Astaxanthin What Is It Good for?

Astaxanthin is a red natural carotenoid that is widely found in nature, especially in the marine environment[1].The chemical name of astaxanthin is 3,3/-dihydroxy-beta,beta /-carotene-4,4/-dione, and the specific configuration is shown in Figure 1.

The unique molecular structure of natural astaxanthin gives it the ability to strongly scavenge oxygen free radicals and inhibit singlet oxygen [2]. It is a more effective antioxidant than β-carotene and vitamin E [3]. The antioxidant activity of natural astaxanthin is 10 times higher than that of other carotenoids and 550 times higher than that of vitamin E, so it is called “super vitamin E” [2, 4].

Numerous studies have proven that astaxanthin has important physiological functions such as anti-cancer, anti-aging, and immunity enhancement, and is absolutely safe for the human body [5, 6]. The main effects are as follows: protecting the skin from ultraviolet rays, improving aging, enhancing the function of the immune system, reducing the incidence of cardiovascular disease and cancer induced by chemical factors, and increasing resistance to filterable viruses, bacteria, fungi and parasites [1, 7-9]; maintaining eye and central nervous system health [10, 11]; strengthening the body's aerobic metabolism and enhancing muscle strength and endurance.

Given the outstanding physiological functions of astaxanthin, it has been used in the food, pharmaceutical and feed industries abroad, and has a broad market prospect. However, for the time being, astaxanthin is mainly used in the aquaculture industry as a new and highly effective feed additive. In view of the above, this paper mainly introduces in more detail the application and research progress of natural astaxanthin in aquaculture.

1 Application of astaxanthin in aquaculture

A large number of studies have proved that astaxanthin has a positive effect on increasing the coloration of cultured objects, improving survival rates, and promoting growth, reproduction, and development [12–32]. At present, astaxanthin has been widely used in various cultured objects such as salmon, trout, and shrimp, and has been determined by food monitoring agencies in the United States, the European Union, Canada, Japan, and other countries to be a safe and efficient animal feed [5, 33]. Its role in aquaculture can be summarized as follows.

1.1 Excellent coloring effect

Astaxanthin is the main carotenoid pigment in marine crustaceans and fish [34]. The rosy flesh color of seafood such as salmon and lobster is due to the high accumulation of astaxanthin in their bodies. However, cultured animals cannot synthesize astaxanthin themselves, and there is a lack of natural sources. Therefore, it must be added to their feed to supplement the pigment [1, 13–15]. One of the main uses of astaxanthin today is as a source of pigment in aquaculture. It was first used in salmon and trout, and is now widely used in various cultured objects [1].

1.1.1 Promoting the coloring of cultured shrimp. If the feed of cultured shrimp lacks astaxanthin, it will cause the shrimp to appear unhealthy. Studies have shown that if shrimp lacking astaxanthin are fed a diet containing 50 × 10-6 (m/m) astaxanthin for 4 weeks, their body color will return to a normal dark blue-green, while the control group will still have a sickly color; moreover, the former will appear bright red after cooking, while the latter will appear pale yellow, which is not conducive to marketing [35].

Yamada (1990) compared the coloring effects of three carotenoids, β-carotene, canthaxanthin and astaxanthin, on prawns. The results showed that when prawns were fed the same amount of feed at a concentration of 100 × 10-6 (m/m), astaxanthin accumulated the most in their tissues [16. 5 × 10 — 6 (m/ m)] and is 23% and 43% higher than that of canthaxanthin and β-carotene, respectively. If the amount of astaxanthin used is increased to 200 × 10 — 6 (m/ m), the highest content in the tissue can reach 29 . 1 × 10 — 6 (m/ m), which proves that astaxanthin is the carotenoid with the best coloring effect [12].

1.1.2 Promoting the coloring of cultured fish

Early studies found that adding astaxanthin to the feed can also make the skin and muscles of farmed fish such as salmon and sturgeon appear bright red [1, 13]. The reddish-pink skin coloration of wild pomfrets (Brama brama) is mainly due to the presence of astaxanthin, while the astaxanthin content in farmed pomfrets that have not been fed astaxanthin is only 5% of the wild level. Adding other carotenoids (such as β-carotene, lutein, canthaxanthin and zeaxanthin) to the feed does not cause the bream to become coloured, nor does it convert to astaxanthin. The carotenoids will continue to be lost from the skin and flesh of the bream. Therefore, astaxanthin must be fed to obtain the reddish colouration of farmed bream [1].

In the coloring of ornamental fish farming, there is currently no product that is as effective and long-lasting as natural astaxanthin provided by Haematococcus pluvialis [1]. Ornamental fish can obtain bright colors by eating carotenoids. Ako and Tamaru (1999) found that after one week of feeding an ornamental fish with a diet containing 100 × 10-6 (m/m) astaxanthin, the yellow, maroon and black colors on the fish's body surface were significantly enhanced [14].

In addition, Choubert and Storebakken (1996) showed that the absorption and utilization of astaxanthin by cultured organisms is better than that of other pigments. For example, the digestion and absorption of astaxanthin by rainbow trout is significantly better than that of canthaxanthin, and its maximum apparent absorption coefficient is more than twice that of canthaxanthin. When rainbow trout (Oncorhynchus mykiss) is fed with astaxanthin and canthaxanthin respectively, to achieve the same coloring effect, it is necessary to feed 72 × 10 — 6 (m/ m) of canthaxanthin, while only 60 × 10 — 6 (m/ m) of astaxanthin is needed, indicating that astaxanthin is more efficient than canthaxanthin in coloring [15].

1.2 Improve the survival rate of cultured objects

Adding astaxanthin as a feed additive can improve the survival rate of cultured animals through various channels, such as enhancing immunity, improving tolerance to harsh conditions, and adaptability to changes in environmental conditions. Merchie et al. (1998) studied the demand for carotenoids in feed and found that adding astaxanthin to the feed can greatly improve the immunity of cultured animals, enhance disease resistance, and improve survival rate. It can also enhance the resistance of postlarvae to salinity fluctuations and reduce the harm of ultraviolet radiation to aquatic animals [16].

In addition, chien (1996), when studying the biological effects of astaxanthin on shrimp, pointed out that astaxanthin accumulates in tissues as a pigment, which can store oxygen between cells and enhance the tolerance of fish and shrimp to high nitrogen and low oxygen environments. It has also been reported that the biological function of astaxanthin is stronger than that of β-carotene. When 100 × 10-6 (m/m) of β-carotene is added to shrimp feed, the survival rate is only 40%, while adding the same amount of astaxanthin can increase the survival rate to 77% [1]. Yamada's (1990) research also showed that if 100 × 10-6 (m/m) astaxanthin is added to the daily feed, the survival rate of prawns can reach 91%, while the control group is only 57% [12]. Jin Zhengyu et al. (1999) pointed out in a feeding experiment with natural astaxanthin that the survival rate of Litopenaeus vannamei could be increased by about 21.66% by using astaxanthin as a feed additive [17].

Christiansen et al. (1995) studied the effect of feed on the survival rate of Atlantic salmon (Salmo salar). They found that when the astaxanthin content in the feed was less than 1 × 10-6 (m/m), there was a large number of fish deaths and the survival rate was less than 50%. However, in the control group fed an adequate amount of astaxanthin, the survival rate of the fish fry was over 90% [18].

Pan et al. (2001) studied the effects of astaxanthin feeding and aquaculture water conditions on the coloring, growth and survival rate of Penaeus monodon. They pointed out that in addition to increasing the coloring of prawns, astaxanthin feeding can also promote growth and increase survival rate. Experimental results show that in order to maintain high survival rates during the later stages of growth in Penaeus monodon and when the content of astaxanthin in the body decreases, the shrimp should be fed a certain concentration of astaxanthin [19].

1.3 Promoting growth, reproduction and development of cultured subjects

Astaxanthin has a significant effect on the growth of cultured organisms. Jin Zhengyu et al. (1999) reported that feeding astaxanthin can significantly increase the weight gain rate of Litopenaeus vannamei. Experiments showed that the weight gain rate reached about 14.48% after 5 weeks of feeding [17].

Christiansen et al. (1995) conducted a study on the effects of different feeds on the growth and survival rate of Atlantic salmon. The results showed that when the astaxanthin content in the daily feed of Atlantic salmon fry was higher than 5.3 × 10-6 (m/m), they maintained normal growth, while below this value, the fry grew slowly [18].

In addition, if the astaxanthin content in the feed for cultured shrimp is insufficient, the shrimp will become sick, hindering their normal growth and development. Feeding this sick shrimp with 50 × 10-6 (m/m) astaxanthin for 4 weeks will restore normal growth, and the amount of astaxanthin in its tissues will increase by more than 300%. 26. 3 × 10 — 6 (m/ m) of carotenoids could be isolated from the shells; the increase in the control group was only 14%, and the carotenoid content in the shells was (4 ~ 7) × 10 — 6 (m/ m) [35].

Petit et al. (1997) studied the effect of feeding astaxanthin on the late growth of prawn larvae and their molting cycle, and found that feeding astaxanthin can shorten the molting cycle of late prawn larvae [20]. Astaxanthin can also be used as a fertilization hormone to improve egg quality. Adding astaxanthin to the feed can improve the survival rate of young shrimp and the buoyancy and survival rate of fish eggs. It can also increase fertilization rates, egg survival rates and growth rates during the salmon fry rearing period, and protect eggs from the effects of harsh conditions during their growth and development [21–24]. Vassallo et al. (2001) studied the effect of astaxanthin on the spawning of cultured subjects, and the results showed that adding 10 × 10-6 (m/m) astaxanthin to the feed can increase the spawning rate [25].

1.4 Improving the physiological functions of cultured subjects

Adding astaxanthin to the feed can improve the health of cultured rainbow trout, giving them better liver function and strengthening the structure of red tilapia liver cells and glycogen storage [21, 26]. Rehulka (2000) studied the effects of astaxanthin on the growth rate, various blood indicators and some physiological functions of rainbow trout, and found that feeding astaxanthin can improve the hematopoietic function and lipid and calcium metabolism of rainbow trout [27]. Amar et al. (2001) added various carotenoids such as astaxanthin to the diet of rainbow trout to study the effect of these additives on the immunity of the fish. The experiment showed that among various carotenoids, carotenoids, astaxanthin and β-carotene can improve both humoral indicators such as serum defensins and lysozyme activity, and cellular indicators such as bacteriophage phagocytosis and non-specific cytotoxicity [28].

1.5 Improve the nutritional value of cultured objects

The nutritional value of fish and shrimp is also increased by the addition of astaxanthin. Christiansen et al. (1995) studied the effect of astaxanthin added to feed on the physiological status of Atlantic salmon, such as immunity. It was found that after Atlantic salmon fed astaxanthin-containing feed, the content of vitamins A, C, and E in some tissues increased significantly. Moreover, when the astaxanthin content added to the feed was higher than 5. 3 × 10- 6 (m/ m), the lipid content also increased significantly; when 13 . 7 × 10- 6 (m/ m) astaxanthin was added, the lipid content of Atlantic salmon flesh could be increased by 20% [29]. In the European and American markets, aquatic products with astaxanthin as a feed additive are very popular, and their prices are much higher than those of ordinary fish and shrimp [13].

1.6 Facilitates the transportation and preservation of aquatic products

During the refrigeration of aquatic products, lipid oxidation is the main cause of meat spoilage [30]. Therefore, the strong antioxidant properties of astaxanthin also play a positive role in the transportation and preservation of aquatic products. Jensen et al. (1998) studied the antioxidant function of carotenoids such as astaxanthin in the refrigeration and preservation of aquatic products. The results showed that there were significant differences in lipid oxidation during the refrigeration process in rainbow trout fed different concentrations of astaxanthin. Low-dose rainbow trout had severe lipid oxidation, while high-dose astaxanthin fed rainbow trout could significantly extend the shelf life of raw meat during refrigeration [31].

In addition, during the storage of salmon and trout after capture, salmon are prone to rancidity because they contain little astaxanthin in their flesh [4.9 × 10-6 (m/m)], while trout have a higher astaxanthin content in their flesh [9. 1 × 10- 6 (m/ m)] and the storage effect under the same conditions is better than that of salmon [32]. It can be inferred that adding astaxanthin to the feed and increasing its content in the bodies of aquaculture subjects can, to a certain extent, reduce the use of chemical preservatives. It can also be used as a special and highly effective “biological preservative” to make aquatic products last longer and is absolutely safe for the human body.

2   The advantages of astaxanthin derived from Haematococcus pluvialis

 2 . 1   Differences between natural and synthetic astaxanthin

At present, astaxanthin is produced synthetically or biologically. Synthetic astaxanthin is not only expensive, but also differs significantly from natural astaxanthin in terms of structure, function, application and safety.

In terms of structure, astaxanthin has three conformations: 3S-3,S; 3R-3,S; and 3R-3,R. Synthetic astaxanthin is a mixture of these three structures, mixture, with the cis structure—3R-3,S type being the main type, which is very different from the astaxanthin in cultured organisms such as salmon (mainly the trans structure—3S-3,S type) [36]. In terms of physiological function, the stability and oxidation activity of synthetic astaxanthin is also lower than that of natural astaxanthin [37]. In terms of application results, the bioavailability of synthetic astaxanthin is also lower than that of natural astaxanthin. When the feeding concentration is low, the concentration of synthetic astaxanthin in the blood of rainbow trout is significantly lower than that of natural astaxanthin [38], and it cannot be converted to the natural conformation in vivo [5]. In terms of biosafety, the synthesis of astaxanthin using chemical methods will inevitably introduce impurity compounds, such as unnatural by-products produced during the synthesis process, which will reduce its safety for biological use [3].

With the rise of natural astaxanthin, countries around the world are becoming stricter in their management of chemically synthesized astaxanthin. For example, the US Food and Drug Administration (FDA) has banned chemically synthesized astaxanthin from entering the health food market [5]. At present, the production of astaxanthin generally tends to develop natural astaxanthin from biological sources and conduct large-scale production accordingly.

2.2 Biological sources of natural astaxanthin

Currently, there are generally three biological sources of natural astaxanthin: waste from the aquatic product processing industry, Phaffia rhodozyma, and microalgae (Haematococcus pluvialis). Among them, the astaxanthin content in waste is low, and the extraction cost is high, making it unsuitable for large-scale production. The average astaxanthin content in natural Phaffia rhodozyma is only 0.40%.

In contrast, the astaxanthin content of Haematococcus pluvialis is 1.5% to 3.0%, and it is considered a “concentrated product” of natural astaxanthin. A large number of studies have shown that the rate of astaxanthin accumulation and the total production of Haematococcus pluvialis are higher than those of other green algae, and the ratio of astaxanthin and its esters (about 70% monoesters, 25% diester and 5% monomer) is very similar to the ratio in aquaculture animals themselves, which is an advantage over astaxanthin extracted by chemical synthesis and using Rhodopseudomonas. In addition, the structure of astaxanthin in Haematococcus pluvialis is mainly 3S-3'S, which is basically the same as that in salmon and other aquatic organisms; while the astaxanthin structure in Rhodopseudomonas palustris is 3R-3'R [33].

Currently, Haematococcus pluvialis is recognized as the best organism in nature for producing natural astaxanthin. Therefore, the use of this microalgae to extract astaxanthin undoubtedly has broad development prospects and has become a research hotspot in recent years for the production of natural astaxanthin internationally.

3 Problems and development directions in research on the application of natural astaxanthin in feed

A comprehensive review of domestic and international research shows that there is still some debate about the effectiveness of various carotenoids in aquaculture [42–46]. Yanar and Tekelioglu (1999) showed that the coloring effect of carotenoids such as canthaxanthin on goldfish is superior to that of astaxanthin [42]. Buttle et al. (2001) studied the differences in the effects of different pigments on the coloring of farmed Atlantic salmon and the accumulation of pigments in their bodies. The results showed that the utilization rate of astaxanthin by rainbow trout was much higher than that of canthaxanthin, but this was not the case for Atlantic salmon [43].

Baker et al. (2002) studied the absorption of astaxanthin and other pigments by Atlantic salmon and the differences in coloring effects. They concluded that the coloring effect of canthaxanthin is basically the same as that of astaxanthin, and pointed out that there is a certain linear relationship between the absorption of pigments and the amount of pigments fed. Some other reports also suggest that for Atlantic salmon and rainbow trout, the coloring effect of astaxanthin is superior to that of canthaxanthin [45, 46]. It can be seen that the application effect of astaxanthin on different aquaculture objects is still controversial and needs to be further studied to determine the application cost-effectiveness of astaxanthin on different aquaculture objects.

Gomes et al. (2002) compared the coloring effects of astaxanthin from different sources (synthetic and different natural biological sources) on the sparidae (sparus aurata). The experiment showed that there was no significant difference in the coloring effect of various sources and types of carotenoids on this fish, and it was pointed out that it is still difficult to determine the effect of a certain pigment on the skin coloring of sparidae by feeding alone [39]. However, many other studies have shown that for farmed organisms (rainbow trout, etc.), natural astaxanthin is superior to chemically synthesized astaxanthin in terms of absorption, coloring power and biological efficacy [1, 37]. Therefore, the biological application value of astaxanthin from different sources (synthetic and different natural biological sources) still needs to be further studied, and the mechanism of absorption and utilization of astaxanthin from various sources by cultured organisms needs to be determined.

In astaxanthin feeding studies, different scholars have used different feeding concentrations. The optimal feeding amount and feeding method of astaxanthin should be different for different cultured organisms. In order to carry out more extensive research on the efficacy of astaxanthin applications, it is necessary to continue to carry out research on the optimal use and feeding methods of astaxanthin in aquaculture.

At present, there are almost no reports on the application of astaxanthin in aquaculture in China. Only Jin Zhengyu et al. (1999) added astaxanthin-containing Rhodopseudomonas to the feed of Macrobrachium rosenbergii to study the effect of astaxanthin on the body color and growth of Macrobrachium rosenbergii [17]. However, judging from the development trend of astaxanthin production, Haematococcus pluvialis will undoubtedly become the main natural biological source of astaxanthin. Therefore, there is an urgent need to carry out research on the application of natural astaxanthin in aquaculture in China, especially the research on the application effect of astaxanthin derived from Haematococcus pluvialis in aquaculture.

4 Conclusion

Astaxanthin has strong antioxidant capacity and powerful physiological functions, and has been widely used in aquaculture abroad. This article focuses on the functions of astaxanthin in aquaculture, such as increasing the coloration of cultured objects, improving survival rate, and promoting growth, reproduction and development. In addition, the advantages of using astaxanthin produced by Haematococcus pluvialis are discussed, and the problems in current research are analyzed, and the future research direction is proposed.

The safety of natural astaxanthin has been widely recognized, and it will help the further development of the aquaculture industry in the face of high “green barriers”. Therefore, astaxanthin as a feed additive in aquaculture will inevitably receive more and more attention, be adopted by more aquaculture operators, and have broad application prospects.

At present, the annual demand for astaxanthin products in developed countries is at least tens of tons, and the market demand is far from being met. The global aquatic product market is growing at a rate of 24% per year, and the annual market capacity for astaxanthin in salmon feed alone is more than 185 million US dollars, with an annual growth rate of 8%, showing great market potential [47]. However, due to some bottlenecks in the production of natural astaxanthin, only a few large foreign companies have achieved large-scale production of astaxanthin, resulting in technological monopoly, which has led to the current international price of astaxanthin being as high as more than 2,500 US dollars per kilogram [1]. Therefore, China should accelerate research on the application and production of astaxanthin in aquaculture to meet market requirements.

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