## 2 Microbial C\({}_{30}\) Carotenoids and Their Derivatives  
Like most carotenoids, the isoprene units that comprise C\({}_{30}\) carotenoids are constructed from the isopentyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) precursors from the mevalonate (MVA) pathway in plants and microalgae (Figure 1), and the methylerythritol phosphate (MEP) pathway in bacteria and fungi (Figure 2) [18]. The biosynthesis of C\({}_{30}\) carotenoids occurs via either the condensation of two C\({}_{15}\) farnesyl diphosphate (FPP) molecules, or the condensation of C\({}_{10}\) geranyl diphosphate (GPP) and C\({}_{20}\) geranyligeranyl diphosphate (GGPP). The former results in symmetric 4,4\({}^{\prime}\)-diapocarotenoids, whereas the latter produces asymmetric apo-8\({}^{\prime}\)-carotenoids [10; 19; 20]. The C\({}_{30}\) 4,4\({}^{\prime}\)-diapocarotenoids include 4,4\({}^{\prime}\)-diapocarotene-4-oic acid and fatty acid esters di-(B,D-glucosyl)-4,4\({}^{\prime}\)-diapocarotene-4,4\({}^{\prime}\)-dioate from _Methylobacterium rhodium_ (formerly _Pseudomonas rhodos_) [21]; 4,4\({}^{\prime}\)-diaponeurosporene and OH-diaponeurosporene from Heliobacteria [22; 23]; and staphyloxanthin, 4,4\({}^{\prime}\)-diapophytoene, 4,4\({}^{\prime}\)-diapophytofluene, 4-4\({}^{\prime}\)-diapo-zeta-carotene, 4,4\({}^{\prime}\)-diaponeurosporene, and several of its derivatives from _S. aureus_[24; 25] (Figure 3). Biosynthetic pathways of known carotenoids can be extended or altered through the inclusion of genes that further alter the forms of the carotenoids. Additionally, novel derivatives of these compounds can be produced by modifying reactions such as methylation, cyclization, and oxygenation. Figure 1: Mevalonate (MVA) pathway as taken by microalgae. AACT: Acetoacetyl-CoA thiolase. HMGCS: HMG-CoA synthase. HMGCR: HMG-CoA reductase. M3K: Mevalonate-3-kinase. M5K: Mevalonate-5-kinase. M3P5K: Mevalonate-3-phosphate-5-kinase. PMD: Phosphomevalonate decarboxylase. BMD: Bisphosphonate decarboxylase. IPK: Isopentenyl phosphate kinase. Idi: Isopentenyl pyrophosphate isomerase. FPPS: Farnesyl pyrophosphate synthase. GGPS: Geranylger-anyl pyrophosphate synthase. Figure 2: Methyl erythroid 4-phosphate (MEP) pathway as taken by bacteria and fungi. DXS: Deoxyxyululose 5-phosphate synthase. DXR: Deoxyxyululose 5-phosphate reductio isomerase. MCT: 2-C-methyl-d-erythitol 4-phosphate cytidylyltransferase. CMK: 4-diphosphocytidyl-2-C-methyl-D-erythhitol kinase. MDS: 2-C-methyl-d-erythhitol 2,4-cyclodiphosphate synthase. HDS: 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase. HDR: (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase. IPPI: Isopentenyl diphosphate isomerase. GPPS: Geranyl diphosphate synthase. GGPS: Geranylgeranyl diphosphate synthase. The asymmetrical C\({}_{30}\) apo-8\({}^{\prime}\)-carotenoids have three conjugated double bonds not found in the center of their skeletal structure (Figure 4), and include methyl glucosyl-3,4-dehydro-apo-8\({}^{\prime}\)-lycopenoate from _Planococcus_[26], and hydroxy-3,4-dehydro-apo-8\({}^{\prime}\)-lycopene and methyl hydroxy-3,4-dehydro-apo-8\({}^{\prime}\)-lycopene from _Halobacillus_[27]. Those of _Planococcus_ were isolated products of natural biosynthesis beyond the known pathways. This suggests the possibility of gene clusters with unconfirmed genes that allow for their synthesis or other reactions within the bacteria. However, hydroxy-3,4-dehydro-apo-8\({}^{\prime}\)-lycopene and methyl hydroxy-3,4-dehydro-apo-8\({}^{\prime}\)-lycopenoate were produced after chemical mutagenesis of the microorganisms. Different microbial C\({}_{30}\) carotenoids originate from a limited number of microorganisms. Their properties are dependent on their structures, which can be altered further by changes or extensions in the biosynthetic pathways. Several C\({}_{30}\) carotenoids are presented in Table 1 with their structures, bacterial sources, and antioxidant activities compared to those of C\({}_{40}\) carotenoids and other standard compounds for the determination of antioxidant capacity.