# Marine Microalgae for Potential Lutein Production
## 6 Microalgae Cultivation for Commercial Lutein Production and Challenges  
There are several microalgae species both from marine and freshwater habitats identified as potential lutein producers. Some of these species could produce up to 5 g of free lutein per kg biomass [16]. Microalgae are considered as important industrial candidates for lutein bioproduction. They have higher biomass productivity, high lutein content, are suitable for cultivation in freshwater, wastewater, brackish, or seawater, and/or are dependent neither on arable lands nor on local weather. However, the main challenge in carotenoid production from microalgae in general is low biomass growth rate due to the stress conditions necessary for carotenogenesis. Therefore, large-scale microalgal cultivation mostly operates in two steps, which are the cultivation of microalgae in optimum conditions for fast growth in complete growth medium followed by stress conditions to improve the desired carotenoid in microalgae [45]. There are many strategies to improve microalgal lutein content such as different light intensities, light colour, growth conditions, and nutrient limitations. In the study by [PERSON] et al. [46], _Scenedesmus obliquus_ FSP-3 was subjected to light-related strategies to increase cell growth and lutein production, where the best lutein productivity (4.08 mg L\({}^{-1}\) d\({}^{-1}\)) was observed at a light intensity of 300 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\) with white light. In another study by [PERSON] et al. [47], the effects of nitrogen sources on the cell growth and lutein content of _Scenedesmus obliquus_ FSP-3 were examined, where the highest lutein content (4.61 mg g\({}^{-1}\) ) and lutein productivity (4.35 mg L\({}^{-1}\) d\({}^{-1}\)) were obtained when using 8.0 mM calcium nitrate as the nitrogen source. After determining the nitrogen source condition, [PERSON] et al. [47] applied two bioreactor strategies which were semi-continuous and two-stage; and observed that semi-continuous operation with a 10% medium replacement ratio achieved the highest biomass productivity (1304.8 mg L\({}^{-1}\) d\({}^{-1}\)) and lutein productivity (6.01 mg L\({}^{-1}\) d\({}^{-1}\)). [PERSON] et al. [48] examined the marine microalga _Chlamydomonas_ sp. JSC4 under different environmental conditions for lutein production and concluded that the optimal lutein content was obtained under blue light and a lower temperature of 20-25 \({}^{\circ}\)C. [PERSON] et al. [49] also examined different environmental factors on marine microalga _Tetraselmis_ sp. CTP4 for carotenoid biosynthesis and examined that lutein amount increased 1.5-fold under higher light intensities (170 and 280 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)), but in contrast to [PERSON] et al. [48], found a temperature increase from 20 to 35 \({}^{\circ}\)C, after only two days at a light intensity of 170 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\), yielded the highest lutein productivity (3.17 \(\pm\) 0.18 mg g\({}^{-1}\) dry weight biomass). [PERSON] et al. [50] also examined light stress on the marine microalga _Chlamydomonas_ sp. JSC4 as a potential lutein production. High lutein productivity (5.08 mg L\({}^{-1}\) d\({}^{-1}\)) was attained under high light irradiation of 625 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\), where lutein amount started to decrease at 750 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\) due to downregulation of _lut1_ and _zep_ genes, which respectively encode the enzymes \(\varepsilon\)-carotene hydroxylase and zeaxanthin epoxidase that are responsible for lutein biosynthesis. [PERSON] et al. [51] also examined light-related carotengenesis strategies along with different nitrogen concentrations and growth conditions on _Scenedesmus obliquus_ CWL-1 under mixotrophic cultivation. In contrast to [PERSON] et al. [46], maximum lutein yield (1.43 mg L\({}^{-1}\) d\({}^{-1}\)) was achieved in 12 h/12 h light/darkconditions under blue/red light.