# Caroenoid Production from Microalgae: Biosynthesis, Salinity Responses and Novel Biotechnologies
## 2 Health-Promoting Carotenoids from Microalgae and Their Biofunctions
### Health-Promoting Carotenoids and Their Production from Microalgae
#### 2.2.2 Astaxanthin  
Astaxanthin (3, 3\({}^{\prime}\)-dihydroxy-\(\beta\), \(\beta^{\prime}\)-carotene-4, 4\({}^{\prime}\)-dione) is a natural pigment existing in various species of aquacultures [18]. It is acknowledged as the strongest natural antioxidant, which is 65 times that of vitamin C and 10 times higher than other carotenoids [51]. Astaxanthin has been widely applied in functional foods, pharmaceuticals and cosmetics, with  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Carotenoid** & **Microalgae** & **Content** & **Productivity/Yield** & **Ref. ** \\ \hline \multirow{7}{*}{**Lutein**} & _Chromochloras zofingenis kdt1 (mutant)_ & 13.81 mg/g DW & 33.97 mg/L & [33] \\  & _Parachlorella sp._ JD-076 & 11.87 mg/g DW & 25.0 mg/L/day & [32] \\  & _Chlorella sorokiniana FZU60_ & 11.22 mg/g DW & 8.25 mg/L/day & [29] \\  & _Chlorella vulgaris_ UTEX 265 & 9.82 mg/g DW & 11.98 mg/g/day & [34] \\  & _Chlorella vulgaris_ CS-4 & 9.0 mg/g DW & 1.56 mg/L/day & [35] \\  & _Scenedesmus_ sp. & 7.47 mg/g DW & 19.70 mg/L/day & [36] \\  & _Chlorella sorokiniana_ FBZU60, also achieved an efficient production at 8.25 mg/L/day & [28] \\  & _Chlorella sorokiniana_ MB-1-M12 & 7.39 mg/g DW & 3.43 mg/L/day & [27] \\  & _Chlorella minutissima_ MCC-27 & 7.05 mg/g DW & 6.34 mg/L/day & [26] \\  & _Haematococcus plurisis_ & 5\% DW & 65.8 mg/m2/day & [37] \\
**Astaxanthin** & _Chromochloras zofingenis_ & 6.5 mg/g DW & 0.8 mg/L/day & [38] \\  & _Coelastrum sp._ GI-C1 (mutant) & – & 28.32 mg/L & [39] \\  & _Dualial sulina_ * & 135-DW & – & [40] \\  & _Chromochloras zofingenis kdt1 (mutant)_ & 7.18 mg/g DW & 34.64 \(\pm\) 1.39 mg/L & [33] \\  & _Nannochloras zoscanica_ CCNM 1081 * & 30.2 mg/g DW & – & [41] \\  & _Celeastrumsf._ SP. 360 & 13.15 mg/g DW & 0.72 mg/L/day & [42] \\
**Zeaxanthin** & _Chromochloras zofingenis kdt1 (mutant)_ & 7.00 mg/g DW & 36.79 \(\pm\) 2.23 mg/L & [33] \\  & _Dunialella sulina zat_ **1 (mutant) * & 5.9 mg/g DW & – & [43] \\  & _Chlorella slipsoada_ * & 4.26 mg/g DW & – & [44] \\  & _Malimonas sp._ & 26.6 mg/g DW & – & [45] \\  & _Isclorvisia shzinjangensis_ * & 23.29 mg/g DW & 2.94 mg/L/day & [46] \\
**Fucoxanthin** & _Odutella aurita_ * & 18.47 mg/g DW & 7.96 mg/L/day & [47] \\  & _[PERSON]. Galhma_ * & 18.23 mg/g DW & – & [48] \\  & _Tischopsis lute_ * & 16.39 mg/g DW & 9.81 mg/L/day & [49] \\  & _Phaematits outstanding potential in free radical scavenging and its anti-aging, anti-inflammatory, anti-hypertensive and anti-cancer properties [10]. Currently, the astaxanthin market is mostly occupied by chemically synthesized products. It is a mixture of three stereoisomers, namely (_3R,3\({}^{\prime}\)R_), (_3R, 3\({}^{\prime}\)S_) and (_3S_, 3\({}^{\prime}\)S) astaxanthin at the ratio of 1:2:1 [52], which is structurally heterogeneous and inefficient for biological uptake. For now, microalgae-derived astaxanthin only occupies 1% of the current astaxanthin market. Additionally, the production cost of natural astaxanthin (USD 1800/kg) is much higher than that of the chemically synthesized one (USD 1000/kg) [53]. Thus, improving the productivity of astaxanthin by microalgae is essential for commercialization. Microalgae-derived astaxanthin has been successfully approved by the Food and Drug Administration (FDA) for direct human consumption [8]. In microalgae, astaxanthin is a secondary carotenoid accumulated under adversity. It can mediate cellular redox imbalance for microalgae to survive oxidative stress [14]. As for the current main source of natural astaxanthin, _Haematococcus pluvialis_ can accumulate up to 5% DW of astaxanthin with attached cultivation [37] and has been used for biotechnology industry for astaxanthin production with an annual yield of 300 tons of biomass [54]. Under adverse growth conditions, _H. pluvialis_ transforms from green and motile cells to red cysts cells with high astaxanthin content [52]. Many pieces of research have exploited strategies for boosting astaxanthin accumulation, such as nutrient deficiency, high light, adding extra chemicals and some combined strategies [10].