## 6 Biotechnological Uses and Production Potentiality of Carotenoids from Haloarchaea
### Production Potentiality of Carotenoids from Haloarchae  
There are few examples of studies about haloarchae carotenoids accumulation supporting the idea that these microorganisms might be considered good carotenoids producers [113; 137], specifically for bacterioruberin and its C\({}_{50}\)-related pigments. The culture conditions that are required to promote fast growth of halophilic archaea include high salt concentration (from 20% to 25% w/v). However, promoting massive carotenoid accumulation of these halophilic microorganisms generally requires much lower concentrations or NaCl, normally below 16% w/v [113; 116]. Such lower salt concentrations address slower growth rates or even cell lysis. Therefore, carotenoid accumulation and growth of halophilic archaea are often opposite events. Moreover, besides the culture medium salinity, other key factors as temperature and pH may affect carotenoids accumulation and also growth rates of halophilic microorganisms tremendously [116]. In some cases, changes in the nutrient composition of the culture medium might result in enhanced C\({}_{50}\) carotenoid accumulation [105]. Consequently, if cultivated under suitable conditions halophilic microorganisms may accumulate carotenoids. So far, only few studies have been reported on the accumulation of carotenoids in halophilic archaea, and all of them at laboratory scale systems. The data obtained so far help clarify that the carotenoid accumulation potential of halophilic archaea is worth being studied in terms of bioprocess engineering. The maximal intracellular concentration of carotenoids reported so far for halophilic archaea were obtained in small flasks during laboratory experiments and ranges from 20 to 25 mg\(\cdot\)g\({}^{-1}\) dry weight, with maximal volumetric productions of about 10 mg\(\cdot\)L\({}^{-1}\)[116]. The intracellular concentration of 20-25 mg\(\cdot\)g\({}^{-1}\) dry weight means an accumulation rate per biomass unit of 2.0%-2.5%. These data compare well to those of carotenoid producing microalgae, many of which are below 1% [139] per biomass unit with the exception of _Dunaliella salina_, obviously the most efficient carotenoid producer microorganism [140]. Productivity is the key parameter to understand the potential of a microorganism for production of whatever value compound. For biomass, volumetric productivity is calculated in terms of g\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\), and productivity of a desirable compound can also be calculated per reactor volume (g\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\)) or as mg\(\cdot\)g\({}^{-1}\) dry weight\(\cdot\)day\({}^{-1}\). At large scale, surface is included as a factor to which productivity is referred. For instance, biomass areal productivity is expressed as g\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\). Independently of how productivity is expressed, the rate at which biomass is produced in time obviously determines the potential of the microorganism for accumulation of desirable compounds. To our knowledge, no data have been published on both biomass and carotenoid productivities of halophilic archaea at scales other than just few examples in laboratory flasks. Data such these are needed to approach the potential of halophilic archaea for carotenoids production. An approach to the biomass productivity of halophilic archaea can be done simply with the data above. For instance, the average time taken for a culture of _Halorubrum_ sp. and other halophilic archaea species to grow to the end of the exponential phase is about 10 days [116, 141], in batch systems. Maximal biomass concentration reported is about 0.8 g\(\cdot\)L\({}^{-1}\). Simple calculations give an average biomass productivity of 0.08 g (dry biomass)\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\). This productivity data should be higher if the halophilic archaea were cultured in continuous production systems at optimal growing conditions. Accordingly, there should still be significant room for improving such laboratory biomass productivity data.