Phytosynthetic associations

Green hydra- Chlorella symbiosis

Hydra are common inhabitants of freshwater lakes and ponds, where they feed on small animals. Hydra viridis contains the green alga Chlorella. Algae reproduces asexually within the gastrodermal cells and a single hydra may contain about 150,000 algal cells. Under normal conditions, symbiotic algae are not digested by hydra. There are two reasons for this: first, the cell wall of algae contains sporopollenin, a protein that resists digestive enzymes; second, vacuoles containing algae do not fuse with lysosomes, the organelles that contain digestive enzymes and normally fuse with food particles. But if a digestive cell takes in more algal cells than normal, the extra cells are either digested or ejected. A bilateral movement of nutrients takes place between the symbionts. Algae supplies the animals with photosynthetic products such as maltose. At an acidic pH level, almost 60% of the carbon fixed by the algae is excreted as maltose, but at a neutral pH level, very little maltose is excreted. The rapidly hydrolyzed maltose is converted to glucose, and then glycogen is produced. Algae also provide the animal with oxygen, which they produce during photosynthesis. Hydra provides the algae with nutrients, including precursors of proteins and nucleic acids, and a protected place to live. As digestion is avoided and the host cells are able to regulate algal reproduction, the sym-

A sponge (Suberites ficus) gets more food by travelling with a hermit crab (Pagurus beringanus). The hermit crab profits from the sponge's protective cover. (Photo by ©Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)

biosis that exists between H. viridis and Chlorella appears to be a finely tuned, nonpathogenic equilibrium.

Marine algal-invertebrate symbioses

Many marine invertebrates, such as sea anemones, coral, and flatworms have formed mutualistic symbioses with the photosynthetic algae known as the dinoflagellates. Their chloroplasts have efficient light-harvesting complexes that include chlorophyll a, chlorophyll c, and large amounts of xan-thophylls. A common dinoflagellate of marine invertebrates is Symbiodinium microadriaticum, and this is greatly modified when it lives inside animal cells. The algal cell wall becomes thinner, loses the groove and flagella, and divides only by binary fission. In the host animal the algae excrete large amounts of glycerol, in addition to glucose, alanine, and organic acids. When the algae are isolated from animals and grown in culture, they stop excreting these substances.

Sea anemones and jellyfish

The sea anemone Anthopleura xanthogrammica, contains two types of symbiotic algae: zoochlorellae and zooxanthel-lae. The relative proportion of each algal symbiont in the animal depends on the water temperature. The anemones position themselves in ways to increase the exposure of their symbionts to light.

Cassiopea xamachana is a jellyfish that has been used to study how an invertebrate selects its algal symbionts. The lifecycle of Cassiopea includes a sexual medusoid stage, which contains algae that does not swim freely, but rather lies upside down in shallow waters, a behavioral adaptation that allows the algae in its tentacles to receive maximum daylight for photosynthesis, and gives the animal its common name, the upside-down jellyfish.

Anemone-clownfish symbiosis

Fishes of the genera Amphiprion, Dascyllus, and Premnas, commonly called clownfish, form mutualistic associations with giant sea anemones that live in coral reefs throughout the Pacific Ocean. The association is obligatory for the fish, but facultative for the anemones. The anemones eat prey that have been paralyzed by means of poisonous nematocysts discharged from specialized cells in their tentacles. The clown-fishes are immune to the stinging nematocysts and can nestle among tentacles without harm. Some clownfish go through a period of acclimation before they become immune to the anemones' poison. Symbiosis with the anemone changes the mucous coating around the fish and the fish is no longer recognized as prey by the anemone. Clownfishes are brightly colored and marked, and attract larger fish to the anemone. These fish, if they come too close, are stung by the tentacles and eaten by the anemone. The clownfish share in the meal. A similar relationship exists between the Portugese man-of-war (Physalia physalia) and the horse mackerel (Trachurus tra-churus). The bright blue and silver color of the fish, as well as its small size, attract prey for the man-of-war.

Reef-building corals

The symbiotic association between Symbiodinium-reef-building corals (Scleractinia) is of great importance in marine tropical ecosystems and has been the subject of many studies. Coral reefs support large communities of organisms. Coral polyps excrete a calcium carbonate shell around their body. As the polyp dies, the shells harden, and new polyps grow over them. After many years of this process, coral reefs are formed. Symbiotic dinoflagellates live inside nutrient-rich cells of the gasterodermis of the coral polyp. In some corals more than 90% of photosynthate may be released by the sym-biont to its host cell. The algae supply the coral with oxygen, carbon, and nitrogen compounds. The animal obtains vitamins, trace elements, and other essential compounds from the digestion of old algal symbionts. Animal waste products such as ammonia are converted by the algae into amino acids, which are translocated to the animals. Such a recycling of nitrogen is an important feature in the nitrogen-poor habitats of coral. Coral bleaching is caused by the loss of symbiotic algae from the host and may be caused by environmental stresses such as global warming, pollution, and increased ultraviolet radiation.

Green flatworms

Convoluta roscoffensis is a small marine flatworm that lives in the intertidal zones of beaches in the Channel Islands of the United Kingdom and in western France. The worms are 0.08-0.16 in (2-4 mm) long and deep green in color from the algae they contain. During high tide, the worms are buried in the sand, but at low tide, during daylight, they move up to the surface. During this time the green algal symbiont, Tetraselmis convolutae, photosynthesizes until the next high tide. The Convoluta-algal symbiotic relationship is an early example of detailed studies (1910) that attracted public attention to the broader significance of symbiosis in nature.


Some marine tunicates contain an unusual photosynthetic symbiont, Prochloron, that has characteristics of both cyano-

Coleman's shrimp (Periclimenes colemani) lives exclusively with the venomous fire urchin (Asthenosoma varium). They are found often as a male/female pair on a bare patch that they have created by removing the urchin's tube feet and spines. They can move through the venomous spines without being harmed. (Photo by ©David Hall/Photo Researchers, Inc. Reproduced by permission.)

Coleman's shrimp (Periclimenes colemani) lives exclusively with the venomous fire urchin (Asthenosoma varium). They are found often as a male/female pair on a bare patch that they have created by removing the urchin's tube feet and spines. They can move through the venomous spines without being harmed. (Photo by ©David Hall/Photo Researchers, Inc. Reproduced by permission.)

bacteria and green algae. In some tunicates the symbionts lie within a cellulose matrix that surrounds the outer surface of the animal, whereas in other tunicates symbionts are loosely attached to the cloacal wall. The larvae of some tunicates have specialized pouches that carry Prochloron cells that they obtain from the parent.

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