c) Chlorophytes (Green Algae)
This algal division, better known as the green algae, exhibits a great deal of morphological and reproductive diversity.
The green algae and the true plants share many common features: both are photosynthetic, containing chlorophylls a and b and carotenoids within their chloroplasts; their primary energy sources are stored as starch; both have cell walls composed of cellulose; and their motile cells possess two whip-like flagella. These common features have led botanists to the general agreement that the true plants have evolved from the green algal protists.
Within the green algae, three basic morphological types will be discussed. These are the unicellular, the colonial, and the filamentous forms. Note the importance of the basic morphological adaptations of these algae to their environment. Also, note that some of the adaptations discussed are utilized by the dinoflagellates and the euglenoid algae.
These are the simplest algae. Except for a few that live in damp terrestrial habitats, these organisms are mainly confined to aquatic environments which provide an adequate supply of nutrients, water, a means of dispersal and a degree of support. Thus, these simple organisms can survive and even thrive in this environment. Although they consist of a single cell, botanists are able to place them in literally thousands of different genera and species. The range of cellular morphology is, at least in part, an adaptation for floatation or for predation avoidance. These organisms must have some ability to float at least through a portion of their life cycle because they must remain near enough to the surface to obtain sufficient light for photosynthesis. Large irregular shaped cells have the advantage of being avoided by herbivores in the water. Most of the animals, which eat algae, are very tiny and these cells would be difficult for them to ingest.
Make a wet mount of Chlamydomonas sp. and Chlorella sp. and observe the similarities and differences between these two organisms. In addition, examine slide #77 of Chlamydomonas sp. Also refer to Figure 9.
What particular morphological features do these algae possess which enhance their survival?
Chlamydomonas sp.:
Chlorella sp.:
Is there any feature lacking in these organisms, which they need to survive?
Figure 9: Unicellular Algae - Chlamydomonas sp. (top) and Chlorella sp. (bottom).
These organisms are also found primarily in the aquatic environment. Morphological variation is due to differences in number and plane of cell division only. These algae have only one basic vegetative cell type. Division in definite and consistent planes results in formation of a regular colony while division in random planes results in the formation of irregular colonies. Some colonial algae possess flagella for motility. Again, the major environmental pressure is the need to remain in the photosynthetic zone. Because of the potential increase in sinking rates due to the increase in mass, these algae tend to inhabit shallow waters where the effective light penetration is to the bottom. In addition, many colonial forms have small cells, which secrete mucilage. The mucilage is less dense than water and aids in buoyancy.
Make a wet mount of Gonium sp., and of Scenedesmus sp. Note that Gonium sp. is very fragile and often breaks into isolated cells during slide preparation. Also refer to Figure 10.
Suggest a reason as to why these algae (Gonium sp. and Scenedesmus sp.) would have evolved their particular shape.
Figure 10: Colonial Algae – Gonium sp. (top) and Scenedesmus sp. (bottom).
While the colonial body form appears to be a dead end from an evolutionary viewpoint, the filamentous algae apparently had the morphological flexibility to develop into more complex aquatic and terrestrial plants. The simplest filamentous algae consist of a thallus (body) of a single chain of cells. This is the result of cell division in one plane only. Branching filaments occur when there is periodic division in a second plane (Figure 11). Once this stage of development was reached, the next step was to evolve more complex branching patterns. Some cells in the filaments divide several times in different planes resulting in thick and thin portions. Finally, some filamentous algae began to show some cellular differentiation. For example, thin portions penetrate the substrate, thicker portions lie on the surface, and erect portions of the thallus project about the mass and bear reproductive structures. This could easily lead to a division of labour among the three areas. The algae that evolved these features were ones that attached to the substrate or lived on damp soils. Examine the slide of Ulothrix sp. (#28) and Cladophora sp. (#27) as well as the demonstration slide of Fritschiella sp. Also, refer to Figure 12 illustrating typical filamentous algae.
How many planes of cell division occur in each genus?
Which algae would be most successful in a damp soil environment?
Why would these particular algae be most successful in a damp soil environment?
Figure 11: Planes of cell division.
Figure 12A: Ulothrix sp.
Figure 12B: Cladophora sp.
Figure 12C: Fritschiella sp.
Give examples of unicellular, filamentous, and colonial algae.
How are green algae different from cyanobacteria?