# Antioxidant Production in _Dunaliella_
## 4 Tuning Antioxidant Enzyme Activity in _Dunaliella_
### Light Irradiation  
_D. salina_ exposed to UV-B irradiation enhanced CAT, SOD, POD activities, reaching maximum levels within three days after they levelled off [128], suggesting that CAT, SOD, POD activity could be part of the cell's short-term adaptation against UV-B irradiation. Micosporine-like amino acids (shinorine, porphyra, and palythinol) could act as part of a supportive antioxidants network to protect cells against UV-B irradiation or ROS [129]. The synthesis of these compounds could be the reason for the levelling off enzymatic antioxidant response after three days of growth [128]. _D. tertiolecta_ exposed to static and fluctuating natural UV radiation (400-700 nm) for short-term (1-3 days) or long-term (4-7 days) showed that only short-term irradiation significantly increased SOD and GR activity compared to non-UV irradiated cells. Glutathione content was reduced under both short and long-term irradiation, whilst APX activity did not significantly change, indicating that natural UV radiation-induced O\({}_{2}\)\({}^{-}\) was mitigated by SOD, and that H\({}_{2}\)O\({}_{2}\) was detoxified by the consumption of glutathione [130]. In contrast, _D. salina_ treated with UV-B irradiation (irradiated cultures for 4 h) and cultured for 15 days increased its CAT, POD, and SOD activity compared to untreated cells [34]. Moreover, APX activity (140 umol ascorbate mg\({}^{-1}\) protein h\({}^{-1}\)) was found to increase in _D. bardawil_ cells exposed to UV-A radiation as an addition to cultivation under high light energy (150 umol photons m\({}^{-2}\) s\({}^{-1}\)) compared to cells cultivated at low light intensities (35 umol photons m\({}^{-2}\) s\({}^{-1}\)). Increased APX activation under high light with UV-A radiation may be due to the direct effect of UV-A irradiance on the photosynthetic reactionsthat produce increased ROS within the chloroplast and leads to the increased enzymatic activity [131].  
Exposing _D. salina_ to extremely high light intensities (1000 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)) causes excess electron flow in electron transport chains of the photosystem, which leak electrons onto O\({}_{2}\), thereby generating superoxide ion (O\({}_{2}\)\({}^{\bullet-}\)) and results in enhancing SOD enzyme activity as the expression level of the antioxidant genes (SOD gene, Fe-SOD) increases [132]. No study has yet shown the effect of high light (>920 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)) on CAT activity in _Dunaliella_ strains. Perhaps CAT is not involved in mitigating oxidative stress generated during acclimation of _Dunaliella_ under high light intensities. Rather, peroxididases or other small antioxidant molecules (carotenoid) have been suggested to participate in the detoxification of ROS, as \(\beta\)-carotene content was found to increase when the algal was cultivated under increasing light intensities [133]. Therefore, CAT, POD, SOD and APX activity in _Dunaliella_ can be improved by growing cells under UV radiation combined with a wide range of light irradiation (150-920 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)). However, further studies are needed to evaluate the effect of extreme high light-induced (e.g., 1500 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)) oxidative stress on antioxidant enzyme response.