# Antioxidant Production in _Dunaliella_
## 2 Antioxidants
### Generation and Detoxification of ROS  
ROS is a collective term for oxygen-derived products (free radicals and non-radicals reactive derivatives of oxygen). They are produced in cellular compartments either exogenously (in response to environmental stress such as UV radiation or xenobiotics) or endogenously (from the intracellular metabolic pathway, enzymatic activities, mitochondrial respiration, or photosynthesis) [66]. Accumulation of ROS leads to oxidative stress in cells and causes damage to cellular macromolecules, including proteins, lipids, carbohydrates and DNA [89]. Various sources of ROS and corresponding modes of biochemical metabolism are summarised in Table 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**ROS** & **Reaction** & **Life Span** & **Function** & **Sources of ROS** & **Scavenging** **Antioxidants** \\ \hline \multirow{6}{*}{Singlet (\({}^{1}\)O\({}_{2}\)) \begin{tabular}{} \\ \end{tabular} } & Chlorophyll triplet & & & & \\  & state (Chl) is & & & & \\  & generated sue to & & & & \\  & insufficient energy & & & & \\  & dissipation during & 3 μs (appx\({}_{\text{X}}\)) & & & \\  & first excited & photosynthesis & 4 μs in H\({}_{2}\)O & & & \\  & electronic state of & \(\text{O}_{2}\) & \(\text{O}_{2}\) & 100 μs in polar & & \\  & & Reduction of & solvents & photo-oxidative & & \\  & transition metal & & stress & & proline \\  & (\(\text{G}^{3+}\)) & & & & \\  & \(\text{O}_{2}\text{}^{\bullet-}+\text{Fe}^{3+}\rightarrow\) & & & & \\  & \({}^{1}\)O\({}_{2}\) + Fe\({}^{2+}\) & & & & \\ \hline \multirow{6}{*}{Superoxide radical (\(\text{O}_{2}^{\bullet-}\)) \
\begin{tabular}{} \\ \end{tabular} } & Reduction of & & & & \\  & oxygen (\({}^{3}\)O\({}_{2}\)) & & & & \\  & during electron & & & & \\  & transport during & & & & \\  & the photosynthesis & & & & \\  & process in & & & & \\  & chloroplastss or & & Triggering the formation of more & PSI in Chloroplast & \\  & during oxidative & & & & oxidative \\  & phosphorylation in & 2–4 μs & ROS which further & phosphorylation in & \\  & one electron & & participates in & mitochondria & SOD, ASc, \\  & reduction of \({}^{3}\)O\({}_{2}\) & \({}^{3}\)O\({}_{2}\) + \(\text{e}^{-}\rightarrow\text{O}_{2}^{\bullet-}\) & & & \\  & reduction of \({}^{3}\)O\({}_{2}\) & \({}^{3}\)O\({}_{2}\) + Xanthine & & & \\  & (\(\text{xanthine oxidase})\rightarrow\) & & & & \\  & \(\text{O}_{2}^{\bullet-}+\text{uric acid}\) & & & & \\  & \(\text{O}_{2}\) + NADPH & & & & \\  & (\(\text{x}\Delta\text{EPH oxidase})\rightarrow\) & & & & \\  & \(\text{O}_{2}^{\bullet-}+\text{NADP^{+}}+\) & & & & \\  & \(\text{H}^{\bullet}\) & & & & \\ \hline \multirow{6}{*}{H\({}_{2}\)O\({}_{2}\)
\begin{tabular}{} \\ \end{tabular} } & Protonation & & Act as a signaling & & \\  & reaction (acidic & & & \\  & conditions) & & & \\  & \(\text{O}_{2}^{\bullet-}+\text{H}^{+}+\text{HO}_{2}^{\bullet}\) & & concentration of & & \\  & \(\rightarrow\)H\({}_{2}\)O\({}_{2}\) + 2O\({}_{2}\) & & & \\  & \(\rightarrow\)H\({}_{2}\)O\({}_{2}\) + 2O\({}_{2}\) & & & \\  & \(\text{Glycolate}+\text{O}_{2}\) & & & & \\  & (diyclo oxidase) \(\rightarrow\) & & & \\  & univalent & & & & \\  & reduction of \(\text{O}_{2}^{\bullet-}\) & \({}^{\text{H}}_{2}\)O\({}_{2}\) + Glyoxylate & & & \\  & \(\text{Fatty acids}\) & & & & \\  & (\(\text{}^{\beta-\text{oxidation}}\rightarrow\) & & & \\  & Acetyl coenzyme + & & & & \\  & \(\text{H}_{2}\)O\({}_{2}\) & & & & \\ \end{tabular}
\end{table} 
Table 2: Production of ROS in cells during biological metabolism and their corresponding neutralising antioxidants. The majority of ROS are generated when electrons leak from the chloroplastic electron transport systems during photosynthesis, from the mitochondrial electron transport chain during photorespiration, and from the peroxisomal membrane electron transport chain (Figure 2) [90]. In plant cells, 1-2% of O\({}_{2}\) consumption leads to the formation of superoxide (O\({}_{2}\)\({}^{\bullet-}\)), and 1-5% of mitochondrial O\({}_{2}\) consumption leads to the generation of H\({}_{2}\)O\({}_{2}\)[45].