Brown Algae (Phaeophyceae)
Brown algae belong almost entirely to the sea, only a very few occurring in fresh water. Here are included the conspicuous brown seaweeds, many of which grow to notably large size. The pigments of this class include green chlorophyll, which is masked by the yellow and brown pigments, xanthophyll, carotin, and fucoxanthin.
Plants of this class of algae form the conspicuous offshore growths popularly known as “kelp beds.” They are the giants among the seaweeds, and form the marine forests among whose waving stipes and fronds myriads of neritic fish obtain their food and seek shelter from their aquatic enemies. These, also, are the kelps commonly harvested in many places for the commercial products they yield.
The brown algae possess a great range of size and structure. There are minute, delicate, filamentous branching plants (Ectocarpus, fig. 68e); coarse, hollow, sausage-like chains a foot or more in length (Scytosiphon, fig. 68i); short-stalked forms with broad thalli (Laminaria, Costaria, and Alaria, fig. 68c, some of which become nearly 2 m broad); many branched forms (Fucus, Egregia); and long-stalked giants of the Pacific with long leathery fronds (Macrocystis, Nereocystis, Pelagophycus).
Nereocystis may attain a length of 35 m or more. The plant is anchored to a hard substratum by means of a profusely branched structure known as the holdfast, but there are no true roots. From the holdfast extends the long cylindrical stipe, which is hollow through most of its length and ends distally in a large hollow bulb. This bulb, like the stipe, is filled with gas, giving buoyancy to the plant. Ribbon-like fronds or laminae issue from the distal end of the bulb.
The gross structure and life cycle of Nereocystis. a, sporophyte plant; b, swimming zoospores; c, male and d, female gametophyte plants; e, young sporophyte.
The hollow bulb and stipe maintain the upper portion of the plant near the surface, exposing the fronds to favorable light conditions. In common with other large algae, the parts are tough, flexible, and slippery in order to withstand with least resistance the effect of frequently violent storm waves and strong currents.
Methods of Reproduction. The life cycle of the brown algae includes various types of alternation of generations. Commonly, in the Laminariales, which includes the large kelps, there is an alternation of generations that may be illustrated by the cycle shown in Nereocystis (fig. 69). Here the large, conspicuous sporophyte plant produces a series of sori, or “fruiting areas,” appearing as dark brown patches running longitudinally along the whole length of the fronds. Beginning at the distal end of the frond, these patches are detached at maturity, leaving a broad gap (3 to 10 cm) in the frond. From the mature sori, innumerable ciliated zoospores escape and, upon reaching a suitable substratum, grow to small filamentous plants, the inconspicuous gametophyte stage. Thus the alternation of generations in this form is
In the brown algal group, Fucales, to which Fucus and Sargassum (fig. 68f) belong, the main plant is a sporophyte, but, within the thousands of tiny cup-like conceptacles forming the bladders, gametes are formed like spores. These unite after being discharged free in the water. Thus the alternation of generations is evident only cytologically. In connection with the “spawning” of Fucus, it is interesting to note that it is rhythmic with the tide, taking place after a period of exposure at low tide.
Distribution. The brown algae reach their maximum development in cooler waters, and are therefore typical of the rocky coast of higher latitudes. Sargassum and others of the Fucales are characteristic, however, of tropical or subtropical regions. Tilden (1935) is of the opinion that the Laminariales arose in the North Pacific, while the Fucales had their origin in the South Pacific. Several species or varieties of Sargassum, or “gulfweed,” are found in large quantities in the Sargasso Sea, whence they have drifted and multiplied after being torn loose from coastal areas. They are kept afloat by air bladders and grow vegetatively, propagating by fragmentation, but apparently do not form fruiting bodies. The drifting masses form a characteristic environment with associations including other algal and animal forms of littoral type.
The vertical distribution of brown algae shows many low-growing forms, especially the Fucales, in the rocky intertidal zone. Near the lowest tide level the medium-sized forms with leathery fronds and short limber stipes begin to prevail, and they increase markedly in the next 15 to 20 m of depth, finally diminishing and disappearing below the eulittoral zone.
Intermingled with these short-stiped algae are the giant long-stiped kelps that usually grow most abundantly some distance from the shore and extend to depths of 30 m or more. Macrocystis, one of the giant kelps of the Pacific, is said to reach to the surface from a depth of 80 m off the coast of Chile (Hesse, Allee, and Schmidt, 1937), but in the North Pacific it has its most abundant growth in water of about 15 m. Kelps of this genus are said to be absent from strictly tropical waters and, as a
Mention should also be made of epiphytic forms such as the filamentous Ectocarpus (fig. 68e), which prefers to be attached to other algae growing at various depths.