Microalgae gold microscopic algae grow in either marine or freshwater systems. They are primary producers in the oceans that convert water and carbon dioxide to biomassand 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 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 that increase productivity and are economically profitable.
Commonly cultivated microalgae species
|Chaetoceros sp. ||Aquaculture |
|Chlorella vulgaris ||Source of natural antioxidants |
|Dunaliella salina ||Produce carotenoids ( β-carotene ) |
|Haematococcus sp. ||Produce carotenoids ( β-carotene ), astaxanthin , canthaxanthin |
|Phaeodactylum tricornutum ||Source of antioxidants |
|Porphyridium cruentum ||Source of antioxidants |
|Rhodella sp. ||Colourant for cosmetics |
|Skeletonema sp ||Aquaculture |
|Arthrospira maxima ||High protein content – Nutritional supplement |
|Arthrospira platensis ||High protein content – Nutritional supplement |
Hatchery production techniques
A range of microalgae species are produced in hatcheries and are used in a variety of ways for commercial purposes. Studies Estimated hand-have factors in the success of a hatchery system microalgae have the dimensions of the container / bioreactor Where microalgae is cultured, exposure to light / radiation and concentration of cells dans le reactor. 
Open pond system
This method has been employed since the 1950s. There are two main advantages of microalgae culturing 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 weight systems is also less important than commercially important strains such as Arthrospira sp. , where optimal growth is limited by temperature. 
This method is used in outdoor cultivation and production of microalgae; where is growing in a microalgae is growing.  The culture is grown in transparent tubes that binds horizontally on the ground and is connected by a network of pipes.  Air is passed through the tube such that it 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 the control of the physical, chemical and biological environment of the culture.  This means that evaporation, temperature gradients, and protection from ambient contamination are very difficult to control .  Photobioreactors are the primary example of a closed system where abiotic factors can be controlled for. Several closed systems have been tested for the purpose of culturing microalgae,
This system includes tubes 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 better able to maintain better temperature gradients.  An example noted in higher production of Arthrospira sp. Used as a dietary supplement was increased because of a longer temperature range. 
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 (VAPs) that are a type of photobioreactor .  This photobioreactor is characterized 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 in a flow rate. The two main types of vertical photobioreactors are the VAP Flow-through and the Bubble Column VAP. 
Flat plate reactors
Flat plate reactors (FPR) are built using narrow panels and are placed horizontally to maximize sunlight input to the system.  The concept behind FPR is to increase the area to such a volume that sunlight is efficiently used.   This system of microalgae culture was originally thought to be expensive and incapable of circulating culture.  Therefore, FPRs have been considered to be unfeasible overall for the commercial production of microalgae. However, an experimental FPR system in the 1980s used to circulate across the culture of a gas exchange unit across horizontal panels. This overcomes issues of circulation and provides an advantage of an oxygen transfer.  Examples of successful use of FPRs can be seen in the production of Nannochloropsis sp. used for its high levels of astaxanthin . 
Fermentor-type reactors (FTRs) are where bioreactors are fermented . FTRs have not been developed in the field of microalgae and pose a disadvantage in the area to volume ratio and decreased efficiency in utilizing sunlight.   FTR has been developed using a combination of lower and lower production costs.  However, information available on large scale counterparts to the laboratory-scale systems being developed is very limited.  The main advantage is that the extrinsic factors can be enhanced for the production of pharmaceuticals .
Microalgae is an important source of nutrition and is used widely in the aquaculture of other organisms, directly or as a source of basic nutrients.  Aquaculture farms rearing larvae of molluscs , echinoderms , crustaceans and fishes use microalgae as a source of nutrition.  Low bacteria and high microalgal biomass is a crucial food source for shellfish aquaculture. 
Microalgae can form the start of a chain of further aquaculture processes. For example, microalgae is an important food source in the aquaculture of brine shrimp . Brine shrimp produces eggs, called cysts , which can be stored for long periods of time and provides a convenient method for the production of larval fish and crustaceans.  
Other applications of microalgae within aquaculture include increasing the aesthetic appeal of finfish bred in captivity.  One such example can be noted in the aquaculture of salmon , where microalgae is used to make salmon flesh pinker.  This is achieved by the addition of natural pigments containing carotenoids such as astaxanthin produced from the microalgae Haematococcus to the diet of farmed animals. 
In order to meet the demands of fossil fuels , alternate means of fuels are being investigated. Biodiesel and bioethanol are important with current potential. However, agriculture based renewable fuelsmay not be completely sustainable and thus may not be able to replace fossil fuels.  Microalgae can be remarkably rich in oils (up to 80% dry weight biomass ) suitable for conversion to fuel. Furthermore, microalgae are more productive than land based agricultural crops and could be more sustainable in the long run.  Microalgae forbiofuel production is mainly produced using tubular photobioreactors . 
Cosmetic and health benefits
The main species of microalgae grown as Chlorella sp. and Spirulina sp. The main forms of production occur in small scale ponds with artificial mixers.  Novel bioactive chemical compounds can be isolated from microalgae like sulphated polysaccharides .  These compounds include fucoidans , carrageenans and ulvans That are used for Their beneficial properties. These properties are anticoagulants , antioxidants , anticancer agents that are being tested in research. Red microalgae are characterized by pigments called phycobiliproteins that contain natural colors used in pharmaceuticals and / or cosmetics.  Production of long chain Omega-3 polyunsaturated fatty acids important for human diet can also be achieved through microalgal hatchery systems. 
Blue green algae was first used as a means of fixing nitrogen by allowing cyanobacteria to multiply in the soil. Nitrogen fixation is important as a way of allowing inorganic compounds such as nitrogen to be converted into organicforms which can be used by plants.  The use of cyanobacteria is an economically sound and environmentally friendly method of increasing productivity.  Rice production in India and Iran has employed this method of using cyanobacteria to supplement nitrogen content in soils.  
Microalgae are a source of valuable molecules such as isotopes ie chemical variants of an element that contain different neutrons. Microalgae can effectively incorporate isotopes of carbon ( 13 C), nitrogen ( 15 N) and hydrogen ( 2 H) into their biomass.  13 C and 15 are used to track the flow of carbon between different trophic levels / food webs.  Carbon, nitrogen, and sulfur isotopes can also be used to determine the degree of disturbance.
Cell fragility is the biggest issue that limits the productivity of closed photobioreactors .  Damage to cells Can Be Attributed To the turbulent flow dans le bioreactor qui is required to create mixing so light is available to all cells. 
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