# Caroenoid Production from Microalgae: Biosynthesis, Salinity Responses and Novel Biotechnologies
## 3 Salt-Stress Treatment for Caroenoid Production from Microalgae
### Salt Stress Strategies for Carotenoid Accumulation  
Carotenoid production could be affected by environmental abiotic stress. Under salt stress, carotenoids will accumulate and protect cells as antioxidants to increase the surviving possibility of microalgae. In addition, optimal salt condition varies among different microalgal species. After NaCl treatment (1%, 0.17 M) for 10 days, the astaxanthin content of _H. pluvialis_ climbed from 3.53 mg/g to 17.7 mg/g [87]. _C. zofingiensis_ CCAP 211/14 can tolerate moderate NaCl concentration of 100 mM, and a significant enhancement of astaxanthin content was observed under 200 mM NaCl treatment [88]. The optimal condition to obtain the highest amount of toxoxanthin (79.40 \(\pm\) 0.95 mg/g DW) of _Tisochrysis lutea_ was determined as 36.27 g/L salt addition [64]. As the optimal conditions for cell growth are commonly different from those for secondary metabolite accumulation, two-stage cultivation has been widely applied to microalgae cultivation, a potent strategy to balance cell growth and metabolite accumulation. Acidophilic eukaryotic microcalga _Cocomyxa onubensis_ can endure moderate salt stress and serves as a potential resource for lutein production. Its growth rate and biomass productivity were significantly boosted under salt treatment (100 mM NaCl). By adding 500 mM NaCl, the lutein content was significantly enhanced by 47% to 7.80 mg/g DW, though the cell growth was inhibited [89]. Thus, cultures containing 100 mM salt can be applied at the first stage for higher biomass, then extra salt was added to induce lutein accumulation at the second stage. Moreover, salt stress can also be coupled with other treatments to increase the carotenoid accumulation in microalgae. Light induction and nutrient starvation are widely used methods to boost carotenoid production. Combined with high light, salinity treatment increased the astaxanthin yield of _C. zofingiensis_ 7.53-fold compared with the control [70]. _D. salina_ was able to adapt to NaCl ranging from 0.05 to 5.5 M and the \(\beta\)-carotene content of _D. salina_ achieved 13% of DW under salt stress, combined with high light at high temperature under nutrient deficiency [40]. The addition of chemicals can also boost carotenoid accumulation under salt stress. Photocatalyst TiO\({}_{2}\) can enhance zexaanthin accumulation under salt stress in _Coolastrella sp_. by increasing oxidative stress [42]. The maximum zeaxanthin (13.2 \(\pm\) 4.4 mg/g DW) was achieved under high salinity (3% NaCl) and N-starvation at 40 \({}^{\circ}\)C after TiO\({}_{2}\) treatment. An 0.25 mM amount of \(\gamma\)-aminobutyric acid (GABA) could facilitate astaxanthin productivity by 3.24-fold in _H. pluvialis_ under high light with salinity treatment (2 g/L) [90]. Melatonin (MT) addition enhanced the expression of carotenoid genes of _H. pluvialis_ and induced astaxanthin accumulation by 1.20-fold under N-starvation and salt stress (1 g/L) [91]. NaCl treatment could also amplify the effect of linoleic acid (LA) on boosting astaxanthin accumulation in _Chlorella sorokiniana_, and LA could increase astaxanthin content by 1.25-fold in