## 2 Diversity of Marine Carotenoids
### Diversity of Marine Carotenoids  
Carotenogenic organisms in ocean are algae and bacteria, which possess all the genes for _de novo_ synthesis of carotenoids [2, 3, 4]. Unicellular microalgae _Dunaliella salina_ and _Dunaliella bardawil_ are  
Figure 2: Phylogenic trees of (**A**) phytoene synthases and (**B**) phytoene desaturases. Trees were built using MEGA6.0 software by Neighbor-Joining method [40]. Protein sequences were obtained from National Center for Biotechnology Information (NCBI). rich in the orange pigment \(\beta\)-carotene (H\({}_{\mathrm{a}}\)H\({}_{\mathrm{a}}\), Figure 1) [41,42]. Two rings of \(\beta\)-carotene are often oxidized to form astaxanthin (H\({}_{\mathrm{a1}}\)H'\({}_{\mathrm{a1}}\), Figure 3) in some microalgae by \(\beta\)-carotene hydroxylase and ketolase [43], which can individually catalyze the modification of \(\beta\)-carotene to generate zeaxanthin (H\({}_{\mathrm{a2}}\)H'\({}_{\mathrm{a2}}\), Figure 3) in _Spriulina platensis_ and _Spriulina maxima_[44], and canthaxanthin (H\({}_{\mathrm{a3}}\)H'\({}_{\mathrm{a3}}\), Figure 3) in _Haematococcus pluvialis_, _Clorella vulgaris_ and _Colastrella striolata_[44, 45, 46]. The modifications can just occur in one ring to generate asymmetric intermediates such as \(\beta\)-cryptoxanthin (H\({}_{\mathrm{a}}\)H'\({}_{\mathrm{a2}}\), Figure 3) and echinenone (H\({}_{\mathrm{a}}\)H'\({}_{\mathrm{a3}}\), Figure 3). Chlorophyta _Scenedesmus almeriensis_ and _Muriellosis sp_. accumulate a large amount of lutein (H\({}_{\mathrm{b1}}\)H'\({}_{\mathrm{a2}}\), Figure 3), which is derived from \(\alpha\)-carotene (HbH'\({}_{\mathrm{a}}\), Figure 1) [47]. Cryptophyta also synthesize \(\alpha\)-carotene as well as acetylenic derivatives croocaxanthin (H\({}_{\mathrm{b}}\)H'\({}_{\mathrm{a4}}\), Figure 3) and monadoxanthin (H\({}_{\mathrm{b1}}\)H'\({}_{\mathrm{a4}}\), Figure 3) [28]. Acetylenic groups are also found in \(\beta\)-carotene derivatives alloxanthin (H\({}_{\mathrm{a4}}\)H'\({}_{\mathrm{a4}}\), Figure 3) in Cryptophyta [48], and diatoxanthin (H\({}_{\mathrm{a2}}\)H'\({}_{\mathrm{a4}}\), Figure 3) and epoxy oxidized diadinoxanthin (H\({}_{\mathrm{a4}}\)H'\({}_{\mathrm{a5}}\), Figure 3) in Heterokontophyta, Haptophyta, Dinophyta, and Euglenophyta [28,49,50]. The unique acetylenic carotenoids are only found in algae. In brown algae and diatoms, acetylated and unique allelic modifications produce dinoxanthin (H\({}_{\mathrm{a5}}\)H\({}_{\mathrm{a6}}\), Figure 3) and chain-oxidized fucoxanthin (H\({}_{\mathrm{a6}}\)H'\({}_{\mathrm{a7}}\), Figure 3) [2,51]. Some Chlorophyta species modify the methyl group of lutein to generate loroxanthin (H\({}_{\mathrm{b1}}\)H'\({}_{\mathrm{a8}}\), Figure 3) in _Scenedesmus obliquus_ and _Chlorella vulgaris_[52], and siphonaxanthin (H\({}_{\mathrm{b1}}\)H'\({}_{\mathrm{a9}}\), Figure 3) in _Codium fragile_[53]. Aromatic isorenieratene (H\({}_{\mathrm{c}}\)H'\({}_{\mathrm{c}}\), Figure 1) is a usual biomarker compound, which is synthesized from \(\beta\)-carotene in actinobacteria or \(\gamma\)-carotene (H'\({}_{\mathrm{o}}\)H\({}_{\mathrm{a}}\), Figure 1) in green and purple sulfur bacteria [54,55]. \(\gamma\)-Carotene can also be converted to chlorobactene (H\({}_{\mathrm{c}}\)H'\({}_{\mathrm{o}}\), Figure 1) and OH-chlorobactene (H\({}_{\mathrm{c}}\)H'\({}_{\mathrm{o1}}\), Figure 3). Glycoside modifications generate OH-chlorobactene glucoside (H\({}_{\mathrm{c}}\)H'\({}_{\mathrm{o2}}\), Figure 3) in green sulfur bacteria and myxol 2'-fucoside (H\({}_{\mathrm{o3}}\)H'\({}_{\mathrm{a2}}\), Figure 3) in Cyanophyta [54,56]. Dinophyta can synthesize C37-skeletal carotenoids such as peridinin (H\({}_{\mathrm{d1}}\)H'\({}_{\mathrm{a6}}\), Figure 3) [57].