More than 1,500 species of seaweeds and other algae that are brown to olive green in color are assigned to the class Phaeophyceae in this phylum. Previously, the brown algae were in a phylum of their own (Phaeophyta), but cladistic and structural analyses suggest relatively close relationships with other members of Phylum Chromophyta. Many brown algae are relatively large, and none are unicellular or colonial. Only 6 of the 265 known genera occur in fresh water, the vast majority growing in colder ocean waters, usually in shallower areas, although the giant kelp (Macrocystis pyrifera—Fig. 18.16) may be found in water up to 30 meters (100 feet) or more deep, and one brown alga (Lobophora variegata) was seen in clear tropical waters near the Bahamas growing at a depth of 220 meters (730 feet). One giant kelp measured 274 meters (900 feet) in length—a length believed to be a record for any single living organism.
Many of the brown algae have a tliallus (plural: thalli, the term for multicellular bodies that are usually flattened and not organized into leaves, stems, and roots) that is differentiated into a holdfast, a stipe, and flattened leaflike blades (Fig. 18.17). The holdfast is a tough, sinewy structure resembling a mass of intertwined roots. It holds the seaweed to rocks so tenaciously that even the heaviest pounding of surf will not readily dislodge it.
The stalk that constitutes the stipe is often hollow, with a meristem either at its base or at the blade junctions. Since the meristem produces new tissue at the base, the oldest parts of the blades are at the tips.
The blades and most of the body are photosynthetic and may have gas-filled floats called bladders toward their bases. The bladder gases may include as much as 10% by volume of carbon monoxide. The function of the carbon monoxide, which is deadly to animal life, is not known.
The color of brown algae can vary from light yellow-brown to almost black, reflecting the presence of varying amounts of the brown pigment fucoxanthin, in addition to chlorophylls a and c and several other pigments in the chloroplasts.
The main food reserve is laminarin, a carbohydrate. Algin, or alginic acid (see Table 18.2 and the discussion under "Human and Ecological Relevance of the Algae," page 344) occurs on or in the cell walls and can represent as much as 40% of the dry weight of some kelps. Reproductive cells are unusual in that the two flagella are inserted laterally (i.e., on the side) instead of at the ends. The only motile cells in the brown algae are the reproductive cells.
In some localities off the coast of British Columbia and Washington, herring deposit spawn (eggs) in layers up to 2.5 centimeters (1 inch) thick on both surfaces of giant kelp blades in late spring. In the past, and to a limited extent at present, native North Americans have harvested these spawn-covered blades, sun dried them, and used them for
winter or feast food. Even today, some school children are given small pieces of the dried or preserved material for lunchbox snacks.
The Sargasso Sea gets its name from a floating brown seaweed, Sargassum, that is washed up in abundance on the shores along the Gulf of Mexico after tropical storms (Fig. 18.18). A species occurring in the Pacific Ocean has been used, in chopped form, as a poultice on cuts received from coral by native Hawaiians. This and several other brown algae reproduce asexually by fragmentation, while some produce autospores. Sexual reproduction takes several different forms, depending on the species. The conspicuous phases of the life cycles are usually diploid.
In the common rockweed Fucus (Fig. 18.19), separate male and female thalli are produced, or both sexes may develop on the same thallus. Somewhat puffy fertile areas called receptacles develop at the tips of the branches of the thallus. The surface of each receptacle is dotted with pores
(visible to the naked eye) that open into special, spherical hollow chambers called conceptacles. Within the conceptacles, gametangia (cells or structures where gametes are produced) are formed. Eight eggs are produced in each oogonium (female gametangium) as a result of a single diploid nucleus undergoing meiosis followed by mitosis (Fig. 18.20). Meiosis also occurs in each antheridium (male gametangium), but four mitotic divisions follow meiosis so that 64 sperms are produced. Eventually, both eggs and sperms are released into the water, where fertilization takes place, and the zygotes develop into mature thalli, completing the life cycle.
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