|Algae (Codium Fragile) that has washed up on a beach.|
Green algae are any of the species of algae that are classified under the taxonomic division chlorophyta. Their name comes from two Greek words, chloros (green) and phyto (plant). They are an incredibly diverse division with many differences in physical appearance and structure. The division contains nearly 12,000 different species, making it one of the largest groups of algae. Their contributions to the earth's supply of oxygen and their commercial uses make them one of the most important groups of algae.
Green algae are eukaryotic and contain organelles that are found regularly in similar organisms. A common feature that can be found in most members is a two layer cell wall made of cellulose and pectin. There is a wide variety in structure amongst the green algae, but they can differ in many ways. They are found in various forms including colonies, single celled, filaments, and seaweed. Despite their name, some green algae are not actually green. Those that are green have similar amounts of chlorophylls a and b as land plants. They can also be orange, brown, or reddish in color. The two largest classes (Charophyceae and Chlorophyceae) were originally classified together because of color, but have actually been found that they are only distantly related to each other. While similar to animals in some ways, including being motile, they are classified as plants because of several things. They perform photosynthesis, store energy as starch, and reproduce in ways similar to plants. The majority of motile forms of green algae are unicellular and, as can be seen in the image, have two flagellum to propel them through water. Some reproductive cells (zoospores) also have flagellum which enable them to be motile.
Algae are able to reproduce both sexually and asexually. Sexual reproduction is less common and may have several different forms of gametes including isogamous, anisogamous, oogamous. All three forms of reproduction can be found in just one genus, Chlamydomonas. Many will alternate between forms of reproduction by alteration of generations. During a haploid phase, the algae will reproduce sexually, while during the diploid phase, it will reproduce asexually through meiosis.
- Sexual Reproduction
While less common than asexual reproduction, sexual reproduction is still very important. It helps provide variability in genetic material and occurs in three main forms: isogamy, anisogamy, and oogamy. Isogamy is when two motile gametes, similar in size and structure, fuse and form new organisms. In anisogamy, two different motile gametes fuse. In oogamy a motile male gamete fuses with a sessile female gamete. Most algae are also capable of conjugation, which is where two cells merge and exchange genetic information.
- Asexual Reproduction
Asexual reproduction is the more common form of reproduction among green algae, and there have been several different observed forms used in creating new algae. These forms include zoospory, palmelloids, aplanospores, hypnospores, akinetes, autospores, and cysts. In zoospory, motile spores equipped with flagella are produced in the zoosporangium. Palmelloids follow the same pattern, except the spores never escape and form a colony inside the parent. Aplanospores are non-motile spores. Hypnospores are spores that are released into a dormant state. This is most commonly observed in species living in freezing environments. Their protoplasm can be changed into a gel to help prevent it from freezing. Akinetes are not technically spores, as they are pieces of the actual plant that form a protective covering and enter a dormant state. This is used by some algae to outlive harsh conditions. Autospores are developed within the parent are nearly identical in shape and structure to their parent. Finally, cysts are cells from the parent plant that are enclosed in a thick envelope and remain dormant through harsh conditions.
Some 12,000 species of green algae can be found in habitats all over the world. Charophytes and Clorophytes are both found in marine environments, while Trebouxiophytes are found on land. Others have symbiotic relationships with lichens and other organisms.  Despite the variety of habitats, most are found in fresh water supplies. They have even been found in extreme cold, buried up to seventy feet deep in Antarctic ice. Something unique can be found in the genus Chlorella: some members have the ability to quadruple once every 20 hours, an ability that is rivaled by no other organism on earth. This, along with the medical and health benefits associated with its consumption, has made it the most important and profitable genus in Chlorophyta. Green algae produce their food from photosynthesis. As with most other plants, they are near the base of the food chain and produce most energy for other organisms. They serve as a major food source for plankton, which, in turn, provide food for fish. Since they are found all over the world, they also produce a large portion of the world's supply of oxygen. Negative effects of green algae are felt primarily during algal blooms, which can cause unpleasant odors and tastes, and can also kill other marine organisms because of a decreased oxygen supply.
Several types of algae have also been found to have commercial uses. Dunaliella salina is used in the production of beta-carotene in Australia. Members of the genus Chlorella are grown and used as a yeast like substance. Several types of algae are also used in sewage treatment. They provide oxygen for decomposer bacteria and also accumulate toxins to further help clean the water. They are also the main source of crude oil. Their fossilized remains become oil under the pressure and temperature under the ocean.
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