Marine unsaturated fatty acids: structures, bioactivities, biosynthesis and benefits

Unsaturated fatty acids (UFAs) are an important category of monounsaturated and polyunsaturated fatty acids with nutritional properties. These secondary metabolites have been obtained from multitudinous natural resources, including marine organisms. Because of the increasing numerous biological importance of these marine derived molecules, this review covers 147 marine originated UFAs reported from 1978 to 2018. The review will focus on the structural characterizations, biological properties, proposed biosynthetic processes, and healthy benefits mediated by gut microbiota of these marine naturally originated UFAs.


Introduction
Fatty acids other than saturated fatty acids (fatty acids that do not contain double bonds are called saturated fatty acids, and all animal oils, except sh oils, contain saturated fatty acids) are unsaturated fatty acids. Unsaturated fatty acids are a kind of fatty acid that makes up body fat. Unsaturated fatty acids (UFAs) consist of a long-chain hydrocarbon with the presence of at least one double covalent bond and ending in a carboxyl group (-COOH), and are distinguished into monounsaturated fatty acids and polyunsaturated fatty acids, both of which have numerous benecial properties to human health. 1,2 These secondary metabolites have previously been obtained from a variety of natural resources, including marine sh oils that are a good natural source of these UFAs. 3,4 In previous decades, marine derived UFAs have attracted a great deal of interest because of their structural diversity and potential biological and nutritional functions. 5 In particular, research interest in omega-3 fatty acids, 6 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from marine organisms, has dramatically increased as they are excellent sources of nutrients. These UFAs also can be described as cis fatty acids versus trans fatty acids, which is a description of the geometry of their double bonds. These characteristics in UFAs not only enable them to show a broad range of biological activities, but also allow the development of the nutrient-like physicochemical properties. However, most of marine derived UFAs belong to a relatively unexplored category that may hold a great promise for the potential nutritional application in the future. The structures and potential nutritional applications of UFAs, particularly these with the interesting biological activities have previously been reviewed, 7,8 but there is still lack of a comprehensive review about marine derived UFAs. Thus, this review aims to summarize 147 marine organisms-derived UFAs published from 1978 to 2018. The review will focus on the structural characterizations, biological properties, proposed biosynthetic processes, and benets mediated by gut microbiota of these marine UFAs. In addition, the origin of the isolation of these UFAs is also taxonomically presented.

Monounsaturated fatty acids
Up to date, there are 14 of total monounsaturated fatty acids obtained from marine organisms, linear and branched monounsaturated fatty acids 1-14 (Table 1 and Fig. 1).

Branched chain polyunsaturated fatty acids
Up to date, there are 109 of total linear chain polyunsaturated fatty acids 39-147 obtained from marine organisms (Tables 3-5 and Fig. 3-5).

Biosynthetic pathways
PUFAs are gaining importance due to their innumerable health benets. The most common source of PUFAs is of marine origin. Hence, understanding their biosynthesis in marine origin has attained prominence in recent years. 89,90 Rabbitsh Siganus canaliculatus was the rst marine teleost demonstrated to have the ability to biosynthesize C20-22 long-chain polyunsaturated fatty acid (LC-PUFA) from C18 PUFA precursors, which is generally absent or low in marine teleosts. 91 The marine diatom Phaeodactylum tricornutum accumulates eicosapentaenoic acid (EPA, 20:5n-3) as its major component of fatty acids. To improve the EPA production, delta 5 desaturase, which plays a role in EPA biosynthetic pathway, was characterized in marine diatom Phaeodactylum tricornutum. 90 There is currently considerable interest in understanding how the biosynthetic pathways of highly unsaturated fatty acids (HUFA) are regulated in sh. The aim is to know if it is possible to replace sh oils (FO), rich in HUFA, by vegetable oils (VO), poor in HUFA and rich in their 18 carbon fatty acid precursors, in the feed of cultured sh species of commercial importance. 92 Although many better insights into the synthesis of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in marine microalgae, 93 there are still a little known about  biosynthetic processes of most isolated UFAs of marine resources. 70,94 Thus, more investigation should be carried out for these marine derived UFAs in the coming researches (Fig. 6).

Beneficial application
It is well-known that polyunsaturated fatty acids n-3 (PUFAn-3) are very important for human health and nutrition. 1 As an example, highly unsaturated long-chain omega-3 fatty acids, derived from the liver of white lean sh, esh of fatty sh, and blubber of marine mammals, exhibit important biological activities. 95 They also serve as the building block fatty acids in the brain, retina, and other organs with electrical activity. Hence, inclusion of oils containing docosahexaenoic acid (DHA) in the diet of pregnant and lactating women as well as infants is encouraged. 95 In addition, some polyunsaturated fatty acids from marine microalgae are found to modulate lipid metabolism disorders and gut microbiota. 96 According to the survey results, high saturated fatty acid and high monounsaturated fatty acid diets have an adverse effect on the gut microbiota and high saturated fatty acids are associated with unhealthy metabolic status, while polyunsaturated fatty acid does not have a negative impact on gut microbiota. 97 Through previous studies we nd that connecting with gut microbiota, PUFAs can be more benecial for human health. For example, increasing antiobesogenic microbial species in the gut microbiota population by appropriate n-3 PUFAs can be an effective way to control or prevent metabolic diseases. 98 Furthermore, a link has been established between n-3 PUFAs and gut microbiota especially with respect to inammation (Fig. 7). A few related researchs show that aer omega-3 PUFA supplementation, Faecalibacterium, oen associated with an increase in the Bacteroidetes and butyrate-producing bacteria belonging to the Lachnospiraceae family, has decreased. Omega-3 PUFAs perform a positive action on diseases by reverting the microbiota composition and increasing the production of anti-inammatory compounds like short-chain fatty acids. 99 According to the link between n-3 PUFAs and gut microbiota, which is associated with inammation, some scholars proposing that an optimal level of LC-PUFAs nurtures the suitable gut microbiota that will prevent dysbiosis. The synergy between optimal LC-PUFAs and gut microbiota helps the immune system overcome the immunosuppressive tumour microenvironment. 100 Fig. 8 The distribution of UFAs reported from marine organisms.   Although many scholars have devoted themselves to the study of polyunsaturated fatty acids, they are limited to the more famous unsaturated fatty acids. There is still lack of investigation of the benecial application of these polyunsaturated fatty acid derivatives with similar structural characteristics. Thus, more investigation should focus on fatty acid physiological roles and applications in human health and disease and the interaction with gut microbiota. 101

Conclusions
UFAs are ubiquitous in many marine organisms. 3,102,103 Although these UFA secondary metabolites have been obtained since the early 20th century, they only recently draw signicant interests because of the diverse range of their biological and nutritional properties. 104 However, there is still lack of a comprehensive review about the structural characterizations, biological and nutritional properties, proposed biosynthetic processes, and benecial application of marine derived UFAs. 1978 to 2018, the main structural types of UFAs obtained from marine organisms is branched chain PUFAs, accounting for 74% of the total (Fig. 8), the main natural source of branched monounsaturated fatty acids isolated from marine organisms is coral, accounting for 31% (Fig. 9), while linear chain polyunsaturated fatty acids obtained from marine organisms is mollusc, accounting for 33% (Fig. 10), the preponderant natural marine source of PUPAs is arthropoda, accounting for 49% (Fig. 11). Although omega-3 fatty acid, 6 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from marine organisms, have dramatically increased as excellent sources of nutrients, it is indicated that the biological activities of most of the UPAs are not investigated (Tables 1-3), and the little known about the biosynthetic pathways of these isolated UPAs. In addition, there is no report about new UFAs isolated from marine resources during 2016 to 2018. Thus, the further investigation of marine derived PUPAs should focus on their and benecial application mediated by gut microbiota.

Conflicts of interest
The authors declare no conict of interest.