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Algaculture

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

The majority of algae that are intentionally cultivated fall into the category of microalgae (also referred to as phytoplankton, microphytes, or planktonic algae). Macroalgae, commonly known as seaweed, also have many commercial and industrial uses, but due to their size and the specific requirements of the environment in which they need to grow, they do not lend themselves as readily to cultivation.

Some of the commercial and industrial purposes of algae cultivation are for production of bioplastics, dyes and colorants, feedstock, pharmaceuticals, pollution control, algae fuel and for possible future food sources.

History and uses of algae

Presumably, the first use of algae was food.

Food

One example is the wrapper on a sushi roll. Other species are edible as well, such as Spirulina and dulse (Palmaria palmata).

Dulse is a red species sold particularly in Ireland and Atlantic Canada. It is eaten raw, fresh, dried, or cooked like spinach.

Spirulina is a blue-green microalgae with a long history as a food source in East Africa and pre-colonial Mexico. As it is high in protein and other nutrients it is currently used as a food supplement and as a treatment for malnutrition.

Chlorella, another popular microalgae, has similar nutrition and is an ingredient of "Chlorella Growth Factor," a nutritional supplement which makes the unsubstantiated claim that it can increase growth in animals and children.. As a nutritional supplement it is touted as a method of reducing mercury levels, supposedly by chelation of the mercury to the cell wall of the organism. However, what little scientific study there is on this topic, appears to contradict this claim. Chlorella is very popular in Japan and is currently one of the most prescribed supplements in that country.

Chlorella, particularly a transgenic strain which carries an extra mercury reductase gene, has been studied as an agent for the environmental remediation due to its ability to reduce Hg2+ to the less toxic elemental mercury.

Purple laver (Porphyra) is also collected and used in a variety of ways. In Wales, for example "laverbread" is a traditional food, and in Ireland it is collected and made into a jelly by stewing or boiling. Preparation also can involve frying or converting to a pinkish jelly by heating the fronds with a little water and beating with a fork. It is also harvested along western coast of North America, from California to British Columbia and by Native Hawaiians and the Māori of New Zealand.

Irish moss (Chondrus crispus), often confused with Mastocarpus stellatus, is the source of carrageenan for the stiffening of instant puddings, sauces, and dairy products such as ice cream. Irish moss is also used by brewers as a fining agent; the addition of Irish moss to the wort 15 minutes before the end of the boil produces a clearer beer.

Sea lettuce (Ulva lactuca), is used in Scotland where it is added to soups and salads. Dabberlocks or badderlocks (Alaria esculenta) is eaten either fresh or cooked in Greenland, Iceland, Scotland and Ireland.

Fertilizer and agar

For centuries seaweed has been used as fertilizer. It is also an excellent source of potassium for manufacture of potash and potassium nitrate.

There are commercial uses of algae, such as agar.

Growing, harvesting, and processing algae

Monoculture

Often it is desired to grow just one species of algae in each growing vessel. With mixed cultures, one species tends to dominate over time and if a non dominant species is believed to have particular nutritive value for some larval animal, it is necessary to obtain pure cultures in order to cultivate this species. Individual species cultures are also needed for research purposes.

A common method of obtaining pure cultures is serial dilution. A wild sample or a contaminated lab sample of algae containing the desired algae is diluted with filtered water and small aliquots are introduced into a large number of small growing containers. The dilution is done following a microscopic examination of the source culture to a degree that leads one to expect on average there will be a few of the growing containers with only one cell of the desired species. Following a suitable period on a light table, microscopic examination then selects out the successful growing containers and they are used to start larger cultures.

While algae is often grown in monocultures using microbiological techniques to purify the desired strain, another approach has been used very successfully to produce algae feed for the cultivation of a variety of mollusks. Sea water is passed through filters to remove algae which are too large for the larvae being cultivated. Tanks in a green house, sometimes on a balcony in the mollusk house, are filled with the partially filtered water and nutrients are added. The tanks may be aerated and the water is used after only a day or two of growing. The resulting thin soup of mixed algae has been shown to be an excellent food source for larval mollusks. An advantage of this method of algaculture is the low maintenance requirements.

Growing algae

Factors

When cultivating algae, several factors must be considered, and different algae have different requirements.

Essential factors include water, carbon dioxide, minerals and light (the basic reaction in water is carbon dioxide + light energy = glucose + oxygen ).

Temperature
The water must be in a temperature range that will support the specific algal species being grown.
Light And Mixing
Light must not be too strong nor too weak.

In most algal-cultivation systems, light only penetrates the top to of the water. This is because as the algae grow and multiply, they become so dense that they block light from reaching deeper into the pond or tank. Algae only need about 1/10th the amount of light they receive from direct sunlight. Direct sunlight is often too strong for algae.

In order to have ponds that are deeper than 4 inches algae growers use various methods to agitate the water in their ponds, thus circulating the algae so that it does not remain on the surface, which would cause it to be over-exposed. Paddle wheels can be used to circulate (stir) the water in a pond. Compressed air can be introduced into the bottom of a pond or tank to agitate the water, bringing algae from the lower levels up with it as it makes its way to the surface.

Apart from agitation, another means of supplying light to algae is to place the light in the system. Glow plates are sheets of plastic or glass that can be submerged into a tank, providing light directly to the algae at the right concentration.

Odor and Oxygen
The odor associated with bogs, swamps, or any stagnant waters taken over by algae, can be due to oxygen depletion in the water caused by the decay of deceased algal blooms. Under anoxic conditions, the bacteria inhabiting algae cultures break down the organic material and produce hydrogen sulfide and ammonia which causes the odor. This condition, called hypoxia, often results in the death of all aquatic animals. In a system where algae is intentionally cultivated, maintained, and harvested, neither eutrophication nor aquatic hypoxia are likely to occur.

Some algae also produce odorous chemicals, particularly certain blue-green algae (cyanobacteria) such as Anabaena. The most well-known of these odor-causing chemicals are MIB (2-methylisoborneol) and Geosmin. They give a sort of musty or earthy odor that can be quite strong if an algae bloom is present. Subsequent death of the cyanobacteria will also release MIB, etc. that is trapped in the cells. These chemicals can be smelled at very low levels, in the ppb range, and are responsible for many "taste and odor" issues in drinking water treatment and distribution. There are many good references on taste and odor, MIB, etc. in regards to cyanobacteria, but one example is "A Guide to Geosmin and MIB-producing Cyanobacteria in the United States", Izaguirre and Taylor, Water Science Technology2004, 49(9):19-24. Cyanobacteria can also produce chemical toxins that have been a problem in drinking water in some cases.

Nutrients
Nutrients must be controlled so algae will not be "starved" and nutrients will not be wasted.

Methods

Algae can be mainly cultured in open-ponds (such as raceway-type ponds and lakes) and photobioreactors. Raceway ponds may be less expensive.
Open-ponds
Raceway-type ponds and lakes are open to the elements and so sometimes called "open-pond" systems. They are much more vulnerable to contamination by other microorganisms, such as invasive algal species or bacteria. Because of these factors, the number of species successfully cultivated in an "open-pond" system for a specific purpose (such as for food, for the production of oil, or for pigments) are relatively limited. In open systems one does not have control over water temperature and lighting conditions. The growing season is largely dependent on location and, aside from tropical areas, is limited to the warmer months.

A major benefit to this type of system are that it is one of the cheaper ones to construct, in the very least only a trench or pond needs to be dug. It can also have some of the largest production capacities relative to other systems of comparable size and cost. This type of culture can be viable when the particular algae in question requires (or is able to survive) some sort of extreme condition that other algae can not survive. For instance, Spirulina sp. can grow in water with a high concentration of sodium bicarbonate and Dunaliela salina will grow in extremely salty water. Open culture can also work if there is a simple inexpensive system of selecting out the desired algae for use and to inoculate new ponds with a high starting concentration of the desired algae. Some chain diatoms fall into this category as they can be filtered from a stream of water flowing through an outflow pipe. A "pillow case" of a fine mesh cloth is tied over the outflow pipe and most algae flow right through. The chain diatoms are held in the bag and used to feed shrimp larvae (in Eastern hatcheries) and to inoculate new tanks or ponds.

Photobioreactors
Algae can also be grown in a photobioreactor (PBR). A PBR is a bioreactor which incorporates some type of light source. Virtually any translucent container could be called a PBR, however the term is more commonly used to define a closed system, as opposed to an open tank or pond.

A variation on the basic "open-pond" system is to enclose it with a transparent or translucent barrier, to cover a pond or pool with a greenhouse. A pond covered with a greenhouse could be considered a PBR. While this usually results in a smaller system, for economic reasons, it does take care of many of the problems associated with an open system. It allows more species to be grown, it allows the species that are being grown to stay dominant, and it extends the growing season, only slightly if unheated, and if heated it can produce year round.

Because PBR systems are closed, all essential nutrients must be introduced into the system to allow algae to grow and be cultivated.

A PBR can be operated in "batch mode", but it is also possible to introduce a continuous stream of sterilized water containing nutrients, air, and carbon dioxide. As the algae grows, excess culture overflows and is harvested. If sufficient care is not taken, continuous bioreactors often collapse very quickly, however once they are successfully started, they can continue operating for long periods. An advantage of this type of algae culture is that algae in the "log phase" is produced which is generally of higher nutrient content than old "senescent" algae. It can be shown that the maximum productivity for a bioreactor occurs when the "exchange rate" (time to exchange one volume of liquid) is equal to the "doubling time" (in mass or volume) of the algae.

Different types of PBRs include:

  • tanks provided with a light source
  • polyethylene sleeves or bags
  • glass or plastic tubes.

Harvesting of algae

Algae can be harvested using microscreens, by centrifugation, by flocculation. and by froth flotation.

Alum and ferric chloride are chemical flocculants used to harvest algae. A commercial product called "Chitosan", commonly used for water purification, can also be used as a flocculant but is far more expensive. The shells of crustaceans are ground into powder and processed to acquire chitin, a polysaccharide found in the shells, from which chitosan is derived via de-acetylation. Water that is more brackish, or saline requires additional chemical flocculant to induce flocculation. Harvesting by chemical flocculation is a method that is often too expensive for large operations. Interrupting the carbon dioxide supply to an algal system can cause algae to flocculate on its own, which is called "autoflocculation".

In froth flotation, the water and algae are aerated into a froth, with the algae then removed from the water.

Ultrasound based methods of algae harvesting are currently under development, and other, additional methods are currently being developed.

Oil Extraction

Algae oils have a variety of commercial and industrial uses, and are extracted through a wide variety of methods.

The simplest method is mechanical crushing. Since different strains of algae vary widely in their physical attributes, various press configurations (screw, expeller, piston, etc) work better for specific algae types. Often, mechanical crushing is used in conjunction with chemicals (see below).

Estimates of the cost to extract oil from microalgae vary, but are likely to be around $1.80/kg (compared to $0.50/kg for palm oil).

  • Chemical solvents: Algal oil can be extracted using chemicals. Benzene and ether have been used, oil can also be separated by hexane extraction, which is widely used in the food industry and is relatively inexpensive. The downside to using solvents for oil extraction are the dangers involved in working with the chemicals. Care must be taken to avoid exposure to vapors and direct contact with the skin, either of which can cause serious damage. Benzene is classified as a carcinogen. Chemical solvents also present the problem of being an explosion hazard.

Soxhlet extraction is an extraction method that uses chemical solvents. Oils from the algae are extracted through repeated washing, or percolation, with an organic solvent such as hexane or petroleum ether, under reflux in a special glassware.

  • Enzymatic extraction: Enzymatic extraction uses enzymes to degrade the cell walls with water acting as the solvent, this makes fractionation of the oil much easier. The costs of this extraction process are estimated to be much greater than hexane extraction. The enzymatic extraction can be supported by ultrasonication. The combination "sonoenzymatic treatment" causes faster extraction and higher oil yields.
  • Expression/Expeller press: When algae is dried it retains its oil content, which then can be "pressed" out with an oil press. Many commercial manufacturers of vegetable oil use a combination of mechanical pressing and chemical solvents in extracting oil.
  • Osmotic shock: Osmotic shock is a sudden reduction in osmotic pressure, this can cause cells in a solution to rupture. Osmotic shock is sometimes used to release cellular components, such as oil.
  • Supercritical fluid: In supercritical fluid/CO2 extraction, CO2 is liquefied under pressure and heated to the point that it has the properties of both a liquid and a gas, this liquified fluid then acts as the solvent in extracting the oil.
  • Ultrasonic-assisted extraction: Ultrasonic extraction, a branch of sonochemistry, can greatly accelerate extraction processes. Using an ultrasonic reactor, ultrasonic waves are used to create cavitation bubbles in a solvent material, when these bubbles collapse near the cell walls, it creates shock waves and liquid jets that causes those cells walls to break and release their contents into the solvent.

Other methods are still being developed, including ones to extract specific types of oils, such as those with a high production of long-chain highly unsaturated fatty acids. {| class="wikitable"

Algal Culture Collections

Specific algal strains can be acquired from algal culture collections.

Commercial and industrial uses

Algae are cultivated to serve many commercial and industrial uses.

*CO2 sequestration
*Uranium/Plutonium sequestration
*Fertilizer Runoff reclamation
*Sewage treatment

Nutritional

There are many algae that are cultivated for their nutritional value, either for supplemental use, or as a food source.

Spirulina (Arthrospira platensis) is a blue-green algae (cyanobacteria) that is quite nutritious. This species thrives in open systems and commercial growers have found it well-suited to cultivation. One of the largest production sites for Spirulina is Lake Texcoco in central Mexico. The plants themselves produce a variety of nutrients and high amounts of protein, and is often used commercially as a nutritional supplement. Extracts and oils from algae are also used as additives in various food products. The plants also produce Omega-3 and Omega-6 fatty acids, which are commonly found in fish oils, and which have been shown to have positive medical benefits to humans.

Pollution Control

Much of the carbon dioxide that is released into the atmosphere is from the burning of fossil fuels. With concerns over global warming, new methods for the thorough and efficient capture of CO2 are being sought out. An alternative to carbon capture and storage, by attaching an algae pond, or photobioreactor, to any fuel burning plant, the carbon dioxide produced during combustion can be fed into the algae system. Additional nutrients can be sourced from sewage, thus turning two pollutants into resources for the production of biofuels, with a land requirement much smaller than other crop sources (terrestrial crops) and without competing with food production.

Future food source

Large scale algaculture of the oceans, as well as similar culture of other plankton, is proposed as a future food source in order to support a growing world population. Currently, however, it has limited application, since e.g. technologies of the green revolution has resulted in that land-based food production so far has increased faster than the demand of the growing world population.

See also

References

External links

Algal cultivation

Harvesting

General

Analysis

Photobioreactors

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