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Reorienting Waste Remediation Towards Harnessing Bioenergy 261

generating renewable biodiesel (Devi and Venkata Mohan, 2012; Venkata
Mohan et al., 2014). Algae fix CO 2 during the day via photophosphoryla-
tion (thylakoid) and produce carbohydrate during the Calvin cycle (stroma),
which are converted to various products, including triacyl glycerides
(TAGs) depending on the species of algae or specific conditions pertaining
to cytoplasm and plastids (Liu and Benning, 2013). The biosynthetic path-
way of lipids in algae occur in four steps: (1) carbohydrates accumulate inside
the cell, (2) acetyl-CoA is formed followed by malonyl-CoA, (3) palmitic
acid is synthesized, and (4) finally, higher fatty acids are synthesized by chain
elongation (Venkata Mohan et al., 2014).
Microalgae in the photoautotrophic mode use sunlight as the energy
source and inorganic carbon (CO 2 ) as the carbon source to form biochem-
ical energy through photosynthesis (Huang et al., 2010). This is the most
favorable environmental conditions for the growth of microalgae (Chen
et al., 2011). The autotrophic nutritional mode at the expense of atmo-
spheric CO 2 generates algal oil. In the heterotrophic mode of nutrition,
microalgae utilize the external carbon source in the form of simpler carbo-
hydrates that enter into the cell and participate in other metabolic pathways
such as respiration. Heterotrophic nutrition takes place both in the presence
and absence of light. In photo-heterotrophic nutrition, light acts as an
energy source, but the source of carbon remains organic. Oxidative assim-
ilation takes place by two pathways: (1) the Embden Meyerhof pathway and
(2) the pentose phosphate pathway (Neilson and Lewin, 1974). The
heterotrophic nutritional mode avoids the limitations of light dependency,
one of the major obstruction for gaining high cell density in large-scale
photo-bioreactors (Huang et al., 2010). The heterotrophic nutritional mode
facilitates wastewater treatment along with organic carbon removal, substrate
degradation and lipid productivity. Cost effectiveness and relative simplicity of
operations and easy maintenance are the main attractions of the heterotrophic
mode of operation (Olguı ´n et al., 2012; Perez-Garcia et al., 2011).
Microalgae can also function under mixotrophic nutrition by combining
both the autotrophic and heterotrophic mechanisms that fix atmospheric
CO 2 as well as consume the organic molecules and micronutrients from
the growing environment. The mixotrophic growth regime is a variant
of the heterotrophic growth regime, where CO 2 and organic carbon are
simultaneously assimilated and both respiratory and photosynthetic metab-
olism operate concurrently (Kaplan et al., 1986; Lee, 2004; Perez-Garcia
et al., 2011). Mixotrophs have the ability to utilize organic carbon, and
therefore light energy is not a limiting factor for the biomass growth
(Chang et al., 2011). The acetyl-CoA pool will be maintained from both