Microalgae or microphytes are microscopic algae, typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist individually, or in chains or groups. Depending on the species, their sizes can range from a few micrometers to a few hundred micrometers. Unlike higher plants, microalgae do not have roots, stems, or leaves. They are specially adapted to an environment dominated by viscous forces. Microalgae, capable of performing photosynthesis, are important for life on earth; they produce approximately half of the atmospheric oxygen and use simultaneously the greenhouse gascarbon dioxide to grow photoautotrophically. Microalgae, together with bacteria, form the base of the food web and provide energy for all the trophic levels above them. Microalgae biomass is often measured with chlorophyll a concentrations and can provide a useful index of potential production. The standing stock of microphytes is closely related to that of its predators. Without grazing pressures the standing stock of microphytes dramatically decreases. The biodiversity of microalgae is enormous and they represent an almost untapped resource. It has been estimated that about 200,000-800,000 species in many different genera exist of which about 50,000 species are described. Over 15,000 novel compounds originating from algal biomass have been chemically determined. Most of these microalgae species produce unique products like carotenoids, antioxidants, fatty acids, enzymes, polymers, peptides, toxins and sterols.
Characteristics and uses
The chemical composition of microalgae is not an intrinsic constant factor but varies over a wide range, both depending on species and on cultivation conditions. Some microalgae have the capacity to acclimate to changes in environmental conditions by altering their chemical composition in response to environmental variability. A particularly dramatic example is their ability to replace phospholipids with non-phosphorus membrane lipids in phosphorus-depleted environments. It is possible to accumulate the desired products in microalgae to a large extent by changing environmental factors, like temperature, illumination, pH, CO2 supply, salt and nutrients. Microphytes also produce chemical signals which contribute to prey selection, defense, and avoidance. These chemical signals affect large scale tropic structures such as algal blooms but propagate by simple diffusion and laminar advective flow. Microalgae such as microphytes constitute the basic foodstuff for numerous aquaculture species, especially filtering bivalves. Photosynthetic and chemosynthetic microbes can also form symbiotic relationships with host organisms. They provide them with vitamins and polyunsaturated fatty acids, necessary for the growth of the bivalves which are unable to synthesize it themselves. In addition, because the cells grow in aqueous suspension, they have more efficient access to water, CO2, and other nutrients. Microalgae play a major role in nutrient cycling and fixing inorganic carbon into organic molecules. While fish oil has become famous for its omega-3 fatty acid content, fish don't actually produce omega-3s, instead accumulating their omega-3 reserves by consuming microalgae. These omega-3 fatty acids can be obtained in the human diet directly from the microalgae that produce them.
Aquaculture
A range of microalgae species are produced in hatcheries and are used in a variety of ways for commercial purposes. Studies have estimated the main factors in the success of a microalgae hatchery system to be;
