Algaes are difficult to define. Some classify the group as all eukaryotic photosynthesizing microorganisms. This definition includes the Euglenoid and Dinoflagellates groups, both of which are known to be more closely related to other groups of non-photosynthesizing protozoa than to other algae. For this reason, those two groups are sometimes classified as protozoa rather than algae. In this discussion, we will group euglenoids and dinoflagellates with the algae so that we may compare their photosynthetic characteristics. Keep in mind that this inclusion does not imply close relation to other algae.
Green algae can be either unicellular or multicellular. They live mostly in fresh water, but some can live on land in moist soils. A few green algae are found in marine environments. These organisms often live symbiotically with aquatic and marine animals. They are of particular interest because the group from which land plants evolved, the charophyta, are green algae. The green algae are often classified in the Kingdom Plantae, based on two characteristics shared with higher plants: 1) green algae use chlorophyll a and b in photosynthesis; 2) the chloroplasts of green algae are enclosed in a double membrane.
Yellow-brown Algae, Brown Algae, and Diatoms
These algae are distinguished from other algae and higher plants by the type of chlorophyll they use. While most algae and plants use chlorophyll a and b, these algae use chlorophyll a and c, but not b. Most are unicellular or colonial, and they usually reproduce asexually. Yellow-brown algae are mostly freshwater dwellers, while diatoms live in both fresh- and saltwater. Brown algae are almost exclusively saltwater dwellers. Diatoms are somewhat distinct from other algae in this group. Their cell walls are box-like, with a top and bottom that are fitted together. The cell walls have a high silica content, giving them a glassy appearance. The shells of dead diatoms are used in polishing products and detergents. What makes them truly different from other primitive plant-like organisms is that their non- reproductive cells are normally diploid rather than haploid. All brown algae are all multicellular. In addition, they are the largest of the algae that possess chlorophyll c, growing to lengths of 45 meters or more. The thallus may be flat or three dimensional in structure, but none possess the complex internal tissues of higher plants.
Red algae are mostly multicellular marine seaweeds. Like the green algae and higher plants, their chloroplasts have a two membrane envelope; red algae is often placed in the kingdom Plantae. In addition to chlorophyll a and b, red algae have accessory pigments called phycocyanins and allphycocyanins that contribute to the red coloration of some species. Their reproductive cycle involves alternation of generations like that of the brown algae, though no red algae have flagellated gametes, while some brown algae do.
Blue-green Algae (cyanobacteria)
For many years, cyanobacteria, a group of photoautotrophic eubacteria, were mistakenly classified as algae. They formed the group called blue-green algae. The lack of a defined nucleus and organelles such as chloroplasts make it clear that these are in fact eubacteria rather than algae.