Introduction to Algae and the origin of Eukaryotic cells

Biology is currently dominated by the five-kingdom concept, the theory that there are five main groups of organisms: plants, animals, fungi, protista, and prokaryotes. Among eukaryotes, plants, animals, and fungi are quite distinct and easy to recognize when derived members are considered. But when the simplest eukaryotes are studied, deciding which should be classified as plants, which should be considered animals, and which are fungi can be difficult. Kingdom Protista was established to hold these problematic organisms. It was thought that as our knowledge increased, we would be able to assign each organism to one or another of the other three eukaryotic kingdoms until nothing was left in Protista.

We no longer think so. Green algae are obviously related to land plants, and certain protozoans are clearly related to multicellular animals (Table ; Fig. 1), but many protistans are quite different from plants, animals, or fungi. Among algae, organisms called dinoflagellates (Fig. 1b) have so many relictual features that they may not be truly eukaryotes. Each algal group must have diverged from the others quite early judging by the numerous differences in biochemistry, pigmentation, nutrient reserves, and other features.

FIGURE 1: (a) Euglenoid algae (division Euglenophyta). (Peter Parks Animals Animals/Oxford Scientific Films) (b)A dmoflagellate (division Pyrrhophyta). (Biophoto Associates) (c) Diatoms (division Chrysophyta). (Courtesy of Greta Fryxell, Texas A and M University) (d) Golden brown algae (division Chrysophyta). (e) Yellow-green algae (division Chrysophyta). (d and e, E. R. Degginger) (f) Green algae (division Chlorophyta). (William E. Ferguson) (g) Brown algae (division Phaeophyta). (E. R. Degginger) (h) Red algae (division Rhodophyta). (William E. Ferguson).

We are not certain yet how they are interrelated, and placing them in kingdom Protista emphasizes that uncertainty. Placing the green algae with the Protista is difficult because they are so obviously related to land plants, but this placement seems necessary because green algae constitute a large division with thousands of species, most of which are not closely related to the evolutionary line that gave rise to land plants. Classifying dram Chlorophyta in kingdom Plantae would put many unicellular and colonial algae into the plant kingdom, which would be just as artificial as taking the ancestors of true plants out of the plant kingdom. Taxonomy is a human endeavor, and when we assign groups to categories, we must separate some species from rather close relatives. It is necessary to under­stand why organisms are classified as they are and not merely memorize the classification.

Reproductive structures are the critical factors in distinguishing algae from the plants of kingdom Plantae. Algae are defined as photosynthetic individuals whose reproductive structures are completely converted to spores or gametes that, when released, leave nothing but empty walls (Fig. 2). In true plants (technically known as embryophytes) the reproductive structures are always complex and multicellular, and only some of the inner cells become reproductive. Outer cells constitute a protective layer and persist after the reproductive cells are released. This may seem trivial, but the algal method of reproduction is a cellular process, reflecting the simple, cellular level of organization most species have throughout their bodies. Reproductive structures of true plants involve two tissues- reproductive and sterile—functioning together as an organ; this reflects a more complex organ level of integration in the whole body.

FIGURE 2: Age are distinguished from plants (embryophytes) primarily by the nature of the reproductive structures. (a and b) In algae, the sporangium or gametangium may be either one large cell, as town here, or multicellular, but every cell is converted into a reproductive cell. When reproductive cells are released (b), only the wall of the original cell is left. (c and d) Sporangia and gametangia of plants are always multicellular and only the inner cells differentiate into spores or gametes; after they are liberated (d), a residual layer of sterile cells remains.