**1. Introduction**

Carotenoids are a class of naturally occurring pigments originated in the chloroplasts and chromoplasts of plants, algae and some photosynthetic microorganisms [1–4]. As of 2004, over 750 known carotenoids, which can be  divided into xanthophylls (containing oxygen) and carotenes (pure hydrocarbons), have been isolated from natural sources [5]. These structurally diverse pigments play important biological roles in light capture, protection of cells from the damaging effects of free radicals, and synthesis of many hormones as a precursor [6–10]. Carotenoids are traditionally used as food colorants, animal feed supplements, and, very recently, as nutraceuticals and pharmaceuticals [11,12]. Over the past few decades, researches have supported that the ability of carotenoids to reduce the risk of certain cancers, cardiovascular diseases, and degenerative pathogenesis (e.g., Alzheimer and Parkinson) due to their antioxidative properties [13,14]. According to "Carotenoids: A Global Strategic Business Report" from Global Industry Analysts (GIA), the global market for carotenoids was estimated at approximately \$1.07 billion in 2010 and is projected to top \$1.2 billion by 2015 [15]. Therefore, many efforts have been made to improve the production of these natural compounds for ever-increasing demands [12,16,17]. 

The ocean is a complex aquatic ecosystem covering about 71% of the Earth's surface, which is around 300 times larger than the habitable volume of the terrestrial habitats on Earth. A large proportion of all life on Earth lives in the ocean. Ecologically distinct from the terrestrial ecosystem, the ocean constitutes a unique reservoir of marine biodiversity and provides a vast resource of foodstuffs, medicines, and other useful materials. As such, more than 250 novel carotenoids have originated from marine species [10], many of which show great potential in commercial applications [18]. With the advent of synthetic biology and metabolic engineering, many engineering tools including vectors, genetic controllers, and enzyme designing, have been developed for heterologous production of valuable chemicals. These tools create new opportunities for exploring marine carotenoids for food and health industries. In this review, we describe diverse and novel carotenoids from marine resources and summarize recent progresses in synthetic biology and metabolic engineering which provide great application potential for marine carotenoids. 
