Protists are another group of one-celled organisms, although these cells are larger and more complex than the cells of monerans. There are many types of protists living on the reef. Some of the key players are diatoms, dinoflagellates, and foraminiferans.
Like most groups of protists, diatoms are a highly diverse group. They use all types of nutritional strategies, and their ranks include autotrophs, heterotrophs, and mixotrophs (organisms that are both autotrophic and heterotrophic). Some species are capable of rapid movement, while others are stationary during their entire lives. Despite their differences, all diatoms have some characteristics in common.
The fragile cells of diatoms are covered with protective shells, or frustules. Each frustule contains a large component of silica, the same material that is found in sand and glass; therefore, diatom frustules look like tiny glass hat boxes that are topped with lids. The silica shells of several marine diatoms are visible in this Figure. These delicate silica structures are pierced with openings that permit the organisms inside to interact with their watery environments. The geometric patterns created by openings in frustules are unique to each species and serve as a method of identification.
|Marine diatoms may be round or|
elongated in shape.
(Courtesy of NOAA, Coral Kingdom Collection)
Diatoms create frustules in two basic shapes: round or elongated. Species that make round frustules generally inhabit phytoplankton, so they float in the water column. The elongated, or pinnate, organisms make their homes on the sand, in sea grass, and among algae. Most reef diatoms are elongated forms that inhabit the reef floor.
As in many protists, reproduction in diatoms is by binary fission, an asexual method. Asexual reproduction involves only one parent and forms clones, or exact duplicates, of the parent. Binary fission, however, poses a challenge to diatoms that cells of other species of protists do not face: The frustule must also divide, along with the cell. When a parent diatom splits apart, it forms two “daughter” cells. Each daughter inherits one portion of the parent’s frustule; one daughter gets the larger top portion of the frustule. The other receives the smaller bottom piece. The inherited portions become lids for each daughter cell, and both daughters grow new lower parts. The result is that one cell is the same size as the parent, and one is smaller. Over several generations, frustules become too small to divide further so the organisms leave their old, undersized shells. Afterward, the cells either undergo a period of growth, then secrete new, spacious frustules, or they go through sexual reproduction.
Sexual reproduction can occur in several ways. In some species, a small diatom cell breaks apart into little pieces, each of which swims around until it finds another diatom cell with which it can fuse. The product of their fusion builds the new frustule. In other species, two adult diatom cells line up beside each other. They divide, then exchange one daughter cell. The new pairs of daughter cells fuse, resulting in two new cells. In both cases, the resulting cells have genetic material from two parents.
Most marine diatoms are autotrophs and are critically important producers in reef food chains. For this reason, green diatoms are nicknamed the “grass of the sea.” The few species that cannot photosynthesize live in places that are rich in dissolved organic matter, which they absorb. Some species have the best of both worlds, photosynthesizing when light is available, absorbing nutrition when it is not.
On coral reefs, diatoms serve as meals for other organisms. Larger protists prey on them, and animals that graze sea grass and algae consume them as they feed on the plants. When conditions are good, growth of autotrophic diatom populations can lead to a diatom bloom. Although these blooms can be beneficial to the reef ecosystems by providing plenty of food for grazers, they often create problems. Some species of diatoms manufacture toxins that are designed to discourage predators. Many of these chemicals do not harm the shellfish that eat the diatoms, but they do accumulate in the bodies of those animals. When people eat contaminated shellfish, they can become very sick. One species of diatom causes a condition in humans called amnesic shellfish poisoning, with symptoms such as memory loss, disorientation, and coma. Severe cases can be fatal.
There are hundreds of species of dinoflagellates in the ocean, and several of them inhabit coral reefs. A dinoflagellate has two flagella, long, whiplike structures that propel the organism through the water. One flagellum fits into a groove that wraps around the cell, and the other sticks out behind the cell. Free-living dinoflagellates are covered with protective plates made of cellulose, the same material that forms the woody parts of plants.
Like most species of protists, dinoflagellates usually reproduce by binary fission. In the armored species, each daughter cell inherits half of the armor, then makes the other half. Occasionally, two cells will fuse to form a single large cell that contains the DNA of both parental cells. The large cell splits into two daughter cells, each with new combinations of DNA.
Dinoflagellates have developed several strategies for getting their nutrition. About half of the species contain chlorophyll as well as accessory pigments for photosynthesis. Accessory pigments are responsible for the colors of dinoflagellates, which range from golden brown to green. Most of the heterotrophic species of diatoms are colorless. They feed by engulfing tiny prey or by absorbing dissolved organic matter from seawater. Some species can get their nutrition in both ways, making their own food as well as absorbing it.
In the reef ecosystem, free-living dinoflagellates are prey to many organisms, such as larger protists, fish larvae, and small invertebrates. To discourage their predators, a few species of dinoflagellates use the same strategy adopted by other onecelled autotrophs: They produce potent toxins. Gonyaulax tamarensis is a dinoflagellate that makes a neurotoxin. As reef fish graze on seaweeds, they may consume. G. tamarensis living on the plants. With each feeding the toxin is stored in the grazer’s fatty tissues, building up over time. Toxin levels are highest in the fattest animals, which are usually the most desirable catches. When these animals are eaten by large predators or by humans, the toxin can cause a condition called paralytic shellfish poisoning, whose symptoms include weakness, numbness, dizziness, and slow respiration. Death can occur from respiratory failure.
Another reef-dwelling species, Gambierdiscus toxicus, normally lives quietly on the surfaces of seaweeds in areas of the reef that are protected from waves. However, if predation is high, G. toxicus releases a chemical, ciguatoxin, that can poison fish, shellfish, or humans. Each year, tens of thousands of people worldwide are affected by the toxin, with a 1 percent mortality rate.
In the coral reef environment, some of the most important dinoflagellates are the species that reside inside the bodies of reef-building corals. By living together in a symbiotic relationship, both the host coral and their dinoflagellate guests benefit. The protists supply food and oxygen for the coral. The dinoflagellates provide most of the coral’s nutrition. In addition, the presence of dinoflagellates helps corals form their protective skeletons. In return, the coral provides invaluable services for the dinoflagellates. They supply food and oxygen, protect them from predators, and provide a place to live that is located high in the water, near the sunlight. The relationship between dinoflagellates and corals is not unique; there are other organisms that also support microscopic producers. As a group, symbiotic one-celled autotrophs that live in the tissues of plants or animals are known as zooxanthellae.
In corals, dinoflagellates inhabit the tissues that line the digestive tract. Populations of these protists can be quite dense. With up to 50 tiny green organisms in each coral cell, there may be about 1 million cells per 0.16 square inch (1 sqcm) of coral flesh. Living as zooxanthellae, dinoflagellates lose their cellulose armor and flagella and take on a smaller, rounded shape.
Apparently, the relationship works out well for both organisms. The dinoflagellates are capable of moving out, but they rarely do. In experiments, scientists have removed dinoflagellates from their hosts and found that the little one-celled creatures regained their armor and flagella and flourished as free-living organisms. However, the corals did not fare as well. Without their houseguests, their rate of growth slowed considerably. Foraminifera (forams) are protists that live in shells of different shapes and sizes. Some forams construct one-chambered shells, while others assemble shells that have multiple chambers of increasing size. In a number of species, shells are secreted by the protists, but in others they are constructed of sand and other particles that protists glue together. Many foram shells are large, reaching lengths of several inches.
Foraminifera live all around the coral reef. Some float in the water as single cells or in globules with other cells. Many species live in colonies that encrust rocks or other hard substrates, looking very much like patches of red film or paint. Another type of foram lives on the surfaces of seaweeds. When forams die, their shells settle to the floor. In some parts of the world, foraminifera shells make up 90 percent of reef sediment.
Forams play key roles in the food chains of reefs. They are not photosynthetic, so these protists dine on diatoms, bacteria, and even small fish eggs that they capture with extensions of their cytoplasm called pseudopods. Forams can also absorb dissolved nutrients in the water. Some species take in and nurture zooxanthellae, a strategy that assures plenty of food when prey is scarce. In turn, forams are prey to grazing and sedimentscouring animals such as worms, snails, and some fish.
Individual forams can live as long as two years, quite an advanced age for protists. When it is time to reproduce, cells shed their shells, then break up into many cell fragments. Each of these fragments grows a flagellum and swims away. There are two possible outcomes for these traveling cell parts: They can either develop directly into new forams or fuse with a similar cell fragment to form a cell.
Fungi, like bacterial decomposers, break down organic matter in the coral reef, releasing its nutrients for use by other organisms. As fungi grow on the bodies of dead or decaying organisms, they send out tiny filaments called hyphae. Each filament releases enzymes that dissolve the tissues of the dead organism. Fungi can then absorb these dissolved tissues.
A few species of fungi are capable of causing diseases in organisms that live on the reef. Aspergillus sydowii, a type of fungus related to the kind that grows on old food, has been the source of a tremendous amount of damage to sea fan coral. Although A. sydowii has existed for a long time, researchers are finding that it is becoming more lethal. There are two possible explanations for this change. The fungus may have mutated into a deadlier form, or the corals may be less resistant to disease.