## 7 Bioprocess for Xanthophylls Production by Microalgae
### Cultivation Systems
#### 7.1.2 Closed Systems  
Open system problems have led to the design of closed systems. PBRs represent the most successful approach to attain better control of important culture parameters like pH, light, temperature, loss of H\({}_{2}\)O, capture of CO\({}_{2}\) and biomass productivity [127]. Furthermore, the low contamination risk is a main asset that would permit a higher control and production of molecules with high-commercial values, such as xanthophylls. PBRs were designed for the cultivation of microalgae [123; 126], including the following types:  
* The tubular type is the most appropriate kind of PBR for producing satisfying high-quality cyanobacteria and microalgae biomasses in outdoor environments [128; 123]. It is generally built with glass or plastic tubes, allowing a large illuminated surface area. In this system, the culture homogenization is generally assured by means of air pumps. It is characterized by some defects, such as pH variation, dissolved oxygen, fouling, and CO\({}_{2}\) heterogeneity. There are many studies indicating the suitability of using this PBR kind for high-quality microalgae and cyanobacteria productions. * Flat PBRs have a large surface exposed to light and are characterized by high algal productivities, which is generally greater than those produced by tubular PBR. This culture system is constructed from a rigid transparent material to optimize light capture and to facilitate sterilization. It is suitable for outdoor cultivation, ideal for cell immobilization and is relatively inexpensive. The only drawback of this type of system is the difficulty in controlling the temperature of algal cultures [129]. Flat PBRs have been tested for culturing the marine diatom _Phaeodactylum tricontum_ for the production of fucoxanthin and chrysolaminarin [130]. The AlgaTechnologies industry ([[https://www.algatech.com/](https://www.algatech.com/)]([https://www.algatech.com/](https://www.algatech.com/)), accessed on 15 April 2021) also established a _Haematococcus_ cultivation facility back in the late 1990s. Quite different to other American industries, the AlgaTechnologies Company used glass tubular PBRs for both green and red phases to phototrophically cultivate _Haematococcus_[131]. Problems that are associated with a limited light source that hinders high cell density in large-scale PBRs during photoautotrophic growth can be avoided by using heterotrophic cultivations [132]. The elimination of light restriction led to a higher microalgae cell growth rate and a greater cell mass content can be reached faster. So far, the maximum produced biomass in photoautotrophic conditions was about 40 g/L of microalga dry weight [133] and this content was lower than in that obtained in heterotrophic conditions (150 g/L) [134]. Heterotrophic culture mode is mainly used for the production of high value-added xanthophylls (lutein, ASX...) from microalgae, due to its high cost [79]. The dry weight of the green microalgae _Chorella protothecodes_ and its lutein content attained 19 g/L and 84 mg/L respectively, when it was cultured heterotrophically. These values reached 47 g/L and 225 mg/L using a culture with the fed-batch system [135]. [PERSON] and [PERSON] (2006) found the highest biomass concentrations and a maximum productivity of 105 g/L cell dry weight and 0.613 g/L/h, respectively, when _Chlorella pyrenoidosis_ was grown in heterotrophic conditions [136]. Hybrid systems are deployed for the large-scale production of marine microalgae to produce some molecules of commercial interest. In 2015, The diatom _Staurosina_ sp. and the chlorophyte _Desmodesmus_ sp. were cultured in a hybrid system combining PBRs (25 m\({}^{3}\)) and open basins (400 m\({}^{2}\)) [137]. In this system, the PBRs permanently ensured a source of uncontaminated inoculum for the short-lived batch culture in an open pond. The latter ensures a large-scale biomass production in a competitive cost- and time-consuming operation.