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 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. 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.