The phaeophytes (brown algae) make up roughly 1500-2000 species in 250 genera. They are photosynthetic members of the stramenopiles that range in size from the 50 m giant kelp that produce the kelp forest off the west coast of North America to microscopic filaments. Brown algae are typically epilithic from the intertidal to the deep subtidal and can reach depths of >60 m (Graham et al. 2007). However, they can also grow epiphytic and occasionally endophytic (e.g. Laminariocolax spp.) or pelagic (e.g. Sargassum spp.).
From an ecological perspective, brown algae are the dominant intertidal and shallow subtidal macroalgae in many marine habitats. In the Northern Hemisphere, temperate to arctic coastlines are covered by members of the Laminariales in the Pacific and both the Fucales, and to a lesser extent, Laminariales in the Atlantic. In the tropics, members of the Dictyotales have a worldwide distribution and the coastline of the Southern Hemisphere is dominated by genera of the Fucales and Laminariales largely absent in the Northern Hemisphere. Particularly in cooler waters, brown algae provide important shallow water habitat for an enormous variety of fish and invertebrate species.
CharacteristicsThere are no unicellular brown algae, though nearly all members have characteristic unicellular biflagellated spores in their life cycle, typical of stramenopiles. Flagella are laterally inserted, with the anterior-facing flagellum possessing tripartite mastigonemes and the posterior-facing (whiplash) flagellum naked. Eyespots are often present in the spores, associated with the plastid.
Brown algal plastids are derived from a secondary endosymbiotic event involving a red algal cell (see Archibald, 2009; Keeling 2009; Lane & Durnford, 2010). Thus plastids in the phaeophytes (and all photosynthetic stramenopiles) are related to red algae and are surrounded by four membranes with thylakoids in stacks of three (Kim & Archibald 2008). The primary pigments of brown algae are chlorophylls a and c, as well as the carotenoid, fucoxanthin.
Reproduction can be both sexual, or asexual, in brown algae and some form of alternation between free-living haploid and diploid generations (iso- or heteromorphic) is nearly universal. Asexual reproduction can occur in the diploid sporophyte phase of the life history by way of either mitotic divisions within plurilocular sporangia (2n) that result in the recapitulation of the sporophyte, or meiotic divisions in unilocular sporangia, resulting in haploid (1n) spores that germinate into the gametophyte phase. The gametophyte produces gametes through mitotic divisions in plurilocular gametangia. In many cases the haploid gametes (spores) can settle and develop directly back into a gametophyte if they do not fuse with another gamete.
Graham, M. H., Kinlan, B. P., Druehl, L. D., Garske, L. E., Banks, S. (2007) Deep-water kelp refugia as potential hotspots of tropical marine diversity and productivity. Proc. Nat. Acad. Sci. USA. 104:16576-16580
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This page is being developed as part of the Tree of Life Web Project Protist Diversity Workshop, co-sponsored by the Canadian Institute for Advanced Research (CIFAR) program in Integrated Microbial Biodiversity and the Tula Foundation.
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- First online 28 April 2010
- Content changed 24 May 2010
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Tree of Life Web Project. 2010. Phaeophytes. Brown Algae. Version 24 May 2010 (under construction). http://tolweb.org/Phaeophytes/129402/2010.05.24 in The Tree of Life Web Project, http://tolweb.org/