What Are the Benefits of Astaxanthin for Fish Feeding?

Astaxanthin is a keto-type carotenoid that is widely found in algae, shrimp, crabs and other organisms. It has powerful antioxidant properties and various biological activities, including anticancer, anti-inflammatory, anti-hypertensive and anti-obesity properties, and is already widely used [1-3]. Astaxanthin also plays a key role in the survival, growth, reproduction and development of fish [4-6].

As a powerful antioxidant, astaxanthin can protect the cell membranes of fish from the attack of reactive oxygen species and reactive nitrogen species, reduce oxidative stress, improve growth performance and strengthen immunity [7]. At the same time, astaxanthin has a coloring effect, which can improve the muscle and skin color of farmed fish, enhance meat quality and commercial value, and is widely used in ornamental fish feed to meet the demand of ornamental fish for bright body color [8]. Fish cannot synthesize astaxanthin on their own. In the natural environment, fish obtain astaxanthin by eating algae rich in astaxanthin. Under artificial breeding conditions, astaxanthin can only be obtained by adding it to the feed [9]. Therefore, adding astaxanthin during the artificial breeding of fish can effectively promote the healthy growth of fish and improve the breeding efficiency.

1 Physical and chemical properties of astaxanthin

Astaxanthin is an oxygen-containing derivative of carotenoids, containing a long unsaturated conjugated system and existing in multiple optical isomers. Its molecular formula is C40H52O4 and its chemical name is 3,3'-dihydroxy-4,4'-dione-beta-carotene [10, 11]. Astaxanthin is relatively stable, with a melting point of around 215°C. It is fat-soluble and not easily soluble in water [12]. In addition, astaxanthin is more soluble in acidic eutectic solvents than in ethanol or the ionic liquid tributyl(octyl)phosphonium chloride [13]. The molecular structure of astaxanthin has a long chain of conjugated double bonds, and the end has both an unsaturated ketone group and a hydroxyl group (Figure 1). These special structures can attract free radicals or supply electrons to free radicals, thereby removing free radicals and thus exhibiting excellent antioxidant properties [14].

2 Sources of astaxanthin

Currently, astaxanthin is mainly prepared by two methods: biosynthesis and chemical synthesis. Biosynthesis refers to the isolation and obtainment from crustacean shells, algae, yeast or prokaryotes. This source of natural astaxanthin has a clear structure, few by-products and is environmentally friendly. Products made using this method can be used as food additives [15]. Chemical synthesis methods can be divided into chemical semi-synthesis and chemical total synthesis. This method uses carotenoids such as canthaxanthin, lutein, zeaxanthin, or synthetic chemical materials to produce astaxanthin. Although astaxanthin prepared by semi-synthesis has high activity, the yield is relatively low. In contrast, the chemical total synthesis method is commonly used to produce industrial raw materials or feed because the materials are readily available and the overall yield is high [16].

2.1 Microalgae synthesis of astaxanthin

With the development of biotechnology, the application of microalgae has transitioned from the production of simple biomass to the production of valuable products. The synthesis of astaxanthin is one of the important fields [17]. The mature Haematococcus pluvialis (Figure 2) has a high level of astaxanthin in its cells, and the astaxanthin content of wild strains can also reach 4% of the dry weight of the cells.

It has been identified as one of the promising sources of natural astaxanthin for commercial production [18]. Both it and Dunaliella salina can synthesize astaxanthin and the astaxanthin precursor β-carotene [19]. Haematococcus pluvialis can effectively reduce the isomerization of trans-astaxanthin through alkali treatment and saponification, thereby producing high-purity astaxanthin crystals [20]. In addition, Synechocystis can also be transformed into a factory for producing high-value astaxanthin. This microalgae also has the characteristics of being easy to cultivate, easy to genetically manipulate, and has a clear genetic background [21].

2.2 Yeast-based astaxanthin synthesis

At present, the main yeast source for synthesizing natural astaxanthin is Phaffia rhodozyma (Figure 3), and its astaxanthin products have been widely used in fish feed processing [15]. Studies have found that the astaxanthin concentration synthesized by wild red phaffia yeast ranges from 200 to 400 μg/g [22]. Mutagenesis and screening can be used to obtain red phaffia yeast strains with higher yields [15]. Red phaffia yeast cells are rich in lipids, which helps to evenly distribute astaxanthin in the cells and facilitate storage [23]. In addition, this yeast is characterized by its light-independent growth, fast growth rate and high-density cultivation. After the extraction of astaxanthin, its nutrient-rich by-products can also be used as nutritional feed additives [22].

2.3 Chemically synthesized astaxanthin

Chemical methods for the preparation of astaxanthin include total chemical synthesis and semi-synthesis. Total chemical synthesis is a method that starts from raw materials and synthesizes the target compound through a series of chemical reactions. Using α-ionone as the starting material, astaxanthin can be synthesized using the 2C15+C10 →C40 route [24].

In addition, there are routes for synthesizing astaxanthin through steps such as the hydrolysis and isomerization of lutein esters, the bromination and oxidation of zeaxanthin [25], and the oxidation of zeaxanthin by an oxidant to synthesize astaxanthin [26]. Chemical semi-synthesis generally refers to a method that starts with a natural product or its derivatives and partially converts it into the target compound with the help of a series of chemical reactions. This technique involves multiple steps such as the hydrolysis of lutein esters, the isomerization of lutein, and the bromination and oxidation of zeaxanthin [25]. Regardless of the method used, the chemical synthesis of astaxanthin requires precise control of reaction conditions and steps to ensure efficient and high-yield synthesis results.

3 Effect of astaxanthin added to feed on fish

Astaxanthin has many biological functions, and in fish farming, its main application values are its growth-promoting, coloring, antioxidant and immune-enhancing effects.

3.1 Improves growth performance

Adding the right amount of astaxanthin can effectively enhance the growth performance of fish. Different species of fish have different requirements for astaxanthin. Wang Junhui et al. [27] found that with the increase of the amount of astaxanthin in the feed, the final weight, weight gain rate and specific growth rate of the koi showed an upward trend and then a downward trend. When the amount added was 400 mg/kg, the above parameters reached their highest values. Yao Jinming et al. [28] found that the optimal dosage for big-scaled mud carp was 100 mg/kg.

Li Meixin et al. [29] showed that adding 100–200 mg/kg astaxanthin to the feed can significantly increase the growth rate and feed utilization rate of snakeheads. Zatˇkov et al. [30] reported that the weight gain rate, specific growth rate and feed utilization rate of channel catfish were significantly improved after they were fed a diet rich in astaxanthin. The mechanism of astaxanthin's growth-promoting effect can be attributed to its excellent antioxidant properties.

Studies have shown that a specific dose of astaxanthin can maintain the body's normal oxidative-antioxidant balance by scavenging excess oxygen radicals (ROS) in cells, thereby enhancing vitality and reducing stress responses, and ultimately promoting growth [31-33].

Some studies have also shown that excessive astaxanthin supplementation may inhibit the growth performance of fish. Zhao Fuyang et al. [34] showed in a study on zebrafish that if the astaxanthin supplementation exceeds 0.6%, the weight gain rate and specific growth rate will be significantly lower than the control group, and the body length growth value is also significantly lower than the control group. The reason for the growth inhibition caused by high doses of astaxanthin may be that excessive astaxanthin accelerates the fish's metabolism, thereby expelling excess nutrients from the body and consuming the fish's energy at the same time [27]. In summary, there is a certain dose-dependent relationship between the amount of astaxanthin added and fish growth. Adding the right amount of astaxanthin can have a positive effect on fish growth. Lower doses have no significant effect, while higher doses have no significant effect on growth and may even have a suppressive effect.

3.2 Improves body color

At present, in the environment of intensive artificial breeding, because aquatic animals have difficulty obtaining sufficient astaxanthin from the outside, their body color is generally light [35]. For ornamental fish, body color is formed by the accumulation of phytoene and phytofluene in the body. These two pigments cannot be synthesized by themselves and must be ingested from food [4]. Therefore, the feed required for breeding ornamental fish should take into account the needs of their growth and development and maintaining a bright body color [36].

Wang Junhui et al. [27] found that after koi were fed with a diet containing different levels of astaxanthin for a period of time, the redness value (a* value), yellowness value (b* value) and carotenoid content of the skin of the group fed 400 mg/kg of astaxanthin were significantly higher than those of the control group, and the ornamental value was improved. For fish such as salmon and tilapia that are intended for human consumption, the redness value of the muscle is an important indicator of the quality of the fish [37]. Feeding food containing astaxanthin can significantly improve the color of the skin and muscles of food fish, giving them a more reddish hue. It can also increase the umami flavor of the meat to meet consumer demand [38].

Zhang Chunyan et al. [39] studied the effect of adding different doses of astaxanthin to the feed on the body color and meat quality of rainbow trout. The synthetic astaxanthin added to the feed of the Ast group was 1.0 g/kg, while the Haematococcus pluvialis extract added to the feed of the HE group contained 100 mg/kg astaxanthin, which was 4.4 g/kg. The control group feed was the basic daily ration. After 6 weeks of feeding, the muscle a* and b* values, tissue astaxanthin content, and serum carotenoid content of the Ast and HE groups were significantly higher than those of the control group.

The results of Gong Cuiping et al. [40] showed that after being fed with different amounts of astaxanthin, the carotenoid content in various tissues and organs of red tilapia increased, and the body color of the fish also became more vibrant. Adding 400 mg/kg astaxanthin to the feed significantly increased the deposition of carotenoids in the tissues of the fish. In a study by Yi et al. [41] on yellow croaker, the skin carotenoid content was significantly increased by feeding a diet containing 90 mg/kg astaxanthin.

The above studies show that astaxanthin has a significant effect on the growth and body color of fish. Adding an appropriate amount of astaxanthin to the feed can significantly increase the carotenoid content in fish, making their body color more vibrant. It also improves the quality of the fish, increases the taste and improves the economic benefits. The different colors of fish skin are essentially caused by the movement of different pigment cell particles in the body. Among these, melanocytes are the key cell type that controls the color of skin, eyes, fins and other parts. Frank et al. [42] showed that astaxanthin can affect the body color of fish by altering the signaling pathways in melanocytes, for example by affecting cyclic AMP levels, and promoting or inhibiting the aggregation or dispersion of pigments.

3.3 Enhancing antioxidant capacity

Reactive oxygen species (ROS) are products of aerobic metabolism in living organisms. While moderate amounts are beneficial, excessive amounts are harmful [43]. To reduce the harm, organisms have developed a sophisticated antioxidant defense system, of which non-enzymatic carotenoids are a part. Since fish are rich in n-3 polyunsaturated fatty acids and are therefore highly susceptible to attacks by reactive oxygen species [44], adding astaxanthin to the feed is crucial to maintaining the balance of the body's antioxidant defense system. Superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) are involved in the antioxidant process in fish, and their antioxidant effects are achieved by inhibiting and eliminating free radicals. In fish, free radicals react with lipids to cause a peroxidation reaction, producing malondialdehyde (MDA). MDA can be used as a common indicator of oxidative stress and reflects the degree of oxidation in biological tissues. Wang Junhui et al. [27] found that supplementing the feed of koi carp with astaxanthin had a significant effect on the antioxidant capacity of the fish's liver.

The study showed that when comparing the test group with gradually increasing astaxanthin addition with the control group, the activities of SOD, CAT and GSH-Px in the liver of the koi fish gradually increased and then decreased. When the addition amount reached 400 mg/kg, it was significantly higher than that of the other test groups. The MDA content in the liver of koi carp showed a trend of first decreasing and then increasing with the increase in astaxanthin content.

When the addition amount was 400 mg/kg, the content was the lowest, indicating that adding 400 mg/kg astaxanthin can give koi carp the best antioxidant capacity. Yao Jinming et al. [28] showed that the antioxidant capacity of the liver and pancreas of the largescaled armored minnow can be significantly improved by adding the right amount of astaxanthin, as evidenced by the increased activities of SOD, CAT and GSH-Px, increased GSH content and reduced MDA levels. Li Meixin et al. [29] reached the same conclusion in their study of the antioxidant indicators of snakehead serum and liver. The above research results show that a moderate amount of astaxanthin can help enhance the antioxidant capacity of fish, remove oxygen free radicals, reduce oxidative stress, and prevent damage to the body.

In-depth analysis shows that the reason astaxanthin can exert its antioxidant properties may be that its chemical structure allows it to bind tightly to cell membranes, maintain membrane structure and fluidity, and act as an electron “lightning rod” to help electron transport and neutralization, thereby protecting cell membranes from attacks by reactive oxygen species and reactive nitrogen species [45], and it can also act synergistically with other antioxidants [46] to enhance the antioxidant effect.

It should be noted that although astaxanthin can significantly improve the total antioxidant capacity of fish, astaxanthin itself is a strong antioxidant that can strongly scavenge free radicals in the body. Under certain conditions, it can lead to a decrease in the substrate of SOD, GSH-Px, etc. in the body, thereby significantly reducing the activity of antioxidant enzymes [47]. Sun Liu-juan et al. [48] found that after feeding on feed supplemented with astaxanthin, the total antioxidant capacity of blood parrot fish was enhanced, but the activity of SOD was reduced. According to Wang et al. [49], increasing the content of astaxanthin in the feed can lead to a varying degree of reduction in the activity of the antioxidant enzymes SOD and GSH-Px in the serum of fat carp. The reason for this phenomenon may be related to the antioxidant status in the fish, but the exact reason still needs to be further explored.

3.4 Immune system enhancement

Diseases in fish farming are characterised by their rapid spread and difficulty in treatment, and have become an important factor limiting the development of the aquaculture industry. Therefore, improving the immune system of fish and reducing the harm caused by pathogens to the body is particularly important for the healthy and sustainable development of the fish farming industry. Numerous studies have shown that astaxanthin can enhance the immune system of organisms. Wang Junhui et al. [27] found that with the increase of astaxanthin added, the activity of LZM, ACP and AKP in the serum of koi carp and the content of C3 and C4 showed a trend of first increasing and then decreasing. When 400 mg/kg of astaxanthin was added, the above indicators reached a maximum value and were significantly higher than those of the control group without added astaxanthin.

The results of Shubin et al. [50] showed that adding 100 mg/kg of astaxanthin to the feed significantly increased the serum IgG and IgM levels of largemouth black bass. Lim et al. [51] found that after the blood of the largemouth bass was infected with Vibrio, feeding it with astaxanthin-enriched feed significantly increased the levels of C3 and C4 complement in the blood and significantly increased LZM activity. The above studies have shown that astaxanthin can enhance the immune function of fish and improve their resistance to disease. In studies on rats, researchers found that the mechanism by which astaxanthin enhances immunity is to inhibit mitochondrial dysfunction, thereby reducing oxidative damage, and to inhibit oxidative stress by blocking signal transduction and the activity of the transcription activator 3 (STAT3), thereby reducing inflammation and enhancing immunity [52, 53].

4 Summary and outlook

Adding astaxanthin to fish feed not only improves the body color of fish, but also its powerful antioxidant capacity can protect fish cell membranes from active substances, thereby indirectly improving growth performance and immune capacity. Therefore, astaxanthin has broad application prospects and significant economic value in the field of fish farming. However, despite the huge development potential of astaxanthin, there are still some gaps and challenges in practical applications.

First, astaxanthin is unstable and isomerizes when exposed to light, heat, and oxygen. How to maintain its effectiveness and stability in production is an important research topic [54]. Second, there is no fixed rule for the optimal amount of astaxanthin to be added to the feed of various fish species. The concentration of astaxanthin required to achieve the best results varies for different fish species, and this requires a large amount of experimental data for research and determination. However, with the deepening of research on astaxanthin and the continuous improvement of aquaculture technology, its application in the fish farming industry will become more extensive and mature, and it can provide strong support for the sustainable development of the fish farming industry.

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