The name Phaeophyta means brown plants, which is quite appropriate since the majority of the brown algae are darkly colored: chocolate- or olive-brown. Many of the species of algae that we commonly call the seaweeds are in this phylum. There are approximately a thousand species (more recent books say three thousand) in over two hundred and fifty genera in division Phaeophaeta.
Phaeophyta is distinct from the other algae divisions because it has different pigments (fucoxanthin gives it its brown color) and different chlorophyll, as well as the fact that all of the brown algae are multicellular. All the multicellular algae have cell walls which is rarely composed of a single substance, and in Phaeophyta it is commonly pectin mixed with cellulose and other compounds such as hermicellulose, alginic acid, fucoidin, and fucin. It has chlorophyll a and c, but no chlorophyll b, B-carotene and other xanthophylls. It typically stores its food as complex polysaccharides, sugars and higher alcohols, while the principle carbohydrate reserve is a unique soluble glucose polymer laminarine. True starch (as in the green algae) is absent. Cell walls are made of cellulose and alginic acid, a long-chained heteropolysaccharide. Another interesting fact is that most of members of this division have an alternation of haploid and diploid generations. The haploid thalli form gametes while the diplod thalli form zoospores by meiosis. The haploid (or gametangial) and the diploid (sporangial) thalli may either be isomorphic or heteromorphic (which translates into similar or different) in appearance. Another interesting fact is that the kelps, which are the most complex brown algae, are the only algae with internal tissue differentiation and thus with conducting tissue; there is, however, no true xylem tissue such as in the higher plants. Those are the algae that reach over seventy meters in length that we discussed in class.
The brown algae are found all over the world, however the vast majority of the brown algae are marine, and only a few microscopic genera live in fresh water (Pleurocladia, Heribaudiella, and Bodannella) and a few species inhabit brackish water, but all of the most advanced are either in the cold northern oceans or prospering around the southern tips of Africa, South America, and Australia. The largest of the algae are found in this category: the kelps and the giant kelps which can reach from 70 to over 100 meters in length. Practically all the phaeophytes grow attached to substrates along shorelines, but one genus, Sargassum, is a free-floating form that makes up much of the dense mat of floating algae that accumulates in the Sargasso sea. The Sargasso sea is a huge (over five million square kilometers) eddy in the Atlantic ocean between the West Indies and the Azores.
The brown algae have many uses besides providing food and habitat for marine animals. They have been processed for fertilizers and have been used as food in several parts of the world. Several kelp, such as Macrocystis, Laminaria, Sargassum, and Fucus, are important because man has learned how to use alginates from them in many dairy and nondairy foods, pharmaceutical and medical products, and even industrial applications. Algin is a pectin-like material that finds extensive use including the products in the following categories.
Medical & Pharmaceutical
The reproduction of the brown algae is hard to summarize, since it varies so much from species to species. Their life histories are just as varied, but they are not hard to outline when it is done by representative genera. There are six primary life cycles, each one representing a number of genera. In the first (Ectocarpales, Sphacelariales) there is an alternation of similar diploid and haploid generations, what The Biology of the Algae calls isomorphic alternation, which produce isogametes or anisogametes. The second life cycle, which is represented by Dictyotales, also has an alternation of isomorphic (similar) generations, but this time with the production of heterogametes: egg and sperm. The third (Cutleriales)life cycle is an alternation of dissimilar, or heteromorphic, generations in which the sporophyte is small and the gametophyte dominant, producing anisogametes. The forth form is represented by Chordariales, and is an alternation of dissimilar generations in which the sporophyte is dominant and the gametophyte recessive, and thus producing isogametes. As a representative of our penultimate life cycle, Laminariales undergoes an alternation of heteromorphic generations in which the sporophyte is dominant and the gametophyte tiny, this time producing egg and sperm. Fucales has a strange life cycle. It does not have a true alternation of generations at all, and it has a diploid sporophyte. It produces heterospores by meiosis, the spores then develop directly to act as gametes (egg and sperm). The species Fucus can live unattached on muddy flat tides, instead of being firmly attached underwater, like most brown algae.
We have seen many of the peculiarities of the brown algae, such as its distinguishing pigments and chlorophylls, the fact that they are without exception multicellular, and usually (you have to say that about almost everything-there are exceptions for every rule including this one) have cell walls with cellulose and pectin. We have looked at the diverse life cycles of the brown algae, its habitats, and the many uses man has found for them. In conclusion, division Phaeophyta contains many of the most economically important and interesting algae and seaweeds.
Alexopoulos, C. J. And H. C. Bold. Algae and Fungi. New York:
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