Culture of microalgae in hatcheries

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primary producers in the oceans that convert water and carbon dioxide to biomass and oxygen in the presence of sunlight.[2]

The oldest documented use of microalgae was 2000 years ago, when the Chinese used the cyanobacteria Nostoc as a food source during a famine.[3] 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.[4]

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.[5] It is a potential candidate for biofuel production.[6] Microalgae can grow 20 or 30 times faster than traditional food crops, and has no need to compete for arable land.[6][7] Since microalgal production is central to so many commercial applications, there is a need for production techniques which increase productivity and are economically profitable.

Commonly cultivated microalgae species

Microalgae are microscopic forms of algae, like this coccolithophore which are between 5 and 100 micrometres across
Species Application
Chaetoceros sp.[8] Aquaculture[8]
Chlorella vulgaris[9] Source of natural
antioxidants, [9]
high protein content
Dunaliella salina[10] Produce
β-carotene)[10]
Haematococcus sp.[11] Produce
β-carotene), astaxanthin, canthaxanthin[11]
Phaeodactylum tricornutum[9] Source of antioxidants[9]
Porphyridium cruentum[9] Source of
antioxidants[9]
Rhodella sp.[8] Colourant for cosmetics[8]
Skeletonema sp[8] Aquaculture[8]
Arthrospira maxima[12] High protein content – Nutritional supplement[12]
Arthrospira platensis[12] High protein content – Nutritional supplement[12]

Hatchery production techniques

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.[13]

Open pond system

This method has been employed since the 1950s across the CONUS.

bioreactors require a cooling system.[15] 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.[13] That said, it is possible to use waste heat and CO2 from industrial sources to compensate this.[16][17][18][19][20]

Air-lift method

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.[15] The culture is grown in transparent tubes that lie horizontally on the ground and are connected by a network of pipes.[15] 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.[15]

Closed reactors

The biggest advantage of culturing microalgae within a closed system provides control over the physical, chemical and biological environment of the culture.

gradients and protection from ambient contamination make closed reactors favoured over open systems.[13]
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:

Horizontal photobioreactors

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.[13] 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.[13]

Vertical systems

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.[13] 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.[13] 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.[13]

In darkness

By using an electrocatalytic process to produce acetate from water, electricity and carbon dioxide, which is then used by the algae as food source, sunlight and photosynthesis is no longer required. The method is still at an early stage, but experiments with algae like Chlamydomonas reinhardtii have turned out to be promising.[21][22]

Flat plate reactors

Flat plate reactors(FPR) are built using narrow panels and are placed horizontally to maximise sunlight input to the system.

circulation within the culture from a gas exchange unit across horizontal panels.[23] This overcomes issues of circulation and provides an advantage of an open gas transfer unit that reduces oxygen build up.[23] Examples of successful use of FPRs can be seen in the production of Nannochloropsis sp. used for its high levels of astaxanthin.[24]

Fermentor-type reactors

Fermentor-type reactors (FTR) are bioreactors where

pharmaceutical industry.[23]

Commercial applications

Use in aquaculture

Microalgae is used to culture brine shrimp, which produce dormant eggs (pictured). The eggs can then be hatched on demand and feed to cultured fish larvae and crustaceans.

Microalgae is an important source of nutrition and is used widely in the aquaculture of other organisms, either directly or as an added source of basic nutrients. Aquaculture farms rearing larvae of molluscs, echinoderms, crustaceans and fish use microalgae as a source of nutrition. Low bacteria and high microalgal biomass is a crucial food source for shellfish aquaculture.[25]

Microalgae can form the start of a chain of further aquaculture processes. For example, microalgae is an important food source in the

larval fish and crustaceans.[26][27]

Other applications of microalgae within aquaculture include increasing the

carotenoids such as astaxanthin produced from the microalgae Haematococcus to the diet of farmed animals.[28]
Two microalgae species, I. galbana and C. calcitrans are mostly composed of proteins, which are used to brighten the color of salmon and related species.[29]

Human nutrition

The main species of microalgae grown as health foods are

metabolic rate.[33]

Production of

long chain omega-3 fatty acids important for human diet can also be cultured through microalgal hatchery systems.[34]

Australian scientists at

greenhouse gases.[35]

Biofuel production

In order to meet the demands of

photobioreactors.[2]

Pharmaceuticals and cosmetics

Novel

anticancer agents that are being tested medical research.[36]

Red microalgae are characterised by pigments called

pharmaceuticals and/or cosmetics.[37]

Biofertilizer

Blue green alga was first used as a means of fixing nitrogen by allowing

organic forms which can then be used by plants.[38] The use of cyanobacteria is an economically sound and environmentally friendly method of increasing productivity.[39] This method has been use for rice production in India and Iran, using the nitrogen fixing properties of free living cyanobacteria to supplement nitrogen content in soils.[38][39]

Other uses

Microalgae are a source of valuable molecules such as

sulphur isotopes can also be used to determine disturbances to bottom dwelling communities that are otherwise difficult to study.[41]

Issues

Cell fragility is the biggest issue that limits the productivity from closed

photobioreactors.[42] Damage to cells can be attributed to the turbulent flow within the bioreactor which is required to create mixing so light is available to all cells.[42]

See also

References

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  17. ^ Microalgal biorefinery from CO2 and the effects under the Blue Economy: pdf download
  18. ^ Culture of microalga Spirulina platensis in alternative sources of nutrients
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  30. ^ Yenni Kwok. "The Imp With a Mighty Kick". Asia Week. CNN.tv.
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  35. ^ Leckie, Evelyn (14 Jan 2021). "Adelaide scientists turn marine microalgae into 'superfoods' to substitute animal proteins". ABC News. Australian Broadcasting Corporation. Retrieved 17 Jan 2021.
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