HETEROKONTAE

Phaeophyta (fá-O-fa-ta) is made of two Greek roots that mean brown (phaios -φαιός); and plant (phyto -φυτό). The reference is to the dominance of the brown accessory pigments that give the thalli a tawny to dark brown appearance.

Aside from forming the basis of the food web in their respective environments, some of them, particularly the kelps, are of great economic value. Alginic acid is a mucopolysaccharide that has many economically important uses that range from pharmaceuticals, wetting agents in dehydrated foods, thickening agent in foods (particularly yogurts, puddings, ice cream, and chocolate milk), fire-proofing fabrics, and an additive to cosmetics. Along the northeast coast of the US, Laminaria and Ascophyllum are commercial sources of alginic acid (Laminaria also is consumed as a vegetable). The giant kelp (Macrocystis) is harvested along the coast of California for its alginic acid, which makes up about 40% of the dry weight of its thallus. In fact, I have watched the harvesting barges cut the upper meter of Macrocystis in areas where they had harvested only one week earlier. That is, they grow about a meter a week.

*A. Herbarium sheet of the branched filament, Ectocarpus.*	*B. Herbarium sheet of pseudoparenchymatous Chordaria.*	*C. Herbarium sheet of pseudoparenchymatous Sproochnus.*		*D. Herbarium sheet of Desmarestia, a filament of large central cells and corticating cells.*	*E. Herbarium sheet of Cutleria, a parenchymatous sporophyte thallus.*
*F. Laminaria, a kelp of northern shores, shows the holdfast, stipe, and blade.*	*G. Image of Macrocystis, the giant kelp.*	*H. Sphacelaria, a branched pseudoparenchymatous genus.*		*I. Dictyota, a flat, parenchymatous organism.*	*J. Dictyosiphon, a branching parenchymatous filament.*
*K. Fucus, the common shore rockweed shows parenchymatous thallus with dichotomous braching terminating in floats.*	*L. Durvillaea, similar to Fucus, but grows from diffuse cell division rather than apical growth.*	*M. A unilocular meiosporangium of Ectocarpus.*		*N. A pleurilocular isogametangium of Ectocarpus.*	*O. An oogonial conceptacle of Fucus.*
Images taken from: A: http://www.huh.harvard.edu/libraries/Robinson_exhibit/Plates1-5.html B: http://www.huh.harvard.edu/libraries/Robinson_exhibit/Plates16-20.html C: http://ucjeps.berkeley.edu/cgi-bin/get_pr_image.pl?Sporochnus+bolleanus_H D: http://www.biologie.uni-hamburg.de/b-online/e44/fucuherb.htm E: http://www.bio.utexas.edu/faculty/laclaire/bot321/cutleria.gif F: http://www.arches.uga.edu/~kankoku/laminaria.jpg			G: http://www.sanctuaries.nos.noaa.gov/pgallery/pgchannel/living/giantkelp1_100.jpg H: http://www.botany.hawaii.edu/reefalgae/Herbarium%20specimens/ I: http://www.biol.tsukuba.ac.jp/~inouye/ino/st/br/Dictyota-1.GIF J: http://www.huh.harvard.edu/libraries/Robinson_exhibit/plate23.jpg K: http://www.uri.edu/artsci/bio/rishores/rocky.htm L: http://www.unp.edu.ar/museovirtual/Algasmarinas/Algasjpg/pardas/durvilla300pp.jpg M-O: http://www.humboldt.edu/~dll2/bot105/algae/

The taxonomy of the Phaeophyta is based on Clayton (1990), Margulis and Schwartz (1988, Pr-12; 1998, Pr-17), Sleigh et al. (1985), Bold and Wynne (1985), Graham and Wilcox (2000) in which there is uniform agreement that the brown algae occupy a single class and 12-15 orders. Although both Taylor (1976) and Dodge (1973) suggest the the brown algae are associated with the chrysophyte complex (or chromophytes), they have many unique features [e.g. unilocular sporangia, plasmodesmata, alginates, a unique 7(1)/3(1)/7 microtubule arrangement in motile cells, and the loss of the distal fiber and transitional helix from the flagellar apparatus (O'Kelly 1989).

Clayton (1989) considers that the characteristic association of the nucleus, golgi and flagellar bases shows affinities with the Chrysophyta, Xanthophyta, and Oomycota. However, he questions the "tradition" of rooting phaeophyte phylogenies in the Ectocarpines because they have numerous chloroplasts per cell. This is very different from the characteristic 2 chloroplasts per cell in the chrysophytes. O'Kelly (1989) also questions the "custom" of considering the brown algal ancestor to be a motile unicell as is indicated in Scagel et al. (1984) and Bold et al. (1987). Both Clayton (1989) and O'Kelly (1989) believe that the brown algae evolved from multicellular chrysophytes. Clayton (1989) supports his view by pointing out the similarities between the parenchymatous condition of some phaeophytes and the multiseriate form of the gametophytes of chrysophyte genera like Scytosiphon. The more recent molecular sequence trees (e.g. Tan and Druehl 1996) show that the phaeophytes are sisters to the xanthophytes and are rooted in the Fucales.

Perhaps the oddest classification is that of Scagel et al. (1984) in which the brown algae, because they are photosynthetic and multicellular, are considered members of the Plant Kingdom. I agree that the phaeophytes represent a range in size and complexity that is comparable to that observed in the entire Plant Kingdom. However, that argues more for the creation of a new kingdom, the Kingdom Chromista, as proposed by Cavalier-Smith (1989) or Stramenopiles (Patterson 1999). I have followed their systems and grouped the browns together with the other heterokonts.

Estimates of the age of the phaeophytes vary greatly. Clayton (1990) reports that fossil evidence indicates a great age for the brown algae (up to 1,300 million years). However, he indicates that cellular and molecular evidence suggest that the browns may have arisen only 200 million years ago.

SYSTEMATIC BIOLOGY					THE HETEROKONTAE
HOME	SYLLABUS	WEEKLY ASSIGNMENTS	J. SYSTEMATIC BIOLOGY	TAXA OF LIFE
PHYLUM PHAEOPHYTA