264
Nitrogen depletion has also been shown to correlate increased ROS accumulation with higher cellular lipid content in Chlorella sorokiniana C3 (Zhang et al.
2013). Higher oxidative stress indicated by increased level of membrane peroxidation was observed during oil droplet formation. The assays of SOD, POD and CAT
suggested impaired ROS scavenging ability of the cells under nitrogen starvation.
Menon et al. (2013) studied nutrient stress-induced lipid production and its relationship with specific ROS levels in Chlorella vulgaris. They observed 43-folds increase
in the neutral lipid accumulation and 10–15-folds increase in the O 2
•− and OH
•
radicals after shifting the cells from nutrient replete to starvation medium. From the
experiments with different nutrient levels, a power-law correlation between specific
intracellular levels of neutral lipid and OH
•
radicals was observed suggesting a cascade effect of OH
•
radicals on neutral lipids, possibly in the control of lipid synthesis. Changes in the intracellular level of ROS and neutral lipid accumulation in the
marine diatom Nitzschia closterium cultivated under the range of nitrogen concentration (159–932 μM) was studied by Liu et al. (2012). The intracellular ROS content increased largely in all treatments during stationary phase (12 days) compared
to the exponential phase (4 days). The neutral lipid accumulation increased with the
decrease in nitrogen concentration in the medium after 8 and 12 days, which suggests that cells might accumulate TAG as a protective mechanism to survive under
nitrogen-limited stress. It is known that ROS induced by nutrient starvation play a
significant role in the regulation of starvation-induced autophagy (Goodson et al.
2011; Scherz-Shouval et al. 2007), which releases carbon moieties that might contribute to lipid synthesis (Goodson et al. 2011).
Çakmak et al. (2015) investigated the antioxidant response of Chlamydomonas
reinhardtii under element deprivation (nitrogen, sulphur, phosphorus and magnesium) and supplementation (nitrogen and zinc). The total carotenoid content of the
cells increased under all element regimes except magnesium deprivation. Element
deprivation and zinc supplementation significantly increased H 2 O 2 level and lipid
peroxidation in C. reinhardtii. Confocal imaging showed induction in neutral lipid
accumulation by cells under element stress. A dramatic decrease (70%) in the proline content was determined in nitrogen-deprived cells. A similar trend, but to a
lesser extent, was also observed in sulphur-, phosphorus- and magnesium-deprived
cells. Except for magnesium deprivation, oxygen radical absorbance capacity of the
cells decreased under element deprivation, which confirms a state of oxidative damage, while a significant increase under nitrogen and zinc supplementation suggests
sustained cellular homeostasis via increased production of antioxidants. Fluctuations
in the activities of SOD, CAT, APX and GR were also observed under different element regimes, which refer to different metabolic sources of ROS production triggered by the absence or overabundance of the specific element.
Addition of salt in the growth medium is also reported to affect lipid accumulation and alter the cellular antioxidant system of microalgae. Wang et al. (2016a, b)
studied salt stress (addition of 0–50 g/L NaCl)-induced lipid accumulation and oxidative response in heterotrophic culture of C. protothecoides. The highest lipid content of 41.5% was obtained after addition of 30 g/L NaCl in the culture medium.
After 144 h of stress, compared to control, 4.4-folds higher ROS level was observed.
K. Chokshi et al.
Nitrogen depletion has also been shown to correlate increased ROS accumulation with higher cellular lipid content in Chlorella sorokiniana C3 (Zhang et al.
2013). Higher oxidative stress indicated by increased level of membrane peroxidation was observed during oil droplet formation. The assays of SOD, POD and CAT
suggested impaired ROS scavenging ability of the cells under nitrogen starvation.
Menon et al. (2013) studied nutrient stress-induced lipid production and its relationship with specific ROS levels in Chlorella vulgaris. They observed 43-folds increase
in the neutral lipid accumulation and 10–15-folds increase in the O 2
•− and OH
•
radicals after shifting the cells from nutrient replete to starvation medium. From the
experiments with different nutrient levels, a power-law correlation between specific
intracellular levels of neutral lipid and OH
•
radicals was observed suggesting a cascade effect of OH
•
radicals on neutral lipids, possibly in the control of lipid synthesis. Changes in the intracellular level of ROS and neutral lipid accumulation in the
marine diatom Nitzschia closterium cultivated under the range of nitrogen concentration (159–932 μM) was studied by Liu et al. (2012). The intracellular ROS content increased largely in all treatments during stationary phase (12 days) compared
to the exponential phase (4 days). The neutral lipid accumulation increased with the
decrease in nitrogen concentration in the medium after 8 and 12 days, which suggests that cells might accumulate TAG as a protective mechanism to survive under
nitrogen-limited stress. It is known that ROS induced by nutrient starvation play a
significant role in the regulation of starvation-induced autophagy (Goodson et al.
2011; Scherz-Shouval et al. 2007), which releases carbon moieties that might contribute to lipid synthesis (Goodson et al. 2011).
Çakmak et al. (2015) investigated the antioxidant response of Chlamydomonas
reinhardtii under element deprivation (nitrogen, sulphur, phosphorus and magnesium) and supplementation (nitrogen and zinc). The total carotenoid content of the
cells increased under all element regimes except magnesium deprivation. Element
deprivation and zinc supplementation significantly increased H 2 O 2 level and lipid
peroxidation in C. reinhardtii. Confocal imaging showed induction in neutral lipid
accumulation by cells under element stress. A dramatic decrease (70%) in the proline content was determined in nitrogen-deprived cells. A similar trend, but to a
lesser extent, was also observed in sulphur-, phosphorus- and magnesium-deprived
cells. Except for magnesium deprivation, oxygen radical absorbance capacity of the
cells decreased under element deprivation, which confirms a state of oxidative damage, while a significant increase under nitrogen and zinc supplementation suggests
sustained cellular homeostasis via increased production of antioxidants. Fluctuations
in the activities of SOD, CAT, APX and GR were also observed under different element regimes, which refer to different metabolic sources of ROS production triggered by the absence or overabundance of the specific element.
Addition of salt in the growth medium is also reported to affect lipid accumulation and alter the cellular antioxidant system of microalgae. Wang et al. (2016a, b)
studied salt stress (addition of 0–50 g/L NaCl)-induced lipid accumulation and oxidative response in heterotrophic culture of C. protothecoides. The highest lipid content of 41.5% was obtained after addition of 30 g/L NaCl in the culture medium.
After 144 h of stress, compared to control, 4.4-folds higher ROS level was observed.
K. Chokshi et al.