254
More recently, microalgae have been considered as promising feedstocks for the
generation of biofuels including biodiesel from oil, alcohols from carbohydrates
and liquid or gaseous hydrocarbons from whole biomass (Bohutskyi et al. 2015).
The photosynthetic mechanism of microalgae is similar to plants, but due to their
simple cellular structure and submergence in an aqueous environment, in most
cases, they have an efficient access to water, CO 2 and other nutrients. They exhibit
faster growth and higher photosynthetic efficiency compared to terrestrial crop
plants (Chisti 2007). Their shorter generation time allows production of lipids and
carbohydrates in large amounts over short periods of time, which can be easily converted into biodiesel and bioethanol, respectively. Moreover, microalgae can be cultivated in nonarable lands using seawater, brackish water or wastewater, due to
which they do not compete with agricultural farmlands (Pancha et al. 2014). Under
environmental stress conditions, they can accumulate substantial amounts of lipids
and carbohydrates (50–70% of dry weight). Due to these reasons, microalgal biomass is now recognized as an alternative renewable feedstock for biofuel production. Further utilization of de-oiled (lipid extracted) biomass for various applications
like bioethanol, biogas, animal feed, fertilizer, biosorption of dyes (Maurya et al.
2016), nanoparticle synthesis (Chokshi et al. 2016a, b), etc. reduces the overall cost
of microalgal biofuel production, making it economically sustainable.
Bioprospecting comprises of searching and collecting of unique microbial strains
for their potential applications. As microalgae are a potential source of bioactive
compounds with pharmaceutical, biomedical and nutraceutical prospects, they
could play an important role in producing biofuels and bio-based chemicals based
on both, their natural components and refined products (Hu et al. 2013). Therefore,
bioprospecting of microalgal strains is important to select the best strains that can
produce higher amounts of desired metabolic products.
13.2 Oxidative Stress and Reactive Oxygen Species
In any organism, adverse environmental conditions trigger a series of physiological
processes and generation of reactive oxygen species (ROS). These highly reactive
ROS cause severe damage to proteins, lipids, carbohydrates and nucleic acids, often
leading to alterations in cell structure, organelle dysfunction, mutagenesis (Halliwell
and Gutteridge 1999) and oxidative cell injuries, ultimately resulting in cell death
(Gill and Tuteja 2010). The production of ROS is an unavoidable consequence of
aerobic life. It comprises both free radical (superoxide radicals, O 2
•− ; hydroxyl radicals, OH
•
; perhydroxy radicals, HO 2
•
; and alkoxy radicals, RO
•
) and non-radical
(molecular) forms (hydrogen peroxide, H 2 O 2 and singlet oxygen
1
O 2 ). These are the
partially reduced forms of atmospheric oxygen (O 2 ) resulted from the excitation of
O 2 to form
1
O 2 . The single electron reduction of O 2 results in the generation of O 2
•−
, which inactivates several important enzymes having iron-sulphur clusters which
are required for energy production and amino acid metabolism. This inactivation is
K. Chokshi et al.
More recently, microalgae have been considered as promising feedstocks for the
generation of biofuels including biodiesel from oil, alcohols from carbohydrates
and liquid or gaseous hydrocarbons from whole biomass (Bohutskyi et al. 2015).
The photosynthetic mechanism of microalgae is similar to plants, but due to their
simple cellular structure and submergence in an aqueous environment, in most
cases, they have an efficient access to water, CO 2 and other nutrients. They exhibit
faster growth and higher photosynthetic efficiency compared to terrestrial crop
plants (Chisti 2007). Their shorter generation time allows production of lipids and
carbohydrates in large amounts over short periods of time, which can be easily converted into biodiesel and bioethanol, respectively. Moreover, microalgae can be cultivated in nonarable lands using seawater, brackish water or wastewater, due to
which they do not compete with agricultural farmlands (Pancha et al. 2014). Under
environmental stress conditions, they can accumulate substantial amounts of lipids
and carbohydrates (50–70% of dry weight). Due to these reasons, microalgal biomass is now recognized as an alternative renewable feedstock for biofuel production. Further utilization of de-oiled (lipid extracted) biomass for various applications
like bioethanol, biogas, animal feed, fertilizer, biosorption of dyes (Maurya et al.
2016), nanoparticle synthesis (Chokshi et al. 2016a, b), etc. reduces the overall cost
of microalgal biofuel production, making it economically sustainable.
Bioprospecting comprises of searching and collecting of unique microbial strains
for their potential applications. As microalgae are a potential source of bioactive
compounds with pharmaceutical, biomedical and nutraceutical prospects, they
could play an important role in producing biofuels and bio-based chemicals based
on both, their natural components and refined products (Hu et al. 2013). Therefore,
bioprospecting of microalgal strains is important to select the best strains that can
produce higher amounts of desired metabolic products.
13.2 Oxidative Stress and Reactive Oxygen Species
In any organism, adverse environmental conditions trigger a series of physiological
processes and generation of reactive oxygen species (ROS). These highly reactive
ROS cause severe damage to proteins, lipids, carbohydrates and nucleic acids, often
leading to alterations in cell structure, organelle dysfunction, mutagenesis (Halliwell
and Gutteridge 1999) and oxidative cell injuries, ultimately resulting in cell death
(Gill and Tuteja 2010). The production of ROS is an unavoidable consequence of
aerobic life. It comprises both free radical (superoxide radicals, O 2
•− ; hydroxyl radicals, OH
•
; perhydroxy radicals, HO 2
•
; and alkoxy radicals, RO
•
) and non-radical
(molecular) forms (hydrogen peroxide, H 2 O 2 and singlet oxygen
1
O 2 ). These are the
partially reduced forms of atmospheric oxygen (O 2 ) resulted from the excitation of
O 2 to form
1
O 2 . The single electron reduction of O 2 results in the generation of O 2
•−
, which inactivates several important enzymes having iron-sulphur clusters which
are required for energy production and amino acid metabolism. This inactivation is
K. Chokshi et al.