DESCRIPTION OF THE PHYLUM PHAEOPHYTA (KJELLMAN 1891)
| EUKARYA> CHROMALVEOLATA> HETEROKONTAE> PHAEOPHYTA |
PHAEOPHYTA LINKS
| 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. Kjellman (1891) originally coined the class, Phaeophyceae. The emergence of the class as a separate phylum called for changing the name to Phaeophyta, as it appears in Margulis and Schwartz (1998). |
| INTRODUCTION TO THE PHAEOPHYTA The brown algae are the dominant producers in northern temperate and arctic intertidal waters. They range in form from microscopic branched filaments (Figure A), to pseudoparenchymatous (text with tooltip) Pseudoparenchymatous (adj) describes the structure of a multicellular organism that is formed of interwoven filaments rather than parenchyma. and cortical thalli (text with tooltip) Cortications are outer cells of filaments in certain algal groups like the red algae, brown algae, and the charophytes. The outer layer is formed by filaments that lie apprised to the axial filament and produce a striped appearance to the organism. (Figures 2-5), to enormous multicellular parenchymatous (text with tooltip) arenchymatous (adj.) refers to the formation of a multicellular organism or structure by cells that can divide in multiple planes and thus produce a mass of cells. kelps (Figures 6-7), and like plants, those at the multicellular end of organization tend to be diploid in their dominant form (usually sporophyte (text with tooltip) A sporophyte is the diploid phase of an organism that exhibits alternation of generation. This phase produces spores usually in specialized sporangia as the immediate products of meiosis. ). Some exhibit apical growth (8-11). Fucus and similar taxa have lost the gametophyte (text with tooltip) A gametophyte is haploid, gamete-producing phase of an organism that exhibits alternation of generation. entirely and have gametic meiosis (text with tooltip) Gametic meiosis occurs when gametes are the immediate results of meiosis. (Figures 12-13). Most of the phaeophytes, particularly the kelps, exhibit a heteromorphic alternation of generation (text with tooltip) Heteromorphic alternation of generation occurs in organisms in which the haploid gametophyte phase and diploid sporophyte phase are different in form. . |
FILAMENTOUS AND PSEUDOPARENCHYMATOUS TAXA
These taxa all have the same growth form but are not related to each other. In general, they are relatively small as the phaeophytes go, and all form branched filaments. Some of the branched filaments are relatively simple with a single main filament from which lateral branches arise. However, many taxa are made of branched filaments that are woven into a three dimensional structure, usually a tube or flat blade. This more complex condition is called pseudoparenchymatous. though some are pseudoparenchymatous. Most taxa in this group exhibit trichothallic growth, that is cell division occurs in certain well-defined areas of the filament. Because the filaments do not grow from the the tips, these taxa are able to withstand heavy grazing pressure, especially by sea urchins.
Ectocarpus (Figure 1), a common genus exhibits isomorphic alternation of generation (text with tooltip) Isomorphic alternation of generation occurs in organisms in which the haploid gametophyte phase and diploid sporophyte phase are indistinguishable in form. with the gametophyte forming unilocular meiosporangia (text with tooltip) A unilocular zoosporangium is a structure with a single opening in which meiosis occurs and zoospores develop. and the sporophyte forming pleurilocular isogametangia (text with tooltip) A pleurilocular gametangium is a gametangial (gamete-producing) structure that has multiple cells or openings. (See Figure 2).
Chordaria, Sporochnus, and Sphacelaria all are pseudoparenchymatous taxa, but they do differ in some important ways. Chordaria (Figure 3) is isogamous and has a heteromorphic alternation of generation having a microscopic haploid (gametophyte) alternating with a macroscopic diploid (sporophyte). Sporochnus (Figure 4) is similar to Chordaria in its form of alternation of generation; however, it is oogamous and growth of the sporophyte is by means of an intercalary meristem (text with tooltip) An intercalary meristem is a meristematic region that lies between parts of the thallus rather than at the tip (apical meristem). An intercalary mersitem is found between the blade and the stipe of kelps. .
Sphacelaria (Figure 5) grows as a dense tuft of branching, pseudoparenchymatous filaments. Unlike most phaeophytes, this genus is usually found in tropical and subtropical waters. The life history is simple with an isomorphic alternation of generation.
Desmarestia (Figure 6) grows as a special type of pseudoparenchymatous form. In this case, there is a central filament made of large cells and appressed to the filament are smaller filaments called corticating filaments (text with tooltip) Cortications are outer cells of filaments in certain algal groups like the red algae, brown algae, and the charophytes. The outer layer is formed by filaments that lie apprised to the axial filament and produce a striped appearance to the organism. . The gametophyte is microscopic and oogamous.
 |
| FIGURE 1. Herbarium sheet of the branched filament, Ectocarpus. Image from http://www.huh.harvard.edu/libraries/Robinson_exhibit/Plates1-5.html |
 | FIGURE 2. Ectocarpus has a life history with an isomorphic alternation of generation. The diploid filament (right) forms sporangia without internal walls and within which meiosis occurs. Thus, they are called unilocular meiosporangia (or unilocular zoosporangia). The haploid filament (left) forms gametangia with many internal walls (pleurilocular gametangia, or more correctly, pleurilocular isogametangia). Diploid filaments may also form pleurilocular structures in which 2n zoospores are produced. The motile cells in the center show a gamete or zoospore (top left); fusing gametes (bottom left); and a zygote (right). Image from Biodidac. |
 |  |  |  |
| FIGURE 3. Herbarium sheet of pseudoparenchymatous Chordaria. Image from http://www.huh.harvard.edu/libraries/Robinson_exhibit/Plates16-20.html | FIGURE 4. Herbarium sheet of pseudoparenchymatous Sporochnus. Image from http://ucjeps.berkeley.edu/cgi-bin/get_pr_image.pl?Sporochnus+bolleanus_H | FIGURE 5. Sphacelaria, a branched pseudoparenchymatous genus. Image from http://www.botany.hawaii.edu/reefalgae/Herbarium%20specimens/ | FIGURE 6. Herbarium sheet of Desmarestia, a filament of large central cells and corticating cells. Image from http://www.biologie.uni-hamburg.de/b-online/e44/fucuherb.htm |
| KELPS AND PARENCHYMAL TAXA Rather than being filamentous in form, these organisms have parenchymal tissue. That is, the component cells divide in three dimensions producing solid blocks of tissue. In general, these taxa are thalloid or leaf-like, in form. Although some like Cutleria (Figure 7) and Dictyota (Figure 8) have an isomorphic alternation of generation, the kelps have a microscopic gametophyte and a very large, complex sporophyte that may reach lengths of 50 m in some species (Figure 9). The sporophytes of kelps have three obvious regions: a holdfast, stipe, and blade. The meristematic tissue at the base of the blade. 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 U.S., Laminaria (Figure 10) and Ascophyllum are commercial sources of alginic acid (Laminaria also is consumed as a vegetable). The giant kelp (Macrocystis, Figure 11) 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. |
 |  |
| FIGURE 7. Herbarium sheet of Cutleria, a parenchymatous sporophyte thallus. Image from http://www.bio.utexas.edu/faculty/laclaire/bot321/cutleria.gif | FIGURE 8. Dictyota, a flat, parenchymatous organism. Image from http://www.biol.tsukuba.ac.jp/~inouye/ino/st/br/Dictyota-1.GIF |
 | FIGURE 9. Laminaria has a life history with a heteromorphic alternation of generation. The diploid thallus (top) forms unilocular sporangia in regions called sori on the blades. They are called unilocular meiosporangia (or unilocular zoosporangia). The zoospores produce haploid filamentous male or female gametophytes (center) that form gametangia of two types: the smaller male gametangia produce small biflagellate cells that function as sperm, and the female gametangia release large, nonmotile cells that function as eggs. The zygote germinates and forms the typical sporophyte kelp. Image from Biodidac. |
 |  |
| FIGURE 10. Laminaria, a kelp of northern shores, shows the
holdfast
(text with tooltip)
A holdfast is a root-like attachment structure that is characteristic of seaweeds, particularly the kelps.
,
stipe
(text with tooltip)
Supporting stalk of a mushroom cap.
, and
blade
(text with tooltip)
A blade is the leaf-like photosynthetic organ of seaweeds, particularly the kelps.
. Image from http://www.arches.uga.edu/~kankoku/laminaria.jpg | FIGURE 11. Image of Macrocystis, the giant kelp. Image from NOAA |
| FUCOID TAXA These taxa, many of which are the rock weeds that grow in the intertidal zones of cool temperate waters include Fucus (Figure 12). These organisms grow from large apical cells which give rise to flattened parenchymatous thalli that terminate in enlarged, gas-filled floats. They are diploid like the kelp sporophytes; however, unlike the kelps, they do not have a gametophyte stage (see the life cycle in Figure 13). Fucus is dioecious (separate sexes) and undergoes gametic meiosis to produce sperm in antheridia and egg in oogonia, both of which develop within recessed structures called conceptacles (see Figure 14 for an oogonial conceptacle). The release of gametes in Fucus is strongly tied to the tides. High tides allow the zygote and developing sporophyte to have access to the highest parts of the intertidal zone. Although many fucoid taxa do occur in the intertidal zone, Sargassum occurs as large floating islands in the open ocean where it supports a fairly diverse community. Durvillaea (Figure 15) resembles the fucoids in that it is parenchymatous and has a diplontic life cycle (meiosis occurs only in the development of gametes). However, it grows by a more diffuse cell division rather than by the division of an apical cell. Also, the photosynthetic cells are elongate and have only 2 chloroplasts per cell while Fucus and its relatives have many discoid chloroplasts per cell. |
 |
| FIGURE 12. Fucus, the common shore rockweed shows parenchymatous thallus with dichotomous branching terminating in floats. Image from http://www.uri.edu/artsci/bio/rishores/rocky.htm |
 | FIGURE 13. Fucus has a life history with a gametic meiosis. Thus, there is no alternation of generation. Male and female gametes are produced within specialized gametangia clustered together in conceptacles. Eggs and sperms are released and the resulting zygote attaches and develops into the sporophyte. Image from Biodidac |
 |  |
| FIGURE 14. An oogonial
conceptacle
(text with tooltip)
Conceptacles are fertile regions that are in-folded on the thallus of organisms like Fucus such that the gametangia are contained within a pouch on the thallus.
of Fucus. Image from http://www.humboldt.edu/~dll2/bot105/algae/ | FIGURE 15. Durvillaea, similar to Fucus, but grows from diffuse cell division rather than apical growth. Image from http://www.unp.edu.ar/museovirtual/Algasmarinas/Algasjpg/pardas/durvilla300pp.jpg |
| SYSTEMATICS OF THE PHAEOPHYTA Though they clearly have many features in common with the photosynthetic heterokonts, the phaeophytes 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). Most recent molecular phylogenies (e.g. Andersen 2004; Riisberg et al. 2009; Yang et al. 2012) show a strong relationship between the Xanthophytes and the Phaeophytes (see Figure 16). This emerged even in the early molecular sequence trees (e.g. Tan and Druehl 1996). Those early studies, though, suggested that the phaeophyte clade was rooted in the Fucales. Rousseau et al. (2001) demonstrated a set of basal orders (Dictyoales, Sphacelariales, and Syringodermatales) with two crown clades including the Fucales and Ectocarpales, respectively. Phillips et al. (2008) and Silberfeld et al. (2010), through multigene molecular phylogenies, produced trees that had similar topologies for the orders (see Figure 17). The curious outcome of recent molecular work is that growth forms do not seems to cluster. For example, the terminal clade that includes the Fucales includes orders that are crustose (Nemodermatales and Ralfsiales), filamentous (Tilopteridales), and parenchymatous (Fucales). Perhaps the greatest surprise is the association of the filamentous Ectocarpales with the large kelps (Laminariales). Estimates of the age of the phaeophytes vary greatly. Clayton (1990) reports fossil evidence that 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. This latter view was borne out by a molecular clock study (Silberfeld et al. 2010). Taxonomy of the Phaeophyta has been somewhat chaotic as far as the ordinal structure of the phylum is concerned (e.g. Clayton 1990; Margulis and Schwartz 1988, Pr-12; 1998, Pr-17; Sleigh et al. 1985; Bold and Wynne 1985; Graham and Wilcox 2000) with 12 to 15 orders. All systems agree that the phylum should have only one class. De Reviers et al. (2007) reviewed the state of phaeophyte systematics following a decade of molecular phylogenetic work and introduced a system with 17 orders. |
 | FIGURE 16. This is a tree that summarizes Andersen (2004), Riisberg et al. (2009), and Yang et al. (2012). The phaeophytes (shaded box) fall into a clade that includes the xanthophytes and eustigs. |
 | FIGURE 17. A cladogram of the phaeophyte orders according to the topologies of Phillips et al. (2008) and Silberfeld et al. (2010). The ordinal names are color coded to the growth forms, which show no clear phylogenetic relationships. |
| LITERATURE CITED Andersen R. A. 2004a. Biology and systematics of heterokont and haptophyte algae. American Journal of Botany. 91(10): 1508-1508. Bold, H. C. and M. J. Wynne. 1985. Introduction to the Algae. 2nd Edition. Prentice-Hall, Inc. Englewood Cliffs. NJ. Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York. Cavalier-Smith, T. 1989. The Kingdom Chromista. In: Green, J.C., B. S. C. Leadbeater, and W. L. Diver, eds. The Chromophyte Algae: Problems and Perspectives. Systematics Association Special Volume No. 38. Clarendon Press. Oxford. pp. 381-407. Clayton, M. N. 1990. Phaeophyta. In: Margulis, L., J. O. Corliss, M. Melkonian, and D. J. Chapman, eds. 1990. Handbook of the Protoctista; the Structure, Cultivation, Habits and Life Histories of the Eukaryotic Microorganisms and Their Descendants Exclusive of Animals, Plants and Fungi. Jones and Bartlett Publishers. Boston. pp. 698-714. De Reviers, B., F. Rousseau, and S. G. A. Draisma. 2007. Classification of the Phaeophyceae from past to present and current challenges. In: Brodie, J. and J. Lewis., eds. Unravelling the algae, the past, present, and future of algal systematics. The Systematics Association Special Volume Series 75. CRC Press. New York. pp. 267-284. Dodge, J. D. 1973. The fine structure of algal cells. Academic Press. New York. Graham, L. E., and L. W. Wilcox. 2000, Algae: Prentice Hall, Upper Saddle River, NJ. Kjellman, F. R. 1891. Phaeophyceae (Fucoideae). In: Engler, A. and K. Prantl, eds. Die natürlichen Pflanzenfamilien nebst ihren Gattungen und wichtigeren Arten insbesondere den Nutzpflanzen unter Mitwirkung zahlreicher hervorragender Fachgelehrten, Teil 1, Abteilung 2. Verlag von Wilhelm Engelmann. Leipzig. pp. 176-181. Margulis, L. and K. Schwartz. 1988. Five kingdoms, an illustrated guide to the phyla of life on earth. 2nd Edition. W.H. Freeman and Co. New York. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. O’Kelly, C. J. 1989. The evolutionary origins of the brown algae: information from studies of motile cell ultrastructure. In: Green, J.C., B.S.C. Leadbeater, and W.L. Diver, eds. The chromophyte algae, problems and perspectives. Systematics Association Special Volume No. 38. Clarendon Press. Oxford. pp. 255-278. Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist. 154 (Suppl.): S96–S124. Phillips, N., R. Burrowes, F. Rousseau, B. de Reviers, and G. W. Saunders. 2008. Resolving evolutionary relationships among the brown algae using chloroplast and nuclear genes. Journal of Phycology. 44: 394-405. Riisberg, I., R. J. S. Orr, R. Kluge, K. Shalchian-Tabrizi, H. A. Bowers, V. Patil, B. Edvardsen, and K. S. Jakobsen. 2009. Seven gene phylogeny of heterokonts. Protist. 160: 191-204. [C] Scagel, R. F., R. J. Bandoni, G. E. Rouse, W. B. Schofield, J. R. Stein, and T. M. C. Taylor. 1984. Plant Diversity, An Evolutionary Approach. Wadsworth Publishing Co. Belmont, CA. Silberfeld, T., J. W. Leigh, H. Verbruggen, C. Cruaud, B. de Reviers, and F. Rousseau. 2010. A multi-locus time-calibrated phylogeny of the brown algae (Heterokonta, Ochrophyta, Phaeophyceae): investigating the evolutionary nature of the “brown algal crown radiation”. Molecular Phylogenetics and Evolution. 56: 659-674. [C] Sleigh, M.A., J.D. Dodge and D. J. Patterson. 1984. Kingdom Protista. In: Barnes, R.K.S., ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc. Sunderland, Mass. Taylor, F. J. R. 1976. Flagellate phylogeny: a study in conflicts. Journal of Protozoology. 23: 28-40. Van Den Hoek, C., D. G. Mann, and H. M. Jahns. 1995. Algae, an introduction to phycology. Cambridge University Press. Cambridge. Yang, E. C., G. H. Boo, H. J. Kim, S. M. Cho, S. M. Boo, R. A. Andersen, and H. S. Yoon. 2012. Supermatrix data highlight the phylogenetic relationships of photosynthetic stramenopiles. Protist. 163: 217-231. [C] |
| By Jack R. Holt. Last revised: 02/19/2013 |
