Microalgae are microscopic, unicellular organisms that have high potential for use in CO2 mitigation and for use as a biomass source due to their high photosynthetic rate. The ability of microalgae to amass high oil content makes them one of the most researched subjects in various fields, including the biofuel, pharmaceutics, and food sectors. Thus, microalgae are the most suitable biological approach to be to seques­ter CO2 because of their characteristics make them suitable for incorporation into many fields. Optimum CO2 utilization by microalgae is related directly to the estab­lishment of optimum conditions for the photosynthesis process, i. e., a sufficient amount of CO2, appropriate nutrients, and adequate light.

The commonly cultivated species of microalgae that are used to achieve this purpose are discussed in previous study (Sarbatly and Suali 2012; Suali and Sarbatly 2012) and rearranged as listed in Table 14.1. As shown in Table 14.1, the preferred CO2 concentration for cultivation is in the range of 5-20% of an air supply that is either pure or enriched with flue gases, including slight amounts of nitrogen oxides and sulphur oxides. Microalgae species that have been tested and deemed to be suit­able for CO2 fixation include Chlorella sp. H84, Chlorella sp. A2, Chlorella soroki — niana, Chlorella vulgaris, Chlorella pyrenoidosa, Spirulina platensis, Emiliania huxleyi, Nannochloropsis sp., and Phaeodactylum sp. Among these, tests have shown that the Chlorella species are preferred for use in mitigating CO2.

Table 14.1 also shows that some species perform better when flue gases are used instead of just air and added CO2 . Further, it has been noted that the cultivation

system performs equally well irrespective of whether flue gases or pure CO2 is used, i. e., there is no adverse effect on algal growth (Negoro et al. 1993). In the work of Cheng et al. (2006), it was shown that a gas stream that had a CO2 concentration of 1% v/v was introduced into a membrane photobioreactor that had a microalgae density of 2.0 x 107 cells mL-1, the CO2 concentration was reduced to 0.015% v/v. This clearly demonstrated that initial density of the microalgae has effect on the CO2 fixation rate. The biomass yield of such a reactor is indicative of the efficiency of the photosynthesis process in the reactor. The biomass yield is proportional to biomass photosynthesis and the CO2 uptake by microalgae. This shows that micro­algae can be considered for mitigating CO2 emissions from power plants while simultaneously producing a useful, renewable product.