## 2 Diversity of Marine Carotenoids
### Pathways and Diverse Enzymes for Biosynthesis of Carotenoids  
Biosynthetic routes to carotenoids begin with the basic building blocks isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP), although carotenoids are very diverse in chemical structure. Two distinct pathways, the 2-\(C\)-methyl-d-erythritol 4-phosphoahte (MEP) pathway and the mevalonic acid (MVA) pathway, are responsible for the synthesis of IPP and DMAPP. These two pathways have been reviewed in detail elsewhere [19, 20, 21, 22]. IPP and DMAPP are head-to-tail condensed to generate farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP) by isoprenyl diphosphate synthases (e.g., IspA of _Escherichia coli_ and CrtE of _Pantoea agglomerans_) [23,24]. As shown in Figure 1, FPP and GGPP are further head-to-head condensed to produce symmetric hydrosqualene (C30) and phytoene (C40), which are dehydrogenated in a stepwise manner by desaturating enzymes representing an important branch point for pathway diversification [25,26]. Enzymes involved in the biosynthesis of carotenoids have been mainly investigated in carotenoidogenic cyanobacteria and land plants [27, 28]. They are mostly associated with cytoplasmic and organelle membranes where the hydrophobic substrates of carotenoid enzymes are located [29]. So far, very few crystal structures of carotenogenic enzymes have been elucidated because of their association with the membranes [30, 31]. More than 95% of carotenoids have been characterized in nature to be phytoene-based [32], which will be extensively discussed in this review. Phytoene synthase is positioned early in the carotenoid synthesis pathway and is responsible as a pathway gatekeeper to discriminate GGPP substrate from enormous isoprenyl diphosphates [29]. Phylogenetic analysis of 20 phytoene synthases from marine organisms supports the endosymbiotic theory that plastids evolve from a cyanobacterium, which is engulfed and retained by a unicellular protist [33, 34]. Cyanobacteria _Acaryochloris marina_ and _Prochlorococcus marinus_ are clustered with green algae and land plant tomato (Figure 2A). However, phytoene synthases still display a significant diversification by evolution. A consensus position of 24.5% (identity of 0.5%) is remained among phytoene synthases from marine algae, bacteria, Archaea and land plants. There is only a similarity of 31.9% even between the two proteobacteria phyla \(\alpha\)-proteobacteria and \(\gamma\)-proteobacteria. Figure 1: Synthesis pathway of phytoene-based C40 carotenoid backbones. Most C40 marine carotenoids are modified from the backbones of \(\omega\)/\(\beta\)/\(\gamma\)-carotenes or isorenieratene. Carotenoid structures are presented with two symmetric or asymmetric halves (Figure 2), for example, lycopene is shown as \(\mathrm{H_{o}H^{\prime}_{o}}\) in this review. Conjugated double bonds are shown in red. The photochemical properties of a carotenoid depend on the size of the chromophore formed by conjugated double bonds, and a C40 backbone can accumulate up to 15 conjugated double bonds [35]. Thus, six sequential desaturation steps are required to dehydrogenase colorless phytoene, which has three conjugated double bonds in the center [26]. Lycopene containing a chromophore with eleven conjugated double bonds is the direct precursor of \(\omega\)/\(\beta\)/\(\gamma\)-carotenes or isorenieratene, the phytoene-based C40 carotenoid backbone (Figure 1). In general, oxygenic phototrophs require three enzymes, phytoene desaturase, \(\zeta\)-carotene desaturase and _cis_-carotene isomerase to generate lycopene [6]. However, most bacterial phytoene desaturases are able to catalyze all three reactions [30]. There are also some organisms that disobey this general rule. Primitive cyanobacterium _Gloeobacter violaceus_ PCC 7421 uses bacterial type phytoene desaturase, and no homolog of \(\zeta\)-carotene desaturase or _cis_-carotene isomerase is found in its genome [36,37].