Algae (singular alga) are a large group of diverse organisms that use photosynthesis to produce food. Although some forms are large and multicellular, they differ from plants in that their cells are not clearly organized into different types of tissue with different functions. This group includes a wide variety of organisms that are not always closely related to one another — the similarities in form are often due to parallel evolution, where different organisms have adapted in similar ways to fill similar niches. They are described as polyphyletic, meaning that not all members of the group share the same common ancestor.
By the modern definition, all algae are eukaryotes, which means that the DNA in their cells is contained within a nucleus enclosed by a membrane. Organisms whose cells do not have a nucleus are prokaryotes. The eukaryotes also include plants, fungi and animals. Prokaryotes include bacteria and archaea. The algae can be divided into a number of sub-groups, based largely on the types of pigments they use for photosynthesis.
These forms use the green pigment chlorophyll to photosynthesize, and they are thought to be the ancestors of land plants. Some authorities include them in the plant kingdom, while others prefer to regard them as a separate category of life. They may be single celled or multicellular, and some types live in colonies or form long filaments consisting of many cells. A number of the single-celled types are capable of independent movement using flagellae — long, whip-like structures used by many microorganisms for locomotion. Green algae are found in a wide variety of habitats, including freshwater, the sea, soil, tree trunks and damp walls, but the majority are aquatic.
It is thought that land plants evolved from a type of green alga, possibly about 500 million years ago. They contain the same types of chlorophyll and other pigments as land plants. There are further similarities: for example, the chlorophyll is contained in structures called chloroplasts, and many types store sugars in starch granules, as do land plants.
Also known as rhodophyta, these were among the first eukaryotic organisms on the planet, and their signatures have been found in rocks almost 2 billion years old. They are mostly marine organisms, and include many types of seaweed, as well as a number of single-celled species. Their red color comes from the pigments phycoerythrin and phycocyanin, which they use for photosynthesis. These pigments absorb blue light, which reaches further below the ocean surface than the red light trapped by chlorophyll, allowing the rhodophyta to photosynthesize at greater depths. This group also includes coralline algae, which build shells made of calcium carbonate for themselves and can form reefs.
The proper scientific name for this group is chromista. It is an extremely diverse group, with its members ranging from diatoms — microscopic, single-celled forms with silica shells — to “kelp” seaweeds — large, multicellular organisms that can grow to 164 feet (50 meters) in length. They use a different kind of chlorophyll from that used by plants to photosynthesize and often have additional pigments, such as fucoxanthin, which gives many of these organisms a brown color. Diatoms are an important part of the phytoplankton, which produce a great deal of the planet’s oxygen through photosynthesis and form the base of many marine food chains. Kelp seaweeds can form extensive seafloor “forests,” which are of great ecological importance.
Today, these microorganisms are considered to be bacteria; however, they are still sometimes referred to by their old name, “blue-green algae.” They differ mainly in that they are prokaryotes, like all bacteria, but they can make their own food by photosynthesis. Cyanobacteria are a very ancient group and may have been the very first organisms to use photosynthesis. Many experts think that, in the distant past, some single-celled, non-photosynthesizing organisms may have incorporated cyanobacteria in a symbiotic relationship, and that these bacteria may have become the chloroplasts that are seen today in algae and plants.
From time to time, in certain locations, a species of alga may undergo a population explosion, resulting in what is known as an “algal bloom.” These can occur on coastlines and in freshwater lakes. It is not always possible to establish the cause, but often, it seems to be due to agricultural run-off containing fertilizers that stimulate increased growth and multiplication. Algal blooms are often harmful to other aquatic life forms, and occasionally to animals and even humans. The huge numbers of algae can severely reduce the oxygen content of the water, and some species produce toxins that can kill or harm other organisms.
A number of types of seaweed, especially among the red algae, can be eaten. Seaweeds also provide a number of important food additives, and agar — a type of gel used for culturing microorganisms. Another potential use is in the production of biofuels. The organisms are fast growing and undemanding in terms of conditions and nutritional requirements, and so they can provide a cheap and efficient way of accumulating biomass for fuel.