Appearance''M.pyrifera'' is the largest of all algae. The stage of the life cycle that is usually seen is the sporophyte, which is perennial and individuals persist for many years.The giant kelp receives its name from its incredible size. Individuals may grow to up to 50 metres long. The stalks arise from a holdfast and branch three or four times from near the base. Blades develop at irregular intervals along the stipe, with a single pneumatocyst at the base of each blade.
A related and similar-looking, but smaller species, ''M. integrifolia'', grows to only to 6 metres long. It is found on intertidal rocks or shallow subtidal rocks along the Pacific coast of North America and South America.
Distribution''M. pyrifera'' is found in North America (Alaska to California), South America, South Africa, New Zealand, and southern Australia. It thrives in cooler waters where the ocean water temperature remains below 21 °C (70 °F). Also found at Tristan da Cunha in mid South Atlantic Ocean.
HabitatWhere the bottom is rocky and affords places for it to anchor, giant kelp forms extensive kelp beds with large "floating canopies". When present in large numbers, giant kelp forms kelp forests that are home to many marine species who depend upon the kelp directly for food and shelter, or indirectly as a hunting ground for prey. Both the large size of the kelp and the large number of individuals significantly alter the availability of light, the flow of ocean currents, and the chemistry of the ocean water in the area where they grow.
In high-density populations, giant kelp individuals compete with other individuals of the species for space and resources. Giant kelp may also compete with ''Pterygophora californica'' in these circumstances.
Where surface waters are poor in nutrients, nitrogen in the form of amino acids is translocated up the stipe through sieve elements that very much resemble the phloem of vascular plants. Translocation of nutrients along the stipe may be as rapid as 60 centimetres per hour. Most translocation occurs to move carbon-rich photosynthate, and typically transfers material from mature regions to actively growing regions where the machinery of photosynthesis is not yet fully in place. Translocation also moves nutrients downward from light-exposed surface fronds to sporophylls (reproductive fronds) at the base of the kelp, where there is little light and thus little photosynthesis to produce food.
Cultural''M. pyrifera'' has been utilized for many years as a food source; it also contains many compounds such as iodine, potassium, other minerals vitamins and carbohydrates and thus has also used as a dietary supplement. In the beginning of the 20th century California kelp beds were harvested as a source for soda ash. With commercial interest increasing significantly during the 1970s and the 1980s this was primarily due to the production of alginates, and also for biomass production for animal feed due to the energy crisis during that period. However the commercial production for ''M. pyrifera'' never became realty. With the end of the energy crisis and the decline in prices of alginates, the research into farming ''Macrocystis'' also declined.
The demand for ''M. pyrifera'' is increasing due to the newfound uses of these plants such as fertilizers, cultivation for bioremediation purposes, abalone and sea urchin feed. There is current research going into utilizing ''M. pyrifera'' as feed for other aquaculture species such as shrimps. Recently, ''M. pyrifera'' has been examined as a possible feedstock for conversion into ethanol for biofuel use.
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