## 5 Production of Carotenoids by Haloarchae  
### Type, Content and Biosynthesis of Haloarchaeal Carotenoids  
Halophilic archaea are extreme halophilic microorganisms mainly grouped into the _Haloferacaceae_ family, phylum _Euryarchaeota_, _Archaea_ domain. They are (mostly) aerobic and generally red-pigmented. They constitute the predominant microbial communities in extreme halophilic environments as it was mentioned before. To be alive under those conditions they have adopted several strategies: (i) amino acidic residues predominate in halophilic proteins surface; (ii) cells accumulate high KCl intracellular concentrations to deal with high ionic strength or some osmolytes such as 2-sulforehalose [85]; (ii) cellular bilayers have different composition and structure, _etc._ Due to these adaptations, haloarchaea have become a good and innovative source of different molecules of high interest in biotechnology such as enzymes able to be active at high temperature and high ionic strength [86,87], PHB and PHA, carotenoids, _etc._  
Related to carotenoids, there is little information in the literature about the carotenoid profile of extremophile microorganisms compared with the information available from other organisms, and only few of them are focused on carotenoid production by archaea in general, and by haloarchaea in particular [58]. Figure 1 summarises the number of publications focused on carotenoids. It is important to highlight that despite the huge number of publications on that subject, only 1.3% of them are related to haloarchaeal carotenoids (780 papers about haloarchaeal carotenoids _vs._ 61590 papers about carotenoids in general). What is clearly supported by the literature is that most members of the family _Haloferacaceae_ can synthesize C\({}_{50}\) carotenoids, including bacterioruberin (as the most abundant C\({}_{50}\) in most of the analysed haloarchaeal species) and its precursors (2-isopentenyl-3,4-dehydrohodopin (IDR), bisanhydrobacterioruberin (BABR), and monoanhydrobacterioruberin(MABR)) [32,88]. Several other derivatives have been found in minor amounts, such as 3,4-dehidromonoanhydrobacterioruberin, haloxanthin (which is a derivative of the previous one containing a peroxide end group) and 3,4-epoxymonoanhydrobacterioruberin, identified in _Haloferax volcanism_[26,89]. Other carotenoids such as phytoene, lycopene, and \(\beta\)-carotene are also produced by these species but at lower concentration [7]. Those carotenoids are located in the cell membrane and they are in charge of the colour shown by the red colonies when haloarchae cells grow on solid media or the red colour shown by salted coastal ponds (mainly in summer). In fact, the content of bacterioruberin pigments in the biomass has been used to monitor the density of halophilic archaeal communities in halophilic environments [90]. Other carotenoids have been identified at very low concentrations in halophilic archaea: lycopene, _cis-_ and _trans_-phytoene, _cis-_ and _trans_-phytofluene, neo-\(\beta\)-carotene and neo-\(\alpha\)-carotene. The low concentrations of these compounds suggest that they may be used as precursors for the synthesis of other carotenoids including lycopene, retinal and the members of the bacterioruberin group. Some species may also produce the ketocarotenoid canthaxanthin in addition to other carotenoids [58]. Although this is the general carotenoid profile exhibited for most of the haloarchaeal species, it is important to note that some of them can produce high amounts of canthankanthin, \(\beta\)-carotene and _trans_-astaxanthin [91]. The presence of characteristic carotenoids (\(\alpha\)-bacteriourberin and derivatives) in haloarchae cells is easy to identify by Raman spectroscopy [30,92,93]. Thanks to this technique, \(\alpha\)-bacteriourberin has been identified as the mayor carotenoid in the following haloarchaea: _Halobacterium salinarum_ strains NRC-1 and R1, _Halorubrum sodomense_, _Haloarcula vallismortis_[78] and _Haloarcula japonica_ (68.1% of the total carotenoids (mol %) [5].