## 4 Microbial Fermentation for Commercial Production of C\({}_{30}\) Caroenoids
### Abiotic Stresses for the Optimization of C\({}_{30}\) Carotenoid Production
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Controlled Factor** & **Action** & **Effect** & **Ref.** \\ \hline Oxidative stress & Diphenylamine and & & \\ Oxidative stress & duroquinone & Complete inhibition of C\({}_{30}\) carotenoids & [80] \\  & application & & \\ Oxidative stress & Diphenylamine & & \\  & application & Complete inhibition of C\({}_{30}\) carotenoids & [16] \\ Oxidative stress & Hydrogen peroxide exposure & & 5.9-fold increase in carotenoid & [29] \\  & & & production \\  & & Increased 4,4\({}^{\prime}\)-diaponeurosporene & \\  & & antioxidant activity\(-\) & \\ Temperature & Low temperature & DPPH: 1.7-fold vs BHT; & [31] \\  & & ABTS: 7.5-fold vs BHT; & \\  & & FRAP: 8-fold vs BHT & \\ Recombinant & Constitutive lac & Increased 4,4\({}^{\prime}\)-diapolyorene & [81] \\ technique & promoter & production & \\ Recombinant & Optimizing & Increased 4,4\({}^{\prime}\)-diapolycopene and & \\ technique & engineered pathways in & 4,4\({}^{\prime}\)-diaponeurosporene production & [82] \\  & heterologous host & & \\ Recombinant & Gene coexpression & 20-fold increase in 4,4\({}^{\prime}\)-diapolycopene & \\ technique & Farnesyl diphosphate (FDP) & & dialdehyde production & \\ Recombinant & synthase & 3- to 5-fold increase in & \\ technique & overexpression & & diapotorulene-to-diaponeurosporene ratio & [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Effects of applied factors on C\({}_{30}\) carotenoid production. is a relationship between carotenoid production and carbon:nitrogen ratios in certain bacteria [92]. White-light irradiation has a positive effect on carotenoid synthesis in algae, fungi, and bacteria [93]. The carotenoid yield depends on the microorganism strain and light intensity. White light acts as a stimulatory inducer for cell growth, which then follows carotenoid accumulation, owing to increased enzyme activity [94]. White light increases carotenoid production by regulating biosynthetic genes in the pathway. High light intensity can restrict cell development and growth while inducing carotenoid synthesis [85]. Stress can be utilized to improve either cell growth or carotenoid production, and previous studies have shown that the two are not necessarily related or proportional. Different species respond differently to stress factors, either increasing or decreasing carotenoid concentrations [72]. Carotenoid accumulation can be enhanced at lower incubation temperatures [95], although carotenogenesis is not necessarily related to cell growth or biomass yield [72; 96]. In microalgae, stress can stimulate carotenoid yields but production is limited by slow growth rates, required culture volumes, and bacterial contamination risks [97]. Salinity, oxidative stress, and pH have been used to increase carotenoid concentrations [10]. This increase is a result of microbial defense mechanisms triggered via gene regulation to modulate and alleviate the effects of stress factors [98]. However, while stress can trigger and stimulate carotenoid accumulation, stressful environments past the tolerance point will more likely trigger cell death than carotenoid accumulation.