Microalgae or microscopic algae grow in either marine or freshwater systems. They areprimary producers in the oceans that convert water and carbon dioxide tobiomass and oxygen in the presence of sunlight.[2]
The oldest documented use of microalgae was 2000 years ago, when the Chinese used thecyanobacteriaNostoc as a food source during a famine.[3] Another type of microalgae, the cyanobacteriaArthrospira (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 forbiofuel 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.
| Species | Application |
|---|---|
| Chaetoceros sp.[8] | Aquaculture[8] |
| Chlorella vulgaris[9] | Source of naturalantioxidants,[9]high protein content |
| Dunaliella salina[10] | Producecarotenoids (β-carotene)[10] |
| Haematococcus sp.[11] | Producecarotenoids (β-carotene),astaxanthin,canthaxanthin[11] |
| Phaeodactylum tricornutum[9] | Source of antioxidants[9] |
| Porphyridium cruentum[9] | Source ofantioxidants[9] |
| Rhodella sp.[8] | Colourant forcosmetics[8] |
| Skeletonema sp[8] | Aquaculture[8] |
| Arthrospira maxima[12] | Highprotein content –Nutritional supplement[12] |
| Arthrospira platensis[12] | High protein content –Nutritional supplement[12] |
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]
This method has been employed since the 1950s across the CONUS.[14] There are two main advantages of culturing microalgae using theopen pond system.[15] Firstly, an open pond system is easier to build and operate.[15] Secondly, open ponds are cheaper than closed bioreactors because closedbioreactors require a cooling system.[15] However, a downside to using open pond systems is decreased productivity of certain commercially important strains such asArthrospira 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]
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]
The biggest advantage of culturing microalgae within a closed system provides control over the physical, chemical and biological environment of the culture.[13] This means factors that are difficult to control in open pond systems such as evaporation, temperaturegradients 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:
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 aphotobioreactor. 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 ofArthrospira 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]
These reactors use verticalpolyethylene 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 ofphotobioreactor.[13] This photobioreactor is characterised by low productivity. However, this problem can be overcome by modifying thesurface area tovolume ratio; where a higher ratio can increase productivity.[13] Mixing anddeoxygenation 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]
By using an electrocatalytic process to produceacetate 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 likeChlamydomonas reinhardtii have turned out to be promising.[20][21]
Flat plate reactors(FPR) are built using narrow panels and are placed horizontally to maximise sunlight input to the system.[22] The concept behind FPR is to increase the surface area to volume ratio such that sunlight is efficiently used.[15][22] This system of microalgae culture was originally thought to be expensive and incapable of circulating the culture.[22] Therefore, FPRs were considered to be unfeasible overall for the commercial production of microalgae. However, an experimental FPR system in the 1980s usedcirculation within the culture from a gas exchange unit across horizontal panels.[22] This overcomes issues of circulation and provides an advantage of an open gas transfer unit that reduces oxygen build up.[22] Examples of successful use of FPRs can be seen in the production ofNannochloropsis sp. used for its high levels ofastaxanthin.[23]
Fermentor-type reactors (FTR) are bioreactors wherefermentation is carried out. FTRs have not developed hugely in the cultivation of microalgae and pose a disadvantage in the surface area to volume ratio and a decreased efficiency in utilizing sunlight.[15][22] FTR have been developed using a combination of sun and artificial light have led to lowering production costs.[22] However, information available on large scale counterparts to the laboratory-scale systems being developed is very limited.[22] The main advantage is that extrinsic factors i.e. light can be controlled for and productivity can be enhanced so that FTR can become an alternative for products for thepharmaceutical industry.[22]
Microalgae is an important source of nutrition and is used widely in theaquaculture of other organisms, either directly or as an added source of basic nutrients. Aquaculture farms rearing larvae ofmolluscs,echinoderms,crustaceans andfish use microalgae as a source of nutrition. Low bacteria and high microalgal biomass is a crucial food source for shellfish aquaculture.[24]
Microalgae can form the start of a chain of further aquaculture processes. For example, microalgae is an important food source in theaquaculture of brine shrimp. Brine shrimp produce dormant eggs, calledcysts, which can be stored for long periods and then hatched on demand to provide a convenient form of live feed for the aquaculture oflarval fish and crustaceans.[25][26]
Other applications of microalgae within aquaculture include increasing theaesthetic appeal of finfish bred in captivity.[24] One such example can be noted in theaquaculture of salmon, where microalgae is used to make salmon flesh pinker.[24] This is achieved by the addition of natural pigments containingcarotenoids such asastaxanthin produced from the microalgaeHaematococcus to the diet of farmed animals.[27]Two microalgae species,I. galbana andC. calcitrans are mostly composed of proteins, which are used to brighten the color of salmon and related species.[28]
The main species of microalgae grown as health foods areChlorella andSpirulina (Arthrospira platensis). The main forms of production occur in small scale ponds with artificial mixers.[10]Arthrospira platensis is a blue-green microalga with a long history as a food source in East Africa and pre-colonial Mexico. Spirulina is high in protein and other nutrients, finding use as afood supplement and for malnutrition. It thrives in open systems and commercial growers have found it well-suited to cultivation. One of the largest production sites isLake Texcoco in central Mexico.[29] The plants produce a variety of nutrients and high amounts ofprotein, and is often used commercially as a nutritional supplement.[30][31]Chlorella has similar nutrition to spirulina, and is very popular inJapan. It is also used as anutritional supplement, with possible effects onmetabolic rate.[32]
Production oflong chain omega-3 fatty acids important for human diet can also be cultured through microalgalhatchery systems.[33]
Australian scientists atFlinders University inAdelaide have been experimenting with using marine microalgae to produce proteins for human consumption, creating products like "caviar",vegan burgers,fake meat,jams and otherfood spreads. By manipulating microalgae in alaboratory, theprotein and othernutrient contents could be increased, and flavours changed to make them more palatable. These foods leave a much lightercarbon footprint than other forms of protein, as the microalgae absorb rather than producecarbon dioxide, which contributes to thegreenhouse gases.[34]
In order to meet the demands offossil fuels, alternative means of fuels are being investigated.Biodiesel andbioethanol are renewablebiofuels with much potential that are important in current research. However,agriculture-basedrenewable fuels may not be completely sustainable and thus unlikely to completely replace fossil fuels. Microalgae can be remarkably rich in oils (up to 80% dry weight ofbiomass) suitable for conversion to fuel, and are more productive than land-based agricultural crops, and could therefore be more sustainable in the long run. As of 2008, microalgae for biofuel production was mainly being produced using tubularphotobioreactors.[2]
One barrier to the use of microalgae as fuel has been the energy required for pre-treatments necessary to break down complexbiopolymers in the cells of microalgae, in order to produce gaseous biofuels such as methane and hydrogen. A 2016 study looked at using biological methods to effect this pretreatment, using purified enzymes to bring about cell degradation.[35]
Botryococcus braunii is one microalgae that has been studied as a potential source of biofuel, in particular through its naturally occurring residue known ascoorongite, which is found in Australia. Researchers at theWestern Sydney University looked at usingpyrolysis to removecarboxyl before further processing usinghydrocracking andhydrogenation.[36]
Novelbioactive chemical compounds can be isolated from microalgae like sulphatedpolysaccharides. These compounds includefucoidans,carrageenans andulvans that are used for their beneficial properties. These properties areanticoagulants,antioxidants,anticancer agents that are being tested medical research.[37]
Red microalgae are characterised by pigments calledphycobiliproteins that contain natural colourants used inpharmaceuticals and/orcosmetics.[38]
Blue green alga was first used as a means of fixing nitrogen by allowingcyanobacteria to multiply in the soil, acting as abiofertilizer.Nitrogen fixation is important as a means of allowinginorganic compounds such asnitrogen to be converted toorganic forms which can then be used by plants.[39] The use of cyanobacteria is an economically sound and environmentally friendly method of increasing productivity.[40] This method has been use forrice production in India and Iran, using the nitrogen fixing properties of free living cyanobacteria to supplement nitrogen content in soils.[39][40]
Microalgae are a source of valuable molecules such asisotopes i.e. chemical variants of an element that contain different neutrons. Microalgae can effectively incorporate isotopes ofcarbon (13C),nitrogen (15N) andhydrogen (2H) into their biomass.[41]13C and15N are used to track the flow of carbon between different trophic levels/food webs.[42] Carbon, nitrogen andsulphur isotopes can also be used to determine disturbances to bottom dwelling communities that are otherwise difficult to study.[42]
Cell fragility is the biggest issue that limits the productivity from closedphotobioreactors.[43] Damage to cells can be attributed to the turbulent flow within thebioreactor which is required to create mixing so light is available to all cells.[43]
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