**1. Introduction**

Many bacteria that have been isolated from marine environments can synthesize a variety of carotenoid pigments [1]. For example, acyclic C30-type carotenoic acids were identified in some marine bacteria such as *Planococcus maritimus* [2] and *Rubritalea squalenifaciens* [3]. *Algoriphagus* sp. KK10202C of the *Flexibacteraceae* family, which was isolated from a marine sponge, was found to produce flexixanthin ((3*S*)- 3,1<sup>ȝ</sup>-dihydroxy-3<sup>ȝ</sup>,4<sup>ȝ</sup>-didehydro-1<sup>ȝ</sup>2<sup>ȝ</sup>-dihydro-Ά,Μ-caroten-4-one) and deoxyflexixanthin (1ȝ-hydroxy-3<sup>ȝ</sup>,4<sup>ȝ</sup>-didehydro-1<sup>ȝ</sup>2<sup>ȝ</sup>-dihydro-Ά,Μ-caroten-4-one) [4], which are C40-type monocyclic carotenoids containing one Ά-end group ( Ά ring) (called monocyclic carotenoids in this review). Other marine bacteria including strain P99-3, which belong to the *Flavobacteriaceae* family, were shown to produce monocyclic carotenoids, myxol ((3 *<sup>R</sup>*,2<sup>ȝ</sup>*S*)-3<sup>ȝ</sup>,4<sup>ȝ</sup>-didehydro-1<sup>ȝ</sup>,2<sup>ȝ</sup>-dihydro-Ά,Μ-carotene-3,1<sup>ȝ</sup>,2<sup>ȝ</sup>-triol) and saproxanthin ((3 *<sup>R</sup>*)-3<sup>ȝ</sup>,4<sup>ȝ</sup>-didehydro-1<sup>ȝ</sup>,2<sup>ȝ</sup>-dihydro-Ά,Μ-carotene-3,1<sup>ȝ</sup>diol), and zeaxanthin ((3 *R*,3<sup>ȝ</sup>*R*)- Ά,Ά-carotene-3,3<sup>ȝ</sup>-diol) [5,6], which are a C40-type dicyclic carotenoid containing two Ά-end groups (called dicyclic carotenids in this review). Marine bacteria belonging to genus *Paracoccus*, *Brevundimonas* or *Erythrobacter* in the ΅*-Proteobacteria* class have been revealed to synthesize dicyclic carotenoids that are ketolated at the 4(4ȝ)-position(s) (called ketocarotenoids), e.g., astaxanthin ((3*S*,3<sup>ȝ</sup>*S*)-3,3<sup>ȝ</sup>-dihydroxy-Ά,Ά-carotene-4,4<sup>ȝ</sup>-dione) and adonixanthin ((3*S*,3<sup>ȝ</sup>*R*)-3,3<sup>ȝ</sup>-dyhydroxy-Ά,Ά-caroten-4-one) (Figure 1) [7–9]. 

**Figure 1.** Chemical structures of ketocarotenoids produced in marine bacteria, *Paracoccus* sp. and *Brevundimonas* sp., and feasible functions of the carotenoid biosynthesis enzymes. These bacteria synthesize dicyclic carotenoids. *Paracoccus* sp. and *Brevundimonas* sp. are demonstrated to possess the unique genes *crtX* and *crtG*, respectively, in addition to the common genes, *crtE*, *crtB*, *crtI*, *crtY*, *crtZ*, and *crtW* [10,11]. 

Among ketocarotenoids, astaxanthin and canthaxanthin ( Ά,Ά-carotene-4,4<sup>ȝ</sup>-dione) (specifically the former), are commercially important pigments as nutraceuticals and cosmetics that have anti-oxidation and anti-aging effects as well as colorants in aquaculture, while other ketocarotenoids are likely to have industrial potentials [12– 16]. This review focuses on carotenoid Ά-ring 4(4ȝ)-ketolase (4-oxygenase), carotenoid Ά-ring 3(3ȝ)-hydroxylase, and carotenoid Ά-ring 2(2ȝ)-hydroxylase, derived from the marine bacteria that belong to ΅*-Proteobacteria*, and pathway engineering for the production of functional xanthophylls via the incorporation of these Ά-ring-modifying enzyme genes. 
