Petroleum-based fuels are no more sustainable for industrial and transportation pur­poses due to emission of green-house gases (GHGs) and other poisonous gases from their burning, increasing demand and depleting natural resources. Moreover, CO2 (a GHG) buildup due to their combustion, is a serious environmental threat. These facts are leading towards the development of alternative energy sources for ensured environmental sustainability. Therefore, renewable raw materials which include edible plants/seeds (mustard, corn, canola, palm oil, soybean, sunflower, coconut) and non-edible plants/seeds oils (jojoba, castor, jatropha, pongame, Citrus reticu­late, Cucumis melo, Moringa oleifera) and waste oils have been explored for the biodiesel production (Rashid et al. 2008, 2011, 2012, 2013; Sharma et al. 2009; Yadav et al. 2009; Diaz and Borges 2012). However, there are several limitations with these resources; (1) competition with human food demand, (2) use of arable land, (3) requirements of huge amounts of fresh irrigation water, (4) lower yield of biofuel molecules, (5) long cultivation periods and low seasonal production (e. g., once a year). Alternatively, microalgae have received massive attention as an alter­native, among the many options. They are presumably the cheapest source among all other renewable sources for biodiesel production (Chisti 2007; Petkov et al. 2012).

Microalgae are tiny (unicellular or filamentous) photosynthetic factories and their photosynthetic competence is remarkably higher than terrestrial plants. Growth rate and oil productivity of microalgae is considerably higher (Fig. 18.1) than the oil productivity of the best available oil producing crops (Chisti 2007; Wu et al. 2012). They are believed to produce up to 300 times more oil than traditional energy crops on the basis of acreage usage (Chisti 2007; Schenk et al. 2008). The average lipid

Fig. 18.1 Comparison of oil productivity of traditional energy crops with microalgae [we have considered 30 % oil contents in microalgae because of lower lipid contents in waste water]

(Wu et al. 2012)

contents of microalgae range between 1 and 70 % (~30 %, when grown in waste water) but under the optimized conditions some species (Botryococcus braunii) can yield up to 80 % of oil (Table 18.1) on dry biomass basis (Schenk et al. 2008; Wu et al. 2012). Moreover, they lack lignin in contrast to higher plants, so are easily degradable. Most importantly, they do not compete with food crops and can be cul­tivated using non-arable (saline, sodic, water-logged soils) lands, saline/waste water, and artificial beds, e. g., compact bioreactors (Musharraf et al. 2012). They produce remarkable quantity of polysaccharides (sugars) and proteins along with the lipids, so the left-over biomass (after oil extraction) may be exploited for pro­duction of bio-ethanol, biogas, bio-fertilizers as well as to enhance to protein and mineral contents of the animal feed (Gill et al. 2013). Microalgae have potential to
sequester the atmospheric CO2 at the rate as high as 1.8 kg of CO2 per kg of dry biomass (Wang et al. 2008). This makes the algal fuels carbon neutral and in certain cases algal fuel production may earn salable carbon credits to meet Kyoto targets.