262
13.4 Link Between Oxidative Stress and Biofuel Potential
of Microalgae
Biodiesel and bioethanol are the most important renewable fuels which can replace
fossil-based fuels. Microalgal biomass has emerged as a potential alternative of
plant-based biofuel feedstock due to its higher lipid and carbohydrate content
(Chisti 2007; John et al. 2011). The productivities of microalgal biomass, lipid and
carbohydrate are key parameters for the economic feasibility of microalgae-based
biofuel. Microalgal lipid and carbohydrate can be enhanced by chemical stimuli like
nutrient (nitrogen and phosphorous) starvation and salinity stress and physical stimuli like changes in culture pH, temperature, light intensity, photoperiods, etc.
(Pancha et al. 2014; Chokshi et al. 2016a). Microalgal lipid is composed of neutral
and glyco- and phospholipids. Neutral lipids are mainly composed of triacylglycerols (TAGs), which are important for the production of biodiesel; while glycolipids
and phospholipids are the components of cell membranes (Olmstead et al. 2013).
While studies on the oxidative stress-enhanced carotenoid accumulation in microalgae are widely reported, understanding of the connection between ROS and other
cellular metabolites is still very poor. Therefore, increasing attempts are being made
(Table 13.3) to understand the link between accumulation of ROS and synthesis of
lipid and carbohydrate by microalgae.
Xin et al. (2011) firstly correlated cellular ROS level and lipid accumulation in
Scenedesmus sp. grown at different temperature (10–30°C) and suggested that low
cultivation temperature induces ROS accumulation in the cells and increases their
lipid content. Chokshi et al. (2015) studied the effects of cultivation temperature on
growth, biochemical composition and response of stress biomarkers in Acutodesmus
dimorphus. Increase in the cultivation temperature from 35 to 38°C resulted in 28%
higher neutral lipid accumulation after 15 days of growth. The H 2 O 2 content of the
cells increased by fourfolds, while the MDA level increased by twofolds. The activities of APX and CAT were also increased by 1.7- and 2.6-folds, respectively.
Ruiz-Domínguez et al. (2015) evaluated lipid accumulation and antioxidant
activity in the acidophilic microalgae Coccomyxa onubensis under nutrient starvation. The total lipid content of the cultures lacking nitrogen, phosphorous or sulphur
enhanced during the first 48 h of stress. Further increase in the stress duration
increased the lipid accumulation only in the nitrogen-starved cultures. The activities
of CAT, APX and GR increased with time suggesting induction of cellular antioxidant responses under nutrient deprivation. In another study, Yilancioglu et al. (2014)
observed that low nitrogen condition increases lipid accumulation in the halophilic
strain D. salina. There was higher level of lipid peroxidation (MDA level) and
enhanced activation of SOD, CAT and APX enzymes. Further, the exogenous application of H 2 O 2 induced oxidative stress and increased cellular lipid content up to
44% compared to control. This confirms that oxidative stress by itself can cause
lipid accumulation and suggests that lipid accumulation under nitrogen depletion is
mediated by oxidative stress.
K. Chokshi et al.
13.4 Link Between Oxidative Stress and Biofuel Potential
of Microalgae
Biodiesel and bioethanol are the most important renewable fuels which can replace
fossil-based fuels. Microalgal biomass has emerged as a potential alternative of
plant-based biofuel feedstock due to its higher lipid and carbohydrate content
(Chisti 2007; John et al. 2011). The productivities of microalgal biomass, lipid and
carbohydrate are key parameters for the economic feasibility of microalgae-based
biofuel. Microalgal lipid and carbohydrate can be enhanced by chemical stimuli like
nutrient (nitrogen and phosphorous) starvation and salinity stress and physical stimuli like changes in culture pH, temperature, light intensity, photoperiods, etc.
(Pancha et al. 2014; Chokshi et al. 2016a). Microalgal lipid is composed of neutral
and glyco- and phospholipids. Neutral lipids are mainly composed of triacylglycerols (TAGs), which are important for the production of biodiesel; while glycolipids
and phospholipids are the components of cell membranes (Olmstead et al. 2013).
While studies on the oxidative stress-enhanced carotenoid accumulation in microalgae are widely reported, understanding of the connection between ROS and other
cellular metabolites is still very poor. Therefore, increasing attempts are being made
(Table 13.3) to understand the link between accumulation of ROS and synthesis of
lipid and carbohydrate by microalgae.
Xin et al. (2011) firstly correlated cellular ROS level and lipid accumulation in
Scenedesmus sp. grown at different temperature (10–30°C) and suggested that low
cultivation temperature induces ROS accumulation in the cells and increases their
lipid content. Chokshi et al. (2015) studied the effects of cultivation temperature on
growth, biochemical composition and response of stress biomarkers in Acutodesmus
dimorphus. Increase in the cultivation temperature from 35 to 38°C resulted in 28%
higher neutral lipid accumulation after 15 days of growth. The H 2 O 2 content of the
cells increased by fourfolds, while the MDA level increased by twofolds. The activities of APX and CAT were also increased by 1.7- and 2.6-folds, respectively.
Ruiz-Domínguez et al. (2015) evaluated lipid accumulation and antioxidant
activity in the acidophilic microalgae Coccomyxa onubensis under nutrient starvation. The total lipid content of the cultures lacking nitrogen, phosphorous or sulphur
enhanced during the first 48 h of stress. Further increase in the stress duration
increased the lipid accumulation only in the nitrogen-starved cultures. The activities
of CAT, APX and GR increased with time suggesting induction of cellular antioxidant responses under nutrient deprivation. In another study, Yilancioglu et al. (2014)
observed that low nitrogen condition increases lipid accumulation in the halophilic
strain D. salina. There was higher level of lipid peroxidation (MDA level) and
enhanced activation of SOD, CAT and APX enzymes. Further, the exogenous application of H 2 O 2 induced oxidative stress and increased cellular lipid content up to
44% compared to control. This confirms that oxidative stress by itself can cause
lipid accumulation and suggests that lipid accumulation under nitrogen depletion is
mediated by oxidative stress.
K. Chokshi et al.