Microalgae or microphytes are microscopic algae, typically found in freshwater and marine ecosystems. They are single celled species which live either individually, or in chains and groups.
Their sizes can range from a few micrometers (μm) to a few hundred micrometers, depending on the species. Unlike earth plants, microalgae do not have roots, stems, or leaves and they are specially adapted to an environment dominated by viscous forces.
By performing photosynthesis, Microalgae produce approximately half of the atmospheric oxygen on earth  while simultaneously using carbon dioxide to grow photoautotrophically. Microalgae, together with bacteria, form the base of the food chain 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.