The majority of algae that are intentionally cultivated fall into the category of micro-algae, also referred to as phytoplankton, microphytes, or planktonic algae..

There is also macro-algae, 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 on a large scale as microalgae and are most often harvested wild from the ocean.

Growing algae

When cultivating algae, several factors must be considered, and different algae have different requirements. The water must be in a temperature range that will support the specific algal species being grown. Nutrients must be controlled so algae will not be "starved" and so that nutrients will not be wasted. Light must not be too strong nor too weak.

Algae can be cultured in raceway-type ponds and lakes.^[4] Because these systems are open to the elements, 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.

A variation on the basic "open-pond" system is to close it off, to cover a pond or pool with a greenhouse. 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.

Algae can also be grown in a photobioreactor. A photobioreactor is a bioreactor which incorporates some type of light source. Virtually any translucent container could be called a photobioreactor, however the term is more commonly used to define a closed system, as opposed to an open tank or pond. Because these systems are closed, all essential nutrients must be introduced into the system to allow algae to grow and be cultivated. Essential nutrients include carbon dioxide, water, minerals and light. A pond covered with a greenhouse could be considered a photobioreactor. A photobioreactor 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.

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.

Different types of photobioreactors include:

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

Drying of harvested Spirulina

In most algal-cultivation systems, light only penetrates the top 3-4 inches 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 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.

The odor associated with bogs, swamps, or any stagnant waters that have been taken over by algae, is 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 sulphide 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. Living algae does not emit objectionable odors.

Harvesting of algae

Algae can be harvested using microscreens, by centrifugation, or by flocculation.^[5] Froth flotation is another method to harvest algae whereby the water and algae are aerated into a froth, with the algae then removed from the water.^[6] Alum and ferric chloride are chemical flocculants used to harvest algae. A commercial product called "Chitosin", commonly used for water purification, can also be used as a flocculant. The shells of crustaceans are ground into powder and processed to acquire chitin, a polysaccharide found in the shells, from which chitosin is derived. 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". Ultrasound based methods of algae harvesting are currently under development, and other, additional methods are currently being developed.^[7][8]

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).

• 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.^[9]

• 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.^[11] The enzymatic extraction can be supported by ultrasonication. The combination "sonoenzymatic treatment" causes faster extraction and higher oil yields. ^[12]
• 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.^[13][14]
• 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.^[15]

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.^[7][8]

Algae as an energy source

Biofuels production

Currently most research into efficient algal-oil production is being done in the private sector, but if predictions from small scale production experiments bear out then using algae to produce biodiesel, bioethanol and biobutanol may be the only viable method by which to produce enough automotive fuel to displace current world gasoline usage.^[16]

Microalgae have much faster growth-rates than terrestrial crops. The oil yield per unit area of algae is estimated to be 5,000 to 20,000 gallons per acre, per year (4.6 to 18.4 l/m² per year); this is 7 to 30 times greater than the next best crop, Chinese tallow (699 gallons).^[17]

The difficulties in efficient biodiesel production from algae lie in finding an algal strain with a high lipid content and fast growth rate that isn't too difficult to harvest, and a cost-effective cultivation system (ie, type of photobioreactor) that is best suited to that strain.

Open-pond methods have largely been abandoned for the cultivation of algae with high-oil content. Many believe that a major flaw of the Aquatic Species Program was the decision to focus their efforts exclusively on open-ponds. Algae in an open-pond environment are subject to wide swings in temperature and pH, and competition from invasive algae and bacteria. Open systems using a monoculture are also vulnerable to viral infection. The open-pond method makes the entire effort dependent upon the hardiness of the strain chosen, requiring it to be unnecessarily resilient (compared to a closed system) in order to withstand the environmental conditions. For a given amount of photosynthetic energy, an algae strain producing relatively high levels of oil will produce relatively less protein and/or carbohydrate, usually resulting in the species being less hardy, or having a slower growth rate. Algal species with a lower oil content, not having to divert their energies away from growth, have an easier time in the harsher conditions of an open system.

Some open sewage ponds trial production has been done in Marlborough, New Zealand.^[18]

A feasibility study using marine microalgae in a photobioreactor is being done by The International Research Consortium on Continental Margins at the International University Bremen.^[19]

Research into algae for the mass-production of oil is mainly focused on microalgae; organisms capable of photosynthesis that are less than 2 mm in diameter, including the diatoms and cyanobacteria; as opposed to macroalgae, e.g. seaweed. This preference towards microalgae is due largely to its less complex structure, fast growth rate, and high oil content (for some species). Some commercial interests into large scale algal-cultivation systems are looking to tie in to existing infrastructures, such as coal power plants or sewage treatment facilities. This approach not only provides the raw materials for the system, such as CO₂ and nutrients; but it changes those wastes into resources.

In November 8, 2006, an entity called "Green Star Products" announced that it has signed an agreement with "De Beers Fuel Limited" of South Africa to build 90 biodiesel reactors with algae as raw material. Each of the biodiesel reactors will be capable of producing 10 million gallons of biodiesel each year for a total production capacity of 900,000,000 gallons per year when operating at full capacity, which is 4 times greater than the entire U.S. output in 2006. Also, GreenFuel Technologies Corporation has delivered a bioreactor to De Beers Fuel. Doubts have been expressed about Green Star's expertise in biodiesel technology. ^[20] Green Star's president did however answer questions in an online interview with WallSt.net where he claimed that the South African biodiesel production has exceeded the original expectations.^[21]

The corporations Chevron, Honeywell, and Boeing are starting algae businesses. According to Boeing's technology leader for energy and emissions, Dave Daggett, 'In the past two years, we have changed from algae skeptics to proponents'. ^[22] The development challenge is to reduce the cost of producing algae oil in commercial volumes, i.e. billions of gallons.

"'In Europe, refiners are producing 1.4 billion gallons a year from rapeseed, soy, and other plants. In all, the world consumed $1.7 billion worth of biodiesel last year. That should grow to $26 billion by 2020, says market researcher Global Insight.'" ^[22] These figures project an average growth of over 20% per year.

Hydrogen production

Algae can be used as a biological source for the production of hydrogen. In 1939 a German researcher named Hans Gaffron, while working at the University of Chicago, observed that the algae he was studying, Chlamydomonas reinhardtii (a green alga), would sometimes switch from the production of oxygen to the production of hydrogen.^[23] Gaffron never discovered the cause for this change and for many years other scientists failed in their attempts at its discovery. In the late 1990s professor Anastasios Melis, a researcher at the University of California at Berkeley discovered that by depriving the algae of sulfur it will switch from the production of oxygen (normal photosynthesis), to the production of hydrogen. He found that the enzyme responsible for this reaction is hydrogenase, but that the hydrogenase will not cause this switch in the presence of oxygen. Melis found that depleting the amount of sulfur available to the algae interrupted its internal oxygen flow, allowing the hydrogenase an environment in which it can react, causing the algae to produce hydrogen.

Biomass

Algae can be grown to produce biomass, which can then be harvested and burned in the same manner as wood, to produce heat and electricity.^[24]

Methane

Through the use of algaculture grown organisms and cultures, various polymeric materials can be broken down into methane.^[25]

SVO

The algal-oil feedstock that is used to produce biodiesel can also be used for fuel directly as "Straight Vegetable Oil", (SVO). While using the oil in this manner does not require the additional energy needed for transesterification, (processing the oil with an alcohol and a catalyst to produce biodiesel), it does require modifications to a normal diesel engine, whereas biodiesel can be run in any modern diesel engine, unmodified, that is designed to use ultra-low sulfur diesel, the new diesel fuel standard for the United States of America that went into effect in the fall of 2006.

Refining to traditional transport fuels

There are processes for vegetable oil refining that can produce gasoline, diesel, propane, or kerosene from the oil extracted from algae.

This information is from Wikipedia 

Other articles to read would be:

http://wwwlib.murdoch.edu.au/adt/pubfiles/adt-MU20050901.140745/02Whole.pdf

http://www.epobio.net/pdfs/0709AquaticReport.pdf