A class of plants, commonly called brown algae, in the chlorophyll a-c phyletic line (Chromophycota). Brown algae occur almost exclusively in marine or brackish water, where they are attached to rocks, wood, sea grasses, or other algae. Approximately 265 genera and 1500 species are recognized, arranged in about 15 orders. See also Algae; Chromophycota.
Phaeophyceae are characterized primarily by biochemical and ultrastructural features. The cells are typically uninucleate and contain one or more chloroplasts with or without pyrenoids. Photosynthetic pigments include chlorophyll a and c, β-carotene, and several xanthophylls, principally fucoxan-thin. The simplest thallus exhibited by Phaeophyceae is generally considered to be an erect, unbranched or branched, uniseriate filament arising from a prostrate filamentous base, Many Phaeophyceae are crustose or bladelike. Complex thalli differentiated into macroscopic organs are produced by kelps and rockweeds.
The geographic distribution of Phaeophyceae is bimodal. Kelps are most abundant and diverse on surf-swept rocky shores of the North Pacific, but they form an ecologically important vegetation belt in the lower intertidal and upper sub-tidal zones on all cold-water shores except Antarctica. Rockweeds are similarly abundant on cold-water shores, forming conspicuous belts in the upper intertidal zone. They also form extensive stands in salt marshes in the Northern Hemisphere. Tropical waters support a diverse array of Dictyotales and members of the fucalean family Sargassaceae.
