Algae can produce their own food by photosynthesis, so why do they need to associate with other organisms? The answer is simple: protection. Single celled algae are susceptible to predation and water loss. By themselves, they cannot survive out of water and they have no protection against herbivores. So, over millions of years, algae have joined together in mutually beneficial relationships with a number of different organisms. Such a relationship, where both partners benefit, is a form of symbiosis known as commensalism.
The most primitive algae are the blue-green algae, which are basically bacteria with photosynthetic capabilities. They have no nucleus in their cells and their scientific name is Cyanobacteria. Most blue-green algae are free-living but some have developed symbiotic relationships with such animals as sponges and echiuroid worms. They live in the tissues of these animals, where they gain protection from predators and in return, they fix nitrogen and photosynthesise, providing an extra food source for their hosts. They also, secondarily, colour their hosts.
Most algae are true cells, with a nucleus (Eukaryotes). Many of these algae are free-living in both fresh and salt water, but many more have developed symbiotic relationships with animals. The most important in the marine environment are the algae known as Zooxanthellae which, in their relationship with coral animals, form the basis of the rich and diverse coral reef ecosystems.
Zooxanthellae live in the tissues of the tiny coral animals, which resemble sea anenomes. The coral animals can build calcareous shells which form the reef structures, but the tropical waters in which they live are poor in nutrients. Coral polyps are filter feeders, using their tiny tentacles to extract detritus and plankton from the water, but there isn't enough food in the water to support them so they depend instead on their colourful zooxanthellae. These algae are protected by the polyps' calcareous structures and inside, the algae can get on with the business of photosynthesis, feeding themselves and their hosts in the process. The algae can use their host's waste products, such as carbon dioxide, and in turn the host can use the algae's waste product, glycerol, to make lipids and proteins. It is the zooxanthellae that give corals their rich and beautiful colours.
Mention should be made of the phenomenon of coral bleaching. This occurs when the waters around reefs get too hot for the coral animals, which become so stressed that they eject their zooxanthellae. This causes the coral to lose its characteristic colour and turn white. The coral animals do not necessarily die and can recover if water temperatures cool. However if the temperature stays too hot, eventually the coral animals starve to death. Huge areas of reefs have been damaged by coral bleaching and human-caused climate change has been implicated in these events. It is another major reason why we must find alternatives to fossil fuel consumption.
A number of other reef organisms use zooxanthellae as well. Giant clams and their relatives dazzle snorkellers and scuba divers with the colours of their lips. These colours are provided by the symbiotic algae in their cells that provide them with food as well as the wonderful colours. Sea anenomes and even some jellyfish also use zooxanthellae. One jellyfish, known as the upside down jellyfish, depends so completely on its algae that it no longer hunts for a living, simply hanging upside down where the sun shines so the algae can do their job.
In temperate and polar communities zooxanthellae are replaced by zoochlorellae, which live in sea anenomes. Some protozoan members of the marine zooplankton communities also have symbioses with algae. These protozoans, the Foraminifera and the Radiolaria, form beautiful calcareous shells for protection and as in the clams, sea anenomes and corals, the algae benefit from this protection while providing their single celled hosts with food. Another animal that quite independently has taken on a symbiotic relationship with an alga is the marine flatworm, Convoluta roscoffensis. The algae live in the digestive cells of the host, which is totally dependent on them for its food.
In freshwater ecosystems, the tiny animals called Hydra also have algae of the genus Chlorella living in their tissues and providing food. Hydra are related to sea anenomes, with tentacles for hunting, and have also found it useful to have a secondary food source. Once again, the Chlorella cells benefit from the protection offered by their predatory hosts. They have chemicals in their cells that resist the digestive juices of the Hydra and this may indicate how the relationship began. The original cells were probably eaten by the Hydra, resisted digestion and then went on living within the protection of the host.
On land, the lichens are the highly successful products of symbiotic relationships between fungi and algae. It was Beatrix Potter, of Peter Rabbit fame, who discovered this relationship. The fungal hyphae take on the role of protection, this time from water loss. Single celled algae were not able to conquer terrestrial habitats until they formed this association with fungi. Then the lichens became one of the most widespread and successful groups on earth. They are found from the tropics to the polar regions. They are one of the few primary producers that survive in the Antarctic. On rocks baking in the desert sun, lichens survive. On the tops of windswept, ice-scoured mountains, lichens survive. They do this by wrapping the sensitive photosynthetic algal cells in tough fungal hyphae and together they can go where no free-living algae can go.
One of the mysteries of evolution is how higher plants evolved from wet, soft algae to conquer the land. One theory is that it required a symbiotic relationship between algae and fungi where the algae dominated and these organisms eventually evolved the ability to produce cellulose and lignin, which protected their tissues from water loss. If this is so, then algal symbiotic relationships were a key to the development of all terrestrial ecosystems and life forms, including ourselves.
S. Paracer & V. Ahmadjian. 2000. Symbiosis: An Introduction to Biological Associations. Oxford University Press.
C. R. WILKINSON* & PETER FAY 1979. Nitrogen fixation in coral reef sponges with symbiotic cyanobacteria. Nature 279 pp 527-9.
L. Muscatin 1967. Glycerol excretion by symbiotic algae from corals and tridacna and its control by the host. Science 156 pp 516-519.