Phaeophytes

Phaeophytes, commonly called brown algae, are a group of mostly marine photosynthetic eukaryotes that form one of the largest lineages of algae. They belong to the phylum Phaeophyta and the class Phaeophyceae within the stramenopile supergroup. Their characteristic brown color comes from the pigment fucoxanthin, which masks the green of chlorophyll a and c. They store carbohydrates as laminarin and mannitol, and their cell walls often contain the gel-forming polysaccharide alginate.

Most brown algae are multicellular, structurally diverse seaweeds ranging from filamentous forms to large kelp. They typically have a holdfast, a stipe, and blades, with some species bearing gas-filled pneumatic cysts that aid buoyancy. They inhabit cold and temperate marine waters, especially along rocky shores, and form important habitats such as kelp forests and shallow subtidal communities. Notable representatives include Laminaria (kelp), Macrocystis (giant kelp), Fucus (rockweed), and Sargassum (free-floating brown alga).

Brown algae generally display a heteromorphic alternation of generations with a dominant diploid sporophyte that is often large and conspicuous, and a microscopic haploid gametophyte. Meiosis produces zoospores that settle as male or female gametophytes; fertilization yields a diploid zygote that grows into a new sporophyte. Some groups lack prominent alternation or have somewhat different sexual expressions. Reproduction can be sexual, asexual, or involve fragmentation and spores in certain lineages.

Ecologically, phaeophytes are major coastal primary producers and create complex habitats for numerous organisms. They also provide commercial products; alginates extracted from their cell walls are used as thickeners in food and industry, and edible brown seaweeds (e.g., kombu and wakame) are cultivated in Asia. Climate change, pollution, and overharvesting threaten kelp forests and other brown algae populations in some regions.