## 5 Production of Carotenoids by Haloarchae
### Bacterioruberin Is One of the Major Carotenoids Produced by Haloarchae  
As it can be concluded from the previous section, bacterioruberin is the main carotenoid component responsible for the colour of the red archaea of the family _Halobacteriaceae_. This pigment has a rather different molecular structure. It has a primary conjugated isoprenoid chain length of 13 C=C units with  
Figure 2: Comparison between the biosynthesis of isoprenoids in photosynthetic organisms (**A**) and the biosynthesis pathway proposed in haloarchae (**B**). C5 preryl units are synthesized via the mevalonate pathway starting from two acetyl-CoA molecules and C2 unit arising from amino acid degradation. _Cis_- and _trans_-phenyl chains are derived through head-tail (HT) condensation steps with isopentenyl-diphosphate (IPP). C15 and C20 preryl chains are modified by head-head condensations and desaturase reactions. Genes coding for the enzymes catalysing those reactions have been identified in the _Halobacterium salinarum_ genome. \(\beta\)-Carotene is the precursor for retinal synthesis while lycopene is the precursor for bacterioruberines in the pathway proposed from the _Halobacterium salinarum_’s genomic analysis [99]. Preliminary evidence from other genomic analysis (_Haloferax_ sp.) also support this proposal [104]. Details about genes, enzymes and chemical reactions involved in retinal and bacterioruberines synthesis are far from known. There are not reports about the biosynthesis reactions of other carotenoids such as zeaxanthin, canthaxanthin, astaxanthin, _etc_. in haloarchae. GPP = geranyl diphosphate; FPP = farnesyl diphosphate; GGPP = geranyldigmosphate. no subsidiary conjugation arising from terminal groups, which contain four -OH group functionalities only [37, 78]. Table 1 summarises the bacterioruberin chemical structure as well as its derivatives. \begin{table}
\begin{tabular}{c c} \hline
**Name** & **Chemical Structure** \\ \hline Bacterioruberin & \\ Monoanhydrobacterioruberin & \\ Bisanhydrobacterioruberin & \\ Trisanhydrobacterioruberin & \\
2-isopentenyl-3,4-dehydrothodopin & \\
5-_cis_-bacterioruberin & \\
9-_cis_-bacterioruberin & \\ \end{tabular}
\end{table} 
Table 1: Chemical structures of bacterioruberin and its derivatives [79, 105]. This pigment protects the cells against damage produced by high intensities of light in the visible and ultraviolet range of the spectrum and provides aid in photoreactivation [106, 107]. It is also involved in the reinforcement of the cell membrane. It was described for the first time from cells of _Halobacterium_ species [88, 108, 109]. The byosinthesis of C\({}_{50}\) carotenoids in general terms, and the effect of several chemical compounds on this biosynthesis were first described from _Halobacterium cutirubrum_ (_Halobacteriaceae_ family) [88, 97, 110]. A few years later, it was described that bacterioruberin is synthesized from other C\({}_{50}\) carotenoids, such as isopentenyldehydrohodopin, bisanhydrobacterioruberin, and monoanhydrobacterioruberin [5] and the synthesis is induced by (i) low oxygen tension and high light intensity [111, 112]; (ii) osmotic stress [113]; and (iii) the presence of different compounds such as aniline [114] (Figure 2). However, this general pattern has some exceptions, for _example Haloquadratum walsbyi_: cells grown under osmotic stress did not experience changes in terms of either membrane lipid composition or carotenoids production [94]. Composition of the total carotenoids fraction in haloarchae can also change on the basis of the nutritive factors within the culture media [105], the light intensity, oxygen tension, NaCl concentration [91, 105, 111], and other physical-chemical parameters such as pH value of the culture media. Figure 3 shows the effect of pH on carotenoids profile in _Haloferax mediterranean_ cells grown in aerobic complex media. It has been reported that pH significantly influences cell growth and total carotenoid production in a lot of microorganism [115]. [PERSON] _et al_.