Page 288 - Industrial Wastewater Treatment, Recycling and Reuse

262 Industrial Wastewater Treatment, Recycling, and Reuse

the carbon source by CO 2 fixation (Calvin cycle) and intake from the out-
side of cell, which can further make malonyl-CoA. The photosynthetic
metabolism utilizes light and CO 2 for growth and organic photosynthate
production, while respiration uses the organic photosynthates produced
during photosynthesis. Mixotrophic cultures showed reduced photo inhibi-
tion and improved growth rates over autotrophic and heterotrophic cultures
(Devi et al., 2013). Mixotrophic cultivation was shown to be a good strategy
to obtain a large biomass and high growth rates (Lee and Lee, 2002; Ogawa
and Aiba, 1981), with the additional benefit of producing photosynthetic
metabolites and lipid productivity (Chen and Johns, 1996; Perez-Garcia
et al., 2011; Chandra et al., 2014). Algae have the flexibility to switch their
nutritional mode based on substrate availability and light conditions. If sim-
pler carbohydrates are present in the system, the algae shift towards hetero-
trophic nutrition from autotrophic mode to save their energy expenditure
(Venkata Mohan et al., 2014).
Subjecting microalgae to stress microalgae causes the photosynthetic
mechanism to switch from biomass growth to lipid synthesis. The different
stress conditions that trigger lipid synthesis are temperature, light, pH,
salinity and nutrients (Venkata Mohan and Devi, 2014). The intracellular
lipid granules stored under stress conditions act as precursors for fatty acid
biosynthesis (Devi et al., 2012;VenkataMohanetal., 2014).The triglyc-
eride composition of algae upon transesterification with an alcohol pro-
duce algae-derived biodiesel (alkyl esters). Depending on the species,
growth conditions, and growth stages, microalgae have been shown to
produce various types of lipids including triacylglycerides, phospholipids,
glycolipids, and betaine lipids (Greenwell et al., 2010). The influence of
various nutritional modes on the algal biomass growth and subsequent lipid
production was studied by employing a two-phase operation, including a
GP and a stress-induced starvation phase (SP) (Devi et al., 2013). The mix-
otrophic mode of operation showed higher biomass growth during the
GP, while higher lipid productivity was observed with the nitrogen-
deprived autotrophic mode followed by heterotrophic and mixotrophic
operations. Relative increment in lipid productivities were noticed in
the SP operation from GP in mixotrophic operation (2.45) followed by
autotrophic (2.2) and heterotrophic (2.14) mode of operations. Effect of
salinity stress was evaluated at varying salt concentrations for inducing max-
imum lipid production (Venkata Mohan and Devi, 2014). Microalgae-
derived lipids and biomass can be converted into alcohols, methyl esters,
and alkanes for use in spark-ignited engines, compression-ignition
engines, and aircraft gas turbine engines (Harrison et al., 2012). Under