is produced by plants.
Algae, or seaweed make up another major classification of ocean plant, but are much simpler in construct than terrestrial plants. Algae can be single celled or multicellular and the species once included cyanobacteria, which complete photosynthesis from the processes of thylakoid membranes, making some similarities to plants. Algae, however conduct photosynthesis via chloroplasts, which are organs that are bound by membranes. They type of chloroplast is used to determine the different species of algae.