266
correlated with higher accumulation of carotenoid and increased APX activity.
These results suggest that acclimation of microalgae to wastewater environment
involves increased oxidative stress tolerance activity, and oxidative stress-tolerant
microalgae are highly efficient for biofuel feedstock production on wastewater.
Higher oxidative stress tolerance is an indicator of many extremophiles adapted to
toxic environments. Chlamydomonas sp. W80 and HS5 can grow in highly saline
environment and tolerate oxidative damage induced by methyl viologen as shown
by high expression of APX (Tanaka et al. 2011).
Addition of 5 mM glycine betaine (GB), a plant growth elicitor, to the culture
medium increased the lipid content of Monoraphidium sp. QLY-1 by 29.06% (Zhao
et al. 2016). Compared to control, neutral lipid content of the cells increased by
12.83%, while ROS accumulation increased by 29.74%, which suggests that GB
may enhance the absorption of metal trace elements, including iron, to provoke
oxidative stress in microalgae via iron-catalysed Haber-Weiss reaction (Hong et al.
2015). The high level of lipid accumulation may also correlate carbon fixation and
lipid biosynthetic genes with GB induction (Wang et al. 2016a). Cho et al. (2016)
studied the effects of phenol-induced oxidative stress and biodiesel production by
marine microalgae D. salina. Increase in the phenol concentration (0–150 mg/L) in
the culture medium significantly increased the MDA content and SOD activity in
the cells after 10 days of exposure. The CAT activity also increased; however, it was
not significantly correlated with phenol concentration. The total lipid accumulation
in the cells was unaffected, but the overall lipid yield increased by approximately
26%, which might be due to increase in the biomass production by cells under different concentration of phenol.
13.5 Other Recent Applications of Microalgae Linked
with Oxidative Stress
Apart from carotenoids and biofuel, microalgae also have the potential for various
other applications. Biological hydrogen production is being evaluated as a promising substitute for carbonaceous fuels owing to its high conversion efficiency and
high specific energy content (Saenz et al. 2015). Microalgae are among the groups
of organisms able to produce clean, carbon-free energy as hydrogen through the
expression of hydrogenase enzymes. Saenz et al. (2015) reported evidences of oxidative stress during hydrogen photoproduction in sulphur-deprived cultures of C.
reinhardtii. At the beginning of the phase during which hydrogen production starts,
i.e. at 48 h, the activity of CAT and APX was significantly lower than that at 0 h.
However, during the peak of hydrogen production, their activity increased by 45%
and 168%, respectively.
Heavy metal pollution is one of the serious environmental problems all around the
world. Bioremediation is an effective and low-cost approach for the removal of heavy
metals (Chehregani et al. 2009). Microalgae are capable of metal removal from the
K. Chokshi et al.
correlated with higher accumulation of carotenoid and increased APX activity.
These results suggest that acclimation of microalgae to wastewater environment
involves increased oxidative stress tolerance activity, and oxidative stress-tolerant
microalgae are highly efficient for biofuel feedstock production on wastewater.
Higher oxidative stress tolerance is an indicator of many extremophiles adapted to
toxic environments. Chlamydomonas sp. W80 and HS5 can grow in highly saline
environment and tolerate oxidative damage induced by methyl viologen as shown
by high expression of APX (Tanaka et al. 2011).
Addition of 5 mM glycine betaine (GB), a plant growth elicitor, to the culture
medium increased the lipid content of Monoraphidium sp. QLY-1 by 29.06% (Zhao
et al. 2016). Compared to control, neutral lipid content of the cells increased by
12.83%, while ROS accumulation increased by 29.74%, which suggests that GB
may enhance the absorption of metal trace elements, including iron, to provoke
oxidative stress in microalgae via iron-catalysed Haber-Weiss reaction (Hong et al.
2015). The high level of lipid accumulation may also correlate carbon fixation and
lipid biosynthetic genes with GB induction (Wang et al. 2016a). Cho et al. (2016)
studied the effects of phenol-induced oxidative stress and biodiesel production by
marine microalgae D. salina. Increase in the phenol concentration (0–150 mg/L) in
the culture medium significantly increased the MDA content and SOD activity in
the cells after 10 days of exposure. The CAT activity also increased; however, it was
not significantly correlated with phenol concentration. The total lipid accumulation
in the cells was unaffected, but the overall lipid yield increased by approximately
26%, which might be due to increase in the biomass production by cells under different concentration of phenol.
13.5 Other Recent Applications of Microalgae Linked
with Oxidative Stress
Apart from carotenoids and biofuel, microalgae also have the potential for various
other applications. Biological hydrogen production is being evaluated as a promising substitute for carbonaceous fuels owing to its high conversion efficiency and
high specific energy content (Saenz et al. 2015). Microalgae are among the groups
of organisms able to produce clean, carbon-free energy as hydrogen through the
expression of hydrogenase enzymes. Saenz et al. (2015) reported evidences of oxidative stress during hydrogen photoproduction in sulphur-deprived cultures of C.
reinhardtii. At the beginning of the phase during which hydrogen production starts,
i.e. at 48 h, the activity of CAT and APX was significantly lower than that at 0 h.
However, during the peak of hydrogen production, their activity increased by 45%
and 168%, respectively.
Heavy metal pollution is one of the serious environmental problems all around the
world. Bioremediation is an effective and low-cost approach for the removal of heavy
metals (Chehregani et al. 2009). Microalgae are capable of metal removal from the
K. Chokshi et al.