Algae are a diverse group of photosynthetic organisms, ranging from unicellular to multicellular eukaryotic organizations. Algae are classified into the kingdom protista, lacking cellular differentiation into tissues. All algae possess the green pigment chlorophyll a, although other pigments can be present obscuring the greenish color of chlorophyll a. Algae lack the distinct cells and organs that characterize land plants. Algae range in size from microscopic organisms, such as plankton that drift passively near the surfaces of oceans and freshwater bodies, to macroscopic seaweeds several meters long. The structural body of algae, either unicellular or multicellular, is called the thallus.
Algae’s general information
Most algae are photosynthetic, meaning they use energy from the sun to produce organic substances needed for their subsistence; thus, they are autotrophs capable of producing their own food. All algae contain the chlorophyll a, but they can also contain a number of other accessory pigments which usually provide the characteristic color shown in certain species. Most algae require a moist environment, since they lack the waxy cuticle found in land plants. Algae may be microscopic (plankton) and live on the surface of the oceans or freshwater bodies or macroscopic (seaweeds) and grow attached to hard substrates in coastal environments. Their size ranges from 0.5 micrometers to 50 meters in length.
Algae's body structure
The thallus is a term used in botany to refer to a body that lacks true roots, stems, leaves or vascular system, such as those found in plants. The thallus forms the body of algae. For microscopic unicellular algae, the thallus comprises a single cell, whereas for multicellular algae, the thallus comprises an aggregation of filaments of cells. There are some species of multicellular algae which are coenocytes, meaning that they lack cell walls or membranes separating the nuclei. The complete thallus appears to be one large multinucleated cell. In most cases the cell walls are made of cellulose, although some tropical species may contain calcified CaCO3 walls.
Some species of algae, such as kelps, may show plant-like structures resembling roots, stems and leaves. The haptera is a structure that anchors the alga to a substrate; therefore, it functions as a root. The stipes are stem-like structures, which are usually flexible and keep some species of algae from being carried away by the force of waves during low tides. The blades are leaf-like structures. Another important structure in some algal species is a gas-filled bladder for buoyancy, known as a pneumatocyst. The pneumatocyst is usually located between the stipes and the algae’s blades. Each species of algae usually shows a branching pattern, which is often used to identify them.
Most algae lack the rigidity that characterizes land plants. The aquatic environment in which they usually thrive provides support to the algal body, which generally undulates when coming in contact with the force of waves and water currents. Algal cells absorb the necessary minerals and moisture from the watery environment that surrounds them. The leaf-like shape of some species allows them to take full advantage of the surface area by absorbing sunlight for photosynthesis. Holdfasts are specialized root-like structures that maintain the algae attached to a hard aquatic substrate, so the algae can grow in the direction where sunlight is coming in at the surface.
Algae have developed a varied array of body structures. While some algae are unicellular and motile, others form complex aggregations in the form of colonies, filaments or single multinucleated cells, with no cell membranes or walls enclosing the nuclei. According to Plant Biology Algae, there are some distinct and effective features used to classify each species of algae, including the number of photosynthetic pigments, the stored kind of foods, the composition of the cell wall, number of flagella, shape of chloroplasts and the presence of pyrenoids and the molecular sequences of their DNA and RNA.