## 5 Production of Carotenoids by Haloarchae
### Type, Content and Biosynthesis of Haloarchaeal Carotenoids
The last species is also able to produce monoanhydrobacteriourberin (22.5%), bisanhydrobacteriourberin (9.3%), and isopentenyldehidrorhodopin (\(<\)0.1%) [5]. The main carotenoids produced by _Halorubrum_ sp. TBZ126 were bacterioruberin, lycopene and \(\beta\)-carotene [58,79], while the major carotenoid produced by _Halococcus morrhuae_ and _Halobacterium salinarum_ was all-_trans_-bacteriourberin, accounting for 69% of the carotenoids, respectively [79]. [PERSON] colleagues [26] reported that _Haloferax volcanism_ contained the (2\(S\),2\({}^{\prime}\)\(S\))-bacteriourberin (82% of total carotenoid), monoanydrobacteriourberin (7%), (2\(S\),2\({}^{\prime}\)\(S\))-bisanhydrobacteriourberin (3%), 3,4-dihydromonoanhydrobacteriourberin (2%) and two undecaene C\({}_{6}\)H\({}_{4}\)O\({}_{4}\) carotenoids (each 2%), the C\({}_{45}\)-carotenoid (2\(S\))-2-isopentenyl-3,4-dehydrorhodopin (1%) and lycopene (0.3%). The lipid composition of the extremely halophilic archaeon _Haloquadratum walsbyvi_ was investigated by thin layer chromatography and electrospray ionization-mass spectrometry. The results confirmed the presence of  
Figure 1: Bars plot summarizing details about the number of publications per year related to carotenoids. The key words used to perform the search were: carotenoids, carotenoids & human health, carotenoids & antioxidants; carotenoids & archaea and bacterioruberin. Pubmed and Scopus were used as databases to do the search. the carotenoids caroten and bacterioruberin, the C\({}_{30}\)-isoprenoid compound squalene and the menaquinone with eight isoprenoid units vitamin MK-8 [94]. The total carotenoid content in _Haloarcula japonica_ was 335 \(\upmu\)g-g-1 of dry mass, although the contents in _Halobacterium salinarum_ and _Halococcus morrhuae_ were 89 and 45 \(\upmu\)g-g-1, respectively [79]. Although general knowledge about carotenoids biosynthesis and their assimilation in higher plants and human beings is considerable, nutritional functions, as well as metabolic pathways and their regulation, have not been examined in detail in haloarchaea [38]. The biosynthesis of carotenoids in haloarchaea was studied for the first time in the later 1970s. At that time, it was stated that the biosynthetic pathway for the formation of C\({}_{40}\) carotenes in _Halobacterium_ proceeds as follows: isopentenyl pyrophosphate leads to _trans_-phytoene, leads to _trans_-phytofluene, leads to \(\zeta\)-carotene, leads to neurosoprene, leads to lycopene, leads to gamma-carotene, and finally leads to \(\upbeta\)-carotene. This pathway differs from that in higher plants in that the _cis_ isomers of phytoene and phytofluene are not on the main pathway of caroten biosynthesis, as they are in higher plants [95]. On the other hand, it has been suggested that bacterioruberin is synthesised by addition of C\({}_{s}\) isoprene units to each end of the lycopene chain, followed by the introduction of four hydroxyl groups. The evidence supporting these suggestions where reported around 40 years ago from experiments where nicotine was used to inhibit the bacterioruberin synthesis [96, 97]. The presence of multiple genes for several steps in _Halobacterium_ NRC-1 carotenoid production suggests that there may be more than one biosynthetic pathway [98, 99]. Computational genome and pathway analysis of halophilic Archaea done by [PERSON] and co-workers [100] suggested that phytoene is reduced to lycopene by phytoene desaturase. Lycopene is the branching point for the synthesis of bacterioruberins (C\({}_{50}\)) and \(\upbeta\)-carotene (C\({}_{40}\)) [101, 102]. Although the reactions leading from lycopene to bacterioruberins have not been elucidated in detail yet, there is some evidence supporting that the lycopene cyclase (OE3983R) converts lycopene to \(\upbeta\)-carotene in _Halobacterium salinarum_ str. NRC-1 [98].