Culture of microalgae in hatcheries

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Raceway pond used to cultivate microalgae. The water is kept in constant motion with a powered paddle wheel. Microalgenkwekerij te Heure bij Borculo.jpg
Raceway pond used to cultivate microalgae. The water is kept in constant motion with a powered paddle wheel.

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. [2]

Contents

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 Gephyrocapsa oceanica color.jpg
Microalgae are microscopic forms of algae, like this coccolithophore which are between 5 and 100 micrometres across
SpeciesApplication
Chaetoceros sp. [8] Aquaculture [8]
Chlorella vulgaris [9] Source of natural antioxidants, [9] high protein content
Dunaliella salina [10] Produce carotenoids (β-carotene) [10]
Haematococcus sp. [11] Produce carotenoids (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. [14] There are two main advantages of culturing microalgae using the open pond system. [15] Firstly, an open pond system is easier to build and operate. [15] Secondly, open ponds are cheaper than closed bioreactors because closed 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. [13] 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. [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. [23] The concept behind FPR is to increase the surface area to volume ratio such that sunlight is efficiently used. [15] [23] This system of microalgae culture was originally thought to be expensive and incapable of circulating the culture. [23] Therefore, FPRs were considered to be unfeasible overall for the commercial production of microalgae. However, an experimental FPR system in the 1980s used 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 fermentation 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] [23] FTR have been developed using a combination of sun and artificial light have led to lowering production costs. [23] However, information available on large scale counterparts to the laboratory-scale systems being developed is very limited. [23] 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 the 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. Brine shrimp cyst.jpg
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 aquaculture of brine shrimp. Brine shrimp produce dormant eggs, called cysts, which can be stored for long periods and then hatched on demand to provide a convenient form of live feed for the aquaculture of larval fish and crustaceans. [26] [27]

Other applications of microalgae within aquaculture include increasing the aesthetic appeal of finfish bred in captivity. [25] One such example can be noted in the aquaculture of salmon, where microalgae is used to make salmon flesh pinker. [25] 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. [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 Chlorella and Spirulina ( 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 a food 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 is Lake Texcoco in central Mexico. [30] The plants produce a variety of nutrients and high amounts of protein, and is often used commercially as a nutritional supplement. [31] [32] Chlorella has similar nutrition to spirulina, and is very popular in Japan. It is also used as a nutritional supplement, with possible effects on 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 Flinders University in Adelaide have been experimenting with using marine microalgae to produce proteins for human consumption, creating products like "caviar", vegan burgers, fake meat, jams and other food spreads. By manipulating microalgae in a laboratory, the protein and other nutrient contents could be increased, and flavours changed to make them more palatable. These foods leave a much lighter carbon footprint than other forms of protein, as the microalgae absorb rather than produce carbon dioxide, which contributes to the greenhouse gases. [35]

Biofuel production

In order to meet the demands of fossil fuels, alternative means of fuels are being investigated. Biodiesel and bioethanol are renewable biofuels with much potential that are important in current research. However, agriculture based renewable fuels may 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 of biomass) suitable for conversion to fuel. Furthermore, microalgae are more productive than land based agricultural crops and could therefore be more sustainable in the long run. Microalgae for biofuel production is mainly produced using tubular photobioreactors. [2]

Pharmaceuticals and cosmetics

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 medical research. [36]

Red microalgae are characterised by pigments called phycobiliproteins that contain natural colourants used in pharmaceuticals and/or cosmetics. [37]

Biofertilizer

Blue green alga was first used as a means of fixing nitrogen by allowing cyanobacteria to multiply in the soil, acting as a biofertilizer. Nitrogen fixation is important as a means of allowing inorganic compounds such as nitrogen to be converted to 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 isotopes i.e. chemical variants of an element that contain different neutrons. Microalgae can effectively incorporate isotopes of carbon (13C), nitrogen (15N) and hydrogen (2H) into their biomass. [40] 13C and 15N are used to track the flow of carbon between different trophic levels/food webs. [41] Carbon, nitrogen and 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

Related Research Articles

<span class="mw-page-title-main">Algae</span> Diverse group of photosynthetic eukaryotic organisms

Algae is an informal term for a large and diverse group of photosynthetic, eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular microalgae, such as Chlorella, Prototheca and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 metres (160 ft) in length. Most are aquatic and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem that are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and stoneworts. Algae that are carried by water are plankton, specifically phytoplankton.

<span class="mw-page-title-main">Spirulina (dietary supplement)</span> Blue-green algal genus (cyanobacteria) used in food

Spirulina is a biomass of cyanobacteria that can be consumed by humans and animals. The three species are Arthrospira platensis, A. fusiformis, and A. maxima.

<span class="mw-page-title-main">Bioreactor</span> System that supports a biologically active environment

A bioreactor refers to any manufactured device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms. This process can either be aerobic or anaerobic. These bioreactors are commonly cylindrical, ranging in size from litres to cubic metres, and are often made of stainless steel. It may also refer to a device or system designed to grow cells or tissues in the context of cell culture. These devices are being developed for use in tissue engineering or biochemical/bioprocess engineering.

<span class="mw-page-title-main">Microalgae</span> Microscopic algae

Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist individually, or in chains or groups. Depending on the species, their sizes can range from a few micrometers (μm) to a few hundred micrometers. Unlike higher plants, microalgae do not have roots, stems, or leaves. They are specially adapted to an environment dominated by viscous forces.

<span class="mw-page-title-main">Phycocyanin</span> Protein complexes in algae

Phycocyanin is a pigment-protein complex from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is an accessory pigment to chlorophyll. All phycobiliproteins are water-soluble, so they cannot exist within the membrane like carotenoids can. Instead, phycobiliproteins aggregate to form clusters that adhere to the membrane called phycobilisomes. Phycocyanin is a characteristic light blue color, absorbing orange and red light, particularly near 620 nm, and emits fluorescence at about 650 nm. Allophycocyanin absorbs and emits at longer wavelengths than phycocyanin C or phycocyanin R. Phycocyanins are found in cyanobacteria. Phycobiliproteins have fluorescent properties that are used in immunoassay kits. Phycocyanin is from the Greek phyco meaning “algae” and cyanin is from the English word “cyan", which conventionally means a shade of blue-green and is derived from the Greek “kyanos" which means a somewhat different color: "dark blue". The product phycocyanin, produced by Aphanizomenon flos-aquae and Spirulina, is for example used in the food and beverage industry as the natural coloring agent 'Lina Blue' or 'EXBERRY Shade Blue' and is found in sweets and ice cream. In addition, fluorescence detection of phycocyanin pigments in water samples is a useful method to monitor cyanobacteria biomass.

<span class="mw-page-title-main">Algaculture</span> Aquaculture involving the farming of algae

Algaculture is a form of aquaculture involving the farming of species of algae.

<span class="mw-page-title-main">Raceway pond</span>

A raceway pond is a shallow artificial pond used in the cultivation of algae.

<span class="mw-page-title-main">Photobioreactor</span> Bioreactor with a light source to grow photosynthetic microorganisms

A photobioreactor (PBR) refers to any cultivation system designed for growing photoautotrophic organisms using artificial light sources or solar light to facilitate photosynthesis. PBRs are typically used to cultivate microalgae, cyanobacteria, and some mosses. PBRs can be open systems, such as raceway ponds, which rely upon natural sources of light and carbon dioxide. Closed PBRs are flexible systems that can be controlled to the physiological requirements of the cultured organism, resulting in optimal growth rates and purity levels. PBRs are typically used for the cultivation of bioactive compounds for biofuels, pharmaceuticals, and other industrial uses.

<span class="mw-page-title-main">Algae fuel</span> Use of algae as a source of energy-rich oils

Algae fuel, algal biofuel, or algal oil is an alternative to liquid fossil fuels that uses algae as its source of energy-rich oils. Also, algae fuels are an alternative to commonly known biofuel sources, such as corn and sugarcane. When made from seaweed (macroalgae) it can be known as seaweed fuel or seaweed oil.

<i>Nannochloropsis</i> Genus of algae

Nannochloropsis is a genus of algae comprising six known species. The genus in the current taxonomic classification was first termed by Hibberd (1981). The species have mostly been known from the marine environment but also occur in fresh and brackish water. All of the species are small, nonmotile spheres which do not express any distinct morphological features that can be distinguished by either light or electron microscopy. The characterisation is mostly done by rbcL gene and 18S rRNA sequence analysis.

<span class="mw-page-title-main">Algae bioreactor</span> Device used for cultivating micro or macro algae

An algae bioreactor is used for cultivating micro or macroalgae. Algae may be cultivated for the purposes of biomass production (as in a seaweed cultivator), wastewater treatment, CO2 fixation, or aquarium/pond filtration in the form of an algae scrubber. Algae bioreactors vary widely in design, falling broadly into two categories: open reactors and enclosed reactors. Open reactors are exposed to the atmosphere while enclosed reactors, also commonly called photobioreactors, are isolated to varying extents from the atmosphere. Specifically, algae bioreactors can be used to produce fuels such as biodiesel and bioethanol, to generate animal feed, or to reduce pollutants such as NOx and CO2 in flue gases of power plants. Fundamentally, this kind of bioreactor is based on the photosynthetic reaction, which is performed by the chlorophyll-containing algae itself using dissolved carbon dioxide and sunlight. The carbon dioxide is dispersed into the reactor fluid to make it accessible to the algae. The bioreactor has to be made out of transparent material.

<i>Arthrospira</i> Genus of Cyanobacteria

Arthrospira is a genus of free-floating filamentous cyanobacteria characterized by cylindrical, multicellular trichomes in an open left-hand helix. A dietary supplement is made from A. platensis and A. maxima, known as spirulina. The A. maxima and A. platensis species were once classified in the genus Spirulina. Although the introduction of the two separate genera Arthrospira and Spirulina is now generally accepted, there has been much dispute in the past and the resulting taxonomical confusion is tremendous.

Wageningen UR has constructed AlgaePARC at the Wageningen Campus. The goal of AlgaePARC is to fill the gap between fundamental research on algae and full-scale algae production facilities. This will be done by setting up flexible pilot scale facilities to perform applied research and obtain direct practical experience. It is a joined initiative of BioProcess Engineering and Food & Biobased Research of the Wageningen University.

<i>Nannochloropsis</i> and biofuels

Nannochloropsis is a genus of alga within the heterokont line of eukaryotes, that is being investigated for biofuel production. One marine Nannochloropsis species has been shown to be suitable for algal biofuel production due to its ease of growth and high oil content, mainly unsaturated fatty acids and a significant percentage of palmitic acid. It also contains enough unsaturated fatty acid linolenic acid and polyunsaturated acid for a quality biodiesel.

<i>Arthrospira platensis</i> Species of bacterium

Arthrospira platensis is a filamentous, gram-negative cyanobacterium. This bacterium is non-nitrogen-fixing photoautotroph. It has been isolated in Chenghai Lake, China, soda lakes of East Africa, and subtropical, alkaline lakes.

Phycotechnology refers to the technological applications of algae, both microalgae and macroalgae.

<span class="mw-page-title-main">Avigad Vonshak</span>

Avigad Vonshak is a Professor Emeritus at the French Associates Institute for Agriculture and Biotechnology of Drylands at the Jacob Blaustein Institutes for Desert Research at Ben-Gurion University of the Negev, Israel.

Sammy Boussiba is a professor emeritus at the French Associates Institute for Agriculture and Biotechnology of Drylands at the Jacob Blaustein Institutes for Desert Research at Ben-Gurion University of the Negev, Israel.

<i>Chlorella vulgaris</i> Species of green alga

Chlorella vulgaris is a species of green microalga in the division Chlorophyta. It is mainly used as a dietary supplement or protein-rich food additive in Japan.

References

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