Global warming can bring about extreme weather occurrences, rise in sea levels, extinction of species, retreat of glaciers, and many other calamities. The rise in global temperature is attributed to the high amount of carbon dioxide (CO2) gases in the atmosphere [5, 13, 56]. CO2 is emitted from the burning of fossil fuels for elec­tricity, transport, and industrial processes [3]. The Kyoto Protocol in 1997 proposed a reduction of greenhouse gases by 5.2% based on the emissions in 1990.

Different CO2 mitigation options have been considered to meet the proposed tar­get [5]. The various strategies can be classified as either chemical-reaction-based approaches or biological mitigation. Chemical-reaction-based strategy captures CO2 by reaction with other chemical compounds before the CO2 is released into the atmo­sphere. The disadvantage of this method is that the chemical reactions can be very energy-intensive and costly. Furthermore, the wasted chemical compounds will need to be disposed of. On the other hand, biological mitigation seems more favourable as it not only captures CO2 but also generates energy through photosynthesis [56].

Photosynthesis is carried out by all plants and any photosynthetic microorganism. Even though the use of plants to capture CO2 is viable, it is inefficient due to their low photosynthetic rates. In contrast, owing to their structural and functional sim­plicities, microalgae are able to photosynthesize and hence capture CO2 with an efficiency up to 10-50 times greater than that of higher-order plants [56]. Microalgae include both prokaryotic cyanobacteria and eukaryotic unicellular algal species [5]. In addition to CO2 and sunlight, microalgae need nutrients, trace metals, and water to grow [2, 38, 46, 58]. In short, microalgal biomass is produced based on the fol­lowing reaction:

CO2 + H2O + nutrients + light energy ^ biomass + O2

Unlike plants, microalgae can be grown with waste or brackish water as their high adaptability enables them to survive in a hostile environment contaminated with excess nitrogen, excess phosphorous, and heavy metals. In fact, microalgae can directly metabolize the excess nitrogen and phosphorous in waste water as nutrients for their cultivation. As such, microalgal cultivation does not interfere with use of fresh water, a limited resource in many parts of the world [2, 34, 38, 46, 58].

Among the 30,000 species of microalgae on Earth, many of them are known to contain a variety of high-value bio-products that can be commercially harnessed, such as biodiesel-convertible neutral lipids, different isomers of carotenoids, poly­saccharides, polyunsaturated fatty acids, and phycobiliproteins [10]. In addition to being a CO2 bio-sequester, commercial applications of microalgal biomass also include: (1) biodiesel through transesterification of its neutral lipids; (2) bio-ethanol through fermentation of its carbohydrates; (3) nutritional supplements for humans; (4) natural food colourants; (5) natural food source for many aquacultural species; (6) natural colourants in cosmetics; (7) bio-fertilizers through pyrolysis; (8) protein feed for farm animals [10, 14, 52]. Some of these applications require specific compo­nents of the biomass to be recovered while others utilize the entire cellular biomass.