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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.

Another difficulty in classifying algae is determining whether they are protists, plants, or whether they merit their own kingdom. Different classification systems answer this question in different ways, with some even splitting the group between the kingdoms Protista and Plantae. Here we have grouped algae with protozoa and slime molds in Protista because mthe majority of algae are unicellular, and even the multicellular algae are structurally simple compared to true plants.

Within the classification of algae, individual species are divided into five groups, based on characteristics such as type of chlorophyll molecule used in photosynthesis and type of reproductive cycle. The structure of the chloroplast is also used, for a very important reason. According to the endosymbiotic theory of chloroplast evolution, proposed by Lynn Margulis of the University of Massachusetts, Amherst, chloroplasts may have evolved when small photosynthesizing cells were engulfed, but not digested, by larger cells. Instead, the two types of cells developed a symbiotic relationship, with the photoautotroph living inside the larger cell. The number of membranes surrounding the chloroplast allows us to determine what type of organism the original photoautotroph was. If it was a prokaryote, the chloroplast will have two membranes: one from the engulfed cell and one from the engulfing cell. If it came from a eukaryote, the chloroplast will have three membranes: the original organelle membrane, the plasma membrane of the engulfed cell and the membrane from the engulfed cell. These two possible endosymbiotic events are diagrammed below.

Green 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. This second characteristic indicates that the chloroplasts evolved from endosymbiosis of a prokaryote, as is the case with higher plants. Also, analysis of genetic material indicates a high degree of relatedness between green algae and terrestrial plants.

The life cycle of green algae is shown below.

Figure %: Life cycle of the Green Algae
Haploid spores give rise to a multicellular haploid leaf-like structure called a thallus. The thallus produces gametes. Green algae are isogamus, meaning they have only one type of gamete, rather than having separate male and female gametes. When two gametes meet, fertilization takes place and a diploid zygote is formed. The zygote then germinates, undergoes meiosis and forms haploid spores. The diploid phase of the life cycle is brief and unicellular. There are a few exceptions this general life cycle, such as the Ulva (sea lettuce), which has a multicellular diploid phase similar to that found in brown algae.

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.

Unlike green and red algae, brown algae the life cycle of brown algae includes an alternation of generations.

Figure %: Life cycle of the Brown Algae
This term describes a reproductive strategy that involves a succession of haploid and diploid phases. Spores produce a multicellular haploid thallus. The thallus produces isogamus gametes. Fertilization occurs when two gametes meet and a diploid zygote is formed. The zygote then gives rise to a multicellular diploid structure, which in some cases is indistinguishable from the haploid structure. The diploid thallus produces haploid spores through meiosis.


The Euglenoids are the least algae-like of the algae. They are unicellular and motile, and they lack a key plant-like structure: the cell wall. For these reasons, they are often categorized as protests. Most euglenoids are photosynthetic, but some lack chlorophyll and are heterotrophic (requiring complex organic compounds of nitrogen and carbon for metabolic synthesis).

The structure of a typical euglenoid, Euglena is pictured below.

Figure %: Structure of Euglena
The euglena has several organelles typical of eukaryotes. The chloroplasts of the Euglena are surrounded by three membranes, indicating that they are the result of endosymbiosis of a eukaryote, most likely a green algal cell. Euglena also have a light sensitive stigma which allows them to move toward light sources for better photosynthesis. The two flagella found in the anterior invagination are not the same as the flagella found in prokaryotes. gives a comparison of the structures of eukaryotic and prokaryotic flagella. The flagella of Euglena are rooted in the cell membrane, and thus cannot rotate like those of prokaryotes. Instead, they beat back and forth in a whip-like motion.

Figure %: Eukaryotic and Prokaryotic flagella


Dinoflagellates are mostly unicellular. The cell wall of these algae is only present when in the cyst stage. Most species of dinoflagellates have two flagella. One typical flagellum extends behind the cell. The other, usually shorter flagellum lies in a groove encircling the cell.

Dinoflagellates are an important component of plankton, the primary producers of organic material in the oceans. While this makes them important as a food source, some species of dinoflagellates are poisonous. The Red Tides common off the coasts of Florida and Mexico are caused by dinoflagellates and can kill millions of fish.

Red Algae

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. Cyanobacteria are discussed in the SparkNote on Monera.

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