The microalgae are photosynthetic organisms can grow in a wide variety of environments and conditions, including freshwater, salty, and brackish water (Benemann 2012). Their mechanism of photosynthesis is similar to higher plants, with the difference that the conversion of solar energy is generally more efficient because of their simplified cellular structure and more efficient access to water, CO2, and other nutrients.

Its uniqueness that separates them from other microorganisms is due to presence of chlo­rophyll and having photosynthetic ability in a single algal cell, therefore allowing easy operation for biomass generation and effective genetic and metabolic research in a much shorter time period than conventional plants (Singh and Sharma 2012).

In addition, the cultivation requirements are quite small, as most species only need water, CO2, and some essential nutrients such as nitrates, phosphates, and potas­sium, without needing the use of pesticides or fertilizers (Groom et al. 2008; Singh and Sharma 2012). Microalgae can produce lipids, proteins, and carbohydrates in large amounts over short periods of time. For these reasons, microalgae are capa­ble of producing 30 times as much oil per unit of land area compared to terrestrial oilseed (Sheehan et al. 1998). And these oil can be processed into both biofuels and valuable coproducts (Singh and Sharma 2012).

The microalgae cultivation can be heterotrophic or autotrophic. The hetero­trophic method is a biochemical conversion that relies on input feedstock derived from an upstream photosynthetic source. This approach uses closed bioreactor systems in a biochemical conversion process without light inputs. This dark fer­mentation process is based on the consumption of simple organic carbon com­pounds, such as sugars or acetate. The cultivation of algae using cellulosic sugars produced from wood and agricultural wastes or purpose-grown energy crops is an area of active research and development (Buford et al. 2012).

In the other hand, the autotrophic cultivation requires only inorganic com­pounds such as CO2, salts, and a source of light energy for their growth. This photosynthetic conversion involves two main methods: open ponds and closed photobioreactors (PBRs). The biomass produced in these autotrophic processes includes lipids that can be converted to fuels (Brennan and Owende 2010; Buford et al. 2012).

According to Benemann (2012), algae have been essentially produced in open ponds with the main strains currently being cultivated are Spirulina, Chlorella, Dunaliella, and Haematococcus. Most designs include mixing systems that use paddle wheels and carbonation techniques to supply and transfer CO2 (in-ground carbonation pit, bubble covers, and in-pound sumps1).

Microalgae are also grown in tanks and small-scale PBRs, in hundreds of dif­ferent systems around the world, producing from small amounts to huge sums of

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biomass annually. In this closed autotrophic approach, algae grow with sunlight or artificial lighting (Benemann 2012; Buford et al. 2012). Different types of PBRs have been designed and developed for cultivating algae that can be horizontal, vertical, tubular, flat, etc. (Benemann 2012; Singh and Sharma 2012). Each of these PBRs has their own advantages and disadvantages. Several studies are being developed which may overcome their limitations in the years to come (Singh and Sharma 2012).