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Algaculture is a form of aquaculture involving the farming of species of algae. The majority of algae that are intentionally cultivated fall into the category of microalgae (also referred to as phytoplankton, microphytes, or planktonic algae).

Macroalgae (seaweed) also have many commercial and industrial uses, but due to their size and the specific requirements of the environment in which they need to grow, they do not lend themselves as readily to cultivation. However, as the algae grow and multiply, the culture becomes so dense that it blocks light from reaching deeper into the water. Direct sunlight is too strong for most algae, which need only approximately 10% of the amount of light they receive from direct sunlight.

Algae can be cultured in open-ponds which are vulnerable to contamination by other microorganisms, such as other algal species or bacteria. Thus cultivators usually choose closed systems for monocultures. Open systems also do not offer control over temperature and lighting. The growing season is largely dependent on location and, aside from tropical areas, is limited to the warmer months.

Open pond systems are cheaper to construct, at the minimum requiring only a trench or pond. Large ponds have the largest production capacities relative to other systems of comparable cost. Open pond cultivation can exploit unusual conditions that suit only specific algae and can also work if there is a system of culling the desired algae and inoculating new ponds with a high starting concentration of the desired algae.

Enclosing a pond with a transparent or translucent barrier effectively turns it into a greenhouse. This allows more species to be grown; it allows the species that are being grown to stay dominant; and it extends the growing season – and if heated, the pond can produce year round.

Algae can also be cultured in a photobioreactor (PBR) which is a bioreactor that incorporates a light source. However, because photobioreactor systems are closed, the cultivator must provide all nutrients, including carbon dioxide. A photobioreactor can operate in batch mode, which involves restocking the reactor after each harvest, but it is also possible to grow and harvest continuously. Continuous operation requires precise control of all elements to prevent immediate collapse. The grower provides sterilized water, nutrients, air, and carbon dioxide at the correct rates. This allows the reactor to operate for long periods.

See also: Algae, Algae Fuels.