Culture of microalgae in hatcheries

Microalgae or microscopic algae grow in either marine or freshwater systems. They are primary producers in the oceans that convert water and carbon dioxide to biomass and oxygen in the presence of sunlight. Microalgae or microscopic algae grow in either marine or freshwater systems. They are primary producers in the oceans that convert water and carbon dioxide to biomass and oxygen in the presence of sunlight. The oldest documented use of microalgae was 2000 years ago, when the Chinese used the cyanobacteria Nostoc as a food source during a famine. Another type of microalgae, the cyanobacteria Arthrospira (Spirulina), was a common food source among populations in Chad and Aztecs in Mexico as far back as the 16th century. Today cultured microalgae is used as direct feed for humans and land-based farm animals, and as feed for cultured aquatic species such as molluscs and the early larval stages of fish and crustaceans. It is a potential candidate for biofuel production. Microalgae can grow 20 or 30 times faster than traditional food crops, and has no need to compete for arable land. Since microalgal production is central to so many commercial applications, there is a need for production techniques which increase productivity and are economically profitable. A range of microalgae species are produced in hatcheries and are used in a variety of ways for commercial purposes. Studies have estimated main factors in the success of a microalgae hatchery system as the dimensions of the container/bioreactor where microalgae is cultured, exposure to light/irradiation and concentration of cells within the reactor. This method has been employed since the 1950s. There are two main advantages of culturing microalgae using the open pond system. Firstly, an open pond system is easier to build and operate. Secondly, open ponds are cheaper than closed bioreactors because closed bioreactors require a cooling system. However, a downside to using open pond systems is decreased productivity of certain commercially important strains such as Arthrospira sp., where optimal growth is limited by temperature. That said, it is possible to use waste heat and CO2 from industrial sources to compensate this. This method is used in outdoor cultivation and production of microalgae; where air is moved within a system in order to circulate water where microalgae is growing. The culture is grown in transparent tubes that lie horizontally on the ground and are connected by a network of pipes. Air is passed through the tube such that air escapes from the end that rests inside the reactor that contains the culture and creates an effect like stirring. The biggest advantage of culturing microalgae within a closed system provides control over the physical, chemical and biological environment of the culture. This means factors that are difficult to control in open pond systems such as evaporation, temperature gradients and protection from ambient contamination make closed reactors favoured over open systems. Photobioreactors are the primary example of a closed system where abiotic factors can be controlled for. Several closed systems have been tested to date for the purposes of culturing microalgae, few important ones are mentioned below: This system includes tubes laid on the ground to form a network of loops. Mixing of microalgal suspended culture occurs through a pump that raises the culture vertically at timed intervals into a photobioreactor. Studies have found pulsed mixing at intervals produces better results than the use of continuous mixing. Photobioreactors have also been associated with better production than open pond systems as they can maintain better temperature gradients. An example noted in higher production of Arthrospira sp. used as a dietary supplement was attributed to higher productivity because of a better suited temperature range and an extended cultivation period over summer months. These reactors use vertical polyethylene sleeves hung from an iron frame. Glass tubes can also be used alternatively.Microalgae are also cultured in vertical alveolar panels (VAP) that are a type of photobioreactor. This photobioreactor is characterised by low productivity. However, this problem can be overcome by modifying the surface area to volume ratio; where a higher ratio can increase productivity. Mixing and deoxygenation are drawbacks of this system and can be addressed by bubbling air continuously at a mean flow rate. The two main types of vertical photobioreactors are the Flow-through VAP and the Bubble Column VAP.