*3.5. Development of Microalgae for Carotenoid Production*

Algae are a diverse group of aquatic, photosynthetic organisms, generally categorized as macroalgae (*i*.*e*., seaweed) and unicellular microalgae. Microalgae have recently garnered interest for production of valuable chemicals including carotenoids [41,131], because they are generally regarded as safe (GRAS) for human consumption and possess the renewable-energy capturing ability of photosynthesis. Moreover, these organisms can be used for genetic manipulation and highthroughput analysis [132]. Some microalgae are also native carotenoid producers (*i*.*e*., *D. salina* for Ά-carotene and *<sup>H</sup>*. *pluvialis* for astaxanthin). The carotenoid production from microalgae is closely related to culture conditions such as illumination, pH, temperature, nitrogen availability and source, salinity, the oxidant substances, and growth rate [12,133,134]. *<sup>D</sup>*. *salina* is a model species of green microalgae which is widely cultivated outdoors for Ά-carotene production [131]. A  systematic evaluation has been done to decipher the relationship between abiotic stresses (Nitrate concentration, salinity and light quality) and lutein synthesis in *D. salina* [135]. The abiotic stresses can also be applied to adaptive evolution of microalgae [136], in a similar manner to strain evolution in yeast for Ά-carotene production [107]. The freshwater microalga *Chlamydomonas reinhardtii* is the first and the best studied transformed Chlorophyte, and the nuclear genetic manipulation is easy and well established. It has been engineered with Ά-carotene ketolase from *H*. *pluvialis* to synthesize ketolutein (Hb1H'a1, Figure 3) and adonixanthin (Ha1H'a2, Figure 3) [137]. It is possible to produce diverse valuable carotenoids from marine microalgae with the development of more available genetic tools and technologies. 
