Macrocystis C.Agardh, 1820

Holotype species: Macrocystis pyrifera (Linnaeus) C.Agardh

Original publication: Agardh, C.A. (1820 '1821'). Species algarum rite cognitae, cum synonymis, differentiis specificis et descriptionibus succinctis. Volumen primum. Pars prima. pp. [i-iv], [1]-168. Lundae [Lund]: ex officina Berlingiana.

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Type designated in: Bachelot de la Pylaie, A.J.M. (1830 '1829'). Flore de Terre-Neuve et des Iles Saint-Pierre et Miclon. Livraison [Algae]. pp. 1-128. Paris: Typographie de A. Firmin Didot, rue Jacob, No. 24.

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Description: Thallus to 45 m long, perennial, typically 4-8 years. Holdfast conical and consisting of a few central erect stipes bearing terete branching haptera; or holdfast prostrate, subligulate, branched, creeping with short haptera along lateral margins. Stipes numerous, erect, terete, dichotomously branching 2-6 times near base. Each stipe branch subtends a frond, which is composed of a stipe with attached blades and an apical meristematic blade. Apical blade falcate, with several young lateral blades produced unilaterally by splitting through transition region. Blades numerous on each stipe, arranged at regular intervals with a short stipe and a pyriform to subglobose pneumatocyst subtending an undivided lamina, which is narrow to broad with tapering ends, smooth to rugose, and with denticulate margins. Spores produced in unilocular sporangia with paraphyses on sporophylls borne on stipe near holdfast, with sporangia covering most of both surfaces. Sporophylls divide several times dichotomously, with or without pneumatocyst. Gametophytes dimorphic, dioecious, oogamous, branched uniseriate filaments.

Information contributed by: R.J. Lewis & M.D. Guiry. The most recent alteration to this page was made on 2023-07-24 by M.D. Guiry.

Taxonomic status: This name is of an entity that is currently accepted taxonomically.

Most recent taxonomic treatment adopted: Demes, K.W, Graham, M.H. & Suskiewicz, T.S. (2009). Phenotypic plasticity reconciles incongruous molecular and morphological taxonomies: the giant kelp, Macrocystis (Laminariales, Phaeophyceae), is a monospecific genus (note). Journal of Phycology 45(6): 1266-1269.

Comments: A single Macrocystis species occurs in both the Northern and Southern Hemispheres, being found along the Pacific coast of North America in the north and having a cicumsubantarctic distribution in the south. Many species were described based on morphological differences, later reduced to species based mainly on holdfast morphology.

Macrocystis experience a wide variety of environmental conditions because of their perennial nature, their growth habit reaching from deeper water to the surface, and the wide geographic distribution of the single species.

The physiology of Macrocystis has been widely studied in these various situations, particularly in Northern Hemisphere populations, including studies of photosynthesis and nutrient uptake kinetics. Light and nutrient requirements for recruitment of young Macrocystis plants have been determined for M. pyrifera in southern California. These data have been used to model the growth of M. pyrifera individuals and populations in response to these complex factors. Ecotypic differentiation of nitrogen physiology has been found between populations of M. pyrifera from different localities of California.

Macrocystis is of both economic and ecological importance. Between 100,000 and 170,000 wet tons of Macrocystis were harvested annually in California for alginate extraction and abalone feed, but this alginate production has now ceased, although a small amount is still harvested for abalone. Sustainable harvesting is important as the organisms that thrive in Macrocystis forest communities are of great value aesthetically, supporting both commercial fishing operations and recreational activities, particularly SCUBA, and an important habitat for the resurgent Sea Otter.

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Some of the descriptions included in AlgaeBase were originally from the unpublished Encyclopedia of Algal Genera, organised in the 1990s by Dr Bruce Parker on behalf of the Phycological Society of America (PSA) and intended to be published in CD format. These AlgaeBase descriptions are now being continually updated, and each current contributor is identified above. The PSA and AlgaeBase warmly acknowledge the generosity of all past and present contributors and particularly the work of Dr Parker.

Descriptions of chrysophyte genera were subsequently published in J. Kristiansen & H.R. Preisig (eds.). 2001. Encyclopedia of Chrysophyte Genera. Bibliotheca Phycologica 110: 1-260.

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M.D. Guiry in Guiry, M.D. & Guiry, G.M. 24 July 2023. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway.; searched on 15 July 2024

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