In applied phycology, the term microalgae is generally used in its broadest sense to mean both prokaryotic cya nobacteria and eukaryotic microscopic algae. The spread of these organisms is enormous since they inhabit all ecosystems - from cold, arctic regions, through extremely alkaline or saline habitats, to hot springs and arid soils. Cyanobacteria, in particular, represent the oldest group of organisms which started the creation of the Earth's oxygenic atmosphere more than 3 billion years ago. Microalgae also represent important CO2 consumers and primary producers - being the basis of the food chain in aquatic environments. Furthermore, they repre sent one of the most efficient converters of solar energy to biomass.

In nature, water blooms develop in eutrophic reser voirs where phytoplankton populations are only occasionally mixed by wind or flux. In these situations, biomass concentrations are much below 1 g of dry matter per liter. For centuries, natural blooms of the cyanobac terium Spirulina (now referred to as Arthrospira) were harvested in selective environments of alkaline soda lakes in Chad, Mexico, or Myanmar, and used as food. On the other hand, the present nutrient enrichment of surface waters, through several factors such as human waste, industrial effluents, and agricultural fertilizers, has caused massive developments of dangerous microal gal blooms; these represent an ecological threat due to the potential deterioration of water quality and create serious environmental problems.

Although several microalgal strains are cultivated worldwide for different purposes, for example, as health food, feed additives, or as a source of bioactive com pounds for pharmacology, cosmetics, or diagnostic products, the bulk of annual biomass production is repre sented by only three species: the cyanobacterium Spirulina and the green algae Chlorella and Dunaliella.

This article presents a general overview of freshwater microalgae for biomass production: the basic techniques and concepts, cultivation systems, and production strains, as well as its problems and achievements.

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