## 6 Biotechnological Uses and Production Potentiality of Carotenoids from Haloarchaea
### Production Potentiality of Carotenoids from Haloarchae
Combining the average biomass productivity data (0.08 g (dry biomass)\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\)) and the maximal intracellular concentration of carotenoids found for _Halorubrum_ sp. under specific conditions, 25 mg\(\cdot\)g\({}^{-1}\) dry biomass, a maximal productivity of carotenoids of about 2 mg\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\) is obtained. Microalgae, natural carotenoid producers, can be produced at biomass volumetric productivities likely between 0.1 and 0.3 g\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\)[139, 142], which means maximal carotenoids productivity of about 1 to 3 mg\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\) for a microcaliga that accumulates carotenoids at 1% (w/w), therefore giving haloarchaea a chance to be considered in studies for assessing potential of carotenoid production. Of course, these productivity data are far from those carotenoid productivities obtained with _Dunaliella_ species, which are 5 to 10-fold higher. Microalgae have the advantage of using natural light as energy source and carbon dioxide, and haloarchaea, which do not need light to grow, should have the advantage of being produced at a larger volume per surface ratio; this might help to overcome lower volumetric biomass productivities. These aspects might at least be promising enough to study continuous processes of carotenoids production from halophilic archaea in laboratory and pre-pilot scale, particularly for production of other carotenoids than those typically obtained from microalgae (C\({}_{50}\)-bacterioruberin and derivate C\({}_{50}\) pigments). Therefore, it is suggested that haloarchaea might become complementary to those already known to be good carotenoid producers, namely microalgae, in the panel of potential producers. Of course this should just be the starting point of a challenging subject. The use of cheap, raw suitable carbon sources, the economics of cultivation--particularly energy costs for mixing and harvesting--biomass processing and carotenoid purification costs are among those key factors that should be extensively studied. Although the biological roles of the carotenoids produced by haloarchae in haloarchae cells are known (see Section 5), potential benefits of those carotenoids on animal cells (including human beings) have not been tested yet. However, there is some evidence supporting that carotenoids from haloarchae are, at least, as efficient as those antioxidant compounds produced by other microorganism [79,143]. For instance, the halophilic bacteria _Halobacterium salinarum_ produces various pigments such as phytoene, \(\beta\)-carotene, lycopene and derivatives of bacterioruberin and salinixanthin. These pigments have been tested for their free radical scavenging activity by DPPH (di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium) assay and the results validated the known antioxidant activity of carotenoids. A further analysis of the cytotoxic properties against human liver cancer cell lines showed dose-dependent increase in cytotoxicity of the carotenoids on these cells, suggesting the probable anti-cancer properties [143,144]. This is the reason why several research groups around the world as well as some I&D (innovation and development) companies focused on secondary metabolites production have focused their attention on haloarchae as carotenoids source. This interest is not only supported by the huge amount of publications on that subject, but also by the patents related to carotenoid production by haloarchae (wild types as well as genetically modified strains) or methods/technologies to isolate and to purify those carotenoids. Table 2 summarises the patents focused on those subjects during the last 20 years.