Algae are a very large and diverse group of autotrophic organisms, ranging from unicellular to multicellular forms. Algae, like plants, use the energy from the sun to carry on photosynthesis; however, unlike plants, algae don't possess the roots, leaves or stems characteristic of land plants. Algae are aquatic organisms and may grow on salty, brackish and freshwater bodies or attached to hard substrates underwater. They may also thrive in humid soil on land or as endosymbionts of other organisms. Algae are classified according to several structural characteristics, including food storage, cell wall composition and the types of photosynthetic pigments present within them.
Algae carry on photosynthesis within organelles called chloroplasts. It is believed that nearly all algae derived their photosynthetic structure from cyanobacteria, although the nature of the chloroplasts varies within different species, revealing distinctive endosymbiotic events. All algae contain the pigment chlorophyll a, but algae can also contain other accessory pigments, which is what provides the characteristic colors within certain algae species. Although most algae are phototropic, capable of using sunlight for photosynthesis, some species are chemotropic and obtain their energy from chemical reactions, while others are heterotrophs, and a few species form symbiotic relationships with corals, lichens and sea sponges.
Single-celled algae occur mostly in water, most commonly in the form of plankton. Single-celled organisms often have flagella which enable them to move freely through the water. All single-celled algae possess chlorophyll a as the main photosynthetic pigment; however, other accessory pigments may be present. Single-celled algae usually thrive in the upper layers of the ocean and freshwater habitats. This enables them to use sunlight for the process of photosynthesis. While most single-celled organisms are photosynthetic, some species may become heterotrophic when light is scarce. Single-celled algae are important primary producers in many marine food chains, as many small and large marine organisms feed on them.
Single-celled algae may group together to form multicellular structures. Motile unicellular species may form complex aggregations of colonies, filaments or single multinucleated cells. In multicellular algae, the thallus (body) forms one large multinucleated cell. The only groups that form multicellular thallus are the red algae, brown algae and some species of green algae. Studies have demonstrated that green and red algae are closely related to land plants, suggesting a common origin. The first plants on Earth probably evolved from freshwater algae. Fossils resembling green algae have been found from the early Cambrian, around 500 million years ago.
Land and marine algae
The survivability of algae is guaranteed, as long as there is water in their environment. Algae are most commonly found in both the oceans and terrestrial ecosystems, such as lakes, rivers, ponds and humid habitats. Salt concentrations, temperature and depth determine which kind of algae can thrive in a given environment. Algae can attain many colors, including red, brown or green. Only red algae are known to inhabit deep ocean habitats. While some species of algae can thrive outside of the water, they still need a humid environment, such as that found in rocks, mud or other hard humid substrates, to survive.
Algae can reproduce asexually by cell division or sexually by the production of spores, depending on the species and environmental conditions. Asexual reproduction can occur in two distinct ways. A fragment of an alga may become detached from its body and continue to grow as long as the environmental conditions are favorable. In the other way, spores with specialized filaments or flagella may swim freely, and when the appropriate environmental conditions are met, the spores germinate into new algae. Single-celled algae usually live in the upper layer of the ocean. When there are enough sunlight and nutrients needed for reproduction, they multiply asexually. In sexual reproduction, the fertilization of gametes produces a zygote, which develops into new algae.
While most species of algae are unicellular and motile, some species form complex aggregations of single cells into colonial forms and filaments. Some multicellular organisms, such as brown algae (kelp) may reach lengths of up to 45 meters (150 feet). Algae lack the specialized systems to transport internal fluids or give support to vertical growth. Unlike terrestrial plants, algae lack an epidermis to resist dehydration. For this reason, algae must be in permanent contact with water to survive. According to Cliffnotes.com, algae are unicellular or multicellular eukaryotic organisms that lack roots, stems or leaves, but possess chlorophyll and other accessory pigments to carry on photosynthesis.