Recent progress in practical applications of a potential carotenoid astaxanthin in aquaculture industry: a review

Astaxanthin is the main natural C40 carotenoid used worldwide in the aquaculture industry. It normally occurs in red yeast Phaffia rhodozyma and green alga Haematococcus pluvialis and a variety of aquatic sea creatures, such as trout, salmon, and shrimp. Numerous biological functions reported its antioxidant and anti-inflammatory activities since astaxanthin possesses the highest oxygen radical absorbance capacity (ORAC) and is considered to be over 500 more times effective than vitamin E and other carotenoids such as lutein and lycopene. Thus, synthetic and natural sources of astaxanthin have a commanding influence on industry trends, causing a wave in the world nutraceutical market of the encapsulated product. In vitro and in vivo studies have associated astaxanthin’s unique molecular features with various health benefits, including immunomodulatory, photoprotective, and antioxidant properties, providing its chemotherapeutic potential for improving stress tolerance, disease resistance, growth performance, survival, and improved egg quality in farmed fish and crustaceans without exhibiting any cytotoxic effects. Moreover, the most evident effect is the pigmentation merit, where astaxanthin is supplemented in formulated diets to ameliorate the variegation of aquatic species and eventually product quality. Hence, carotenoid astaxanthin could be used as a curative supplement for farmed fish, since it is regarded as an ecologically friendly functional feed additive in the aquaculture industry. In this review, the currently available scientific literature regarding the most significant benefits of astaxanthin is discussed, with a particular focus on potential mechanisms of action responsible for its biological activities.

astaxanthin is supplemented in formulated diets to ameliorate the variegation of aquatic species and eventually product quality.Hence, carotenoid astaxanthin could be used as a curative supplement for farmed fish, since it is regarded as an ecologically friendly functional feed additive in the aquaculture industry.In this review, the currently available scientific literature regarding the most significant benefits of astaxanthin is discussed, with a particular focus on potential mechanisms of action responsible for its biological activities.

Introduction
Carotenoids are the most common class of lipidsoluble pigments that feature a broad group of molecules that are naturally produced by plants and many photosynthetic organisms.These molecules are defined as natural antioxidants, protecting the cells from oxidative stress mediated by either light, free radical-mediated peroxidation, or singlet oxygen (Merhan 2017).Carotenoids have been widely applied in the pharmaceutical, cosmetic, and feed industries owing to their positive biological characteristics in humans and animals (Maoka 2011(Maoka , 2012(Maoka , 2015;;Milani et al. 2017;Rodriguez-Concepcion et al. 2018).Given these properties, several healthy effects have been promoted by carotenoids, such as those on the immune response, reproduction, lipid metabolisms, photoprotection in the skin, and chronic disease prevention such as diabetes, cardiovascular disease, hypertension, atherosclerosis, cancer, and inflammation (Maoka 2015;Komatsu et al. 2017;Rodriguez-Concepcion et al. 2018;Bae et al. 2020;Britton 2020;Donoso et al. 2021).Among them, astaxanthin (AX), an orange-reddish ketocarotenoid pigment with superior antioxidative activity, has been shown to have the highest oxygen radical absorbance capacity in comparison with other carotenoids (Nakagawa et al. 2011;Merhan 2017).Besides, functional supplements from natural sources could be considered safe agents for infectious diseases and environmental stressor prevention in farmed fish and crustaceans and subsequently humans (Fletcher 1997;Gatlin et al. 2007;Nakano et al. 2018).Thus, astaxanthin confers a significant impact on improvements in global fish farming owing to the accumulation of residual antibiotics in fish tissues and side effects on fish health (Elia et al. 2014;Nakano et al. 2018).Considerable work has strengthened the demand for the production and utilization of natural sources of astaxanthin as a pigment coloring agent which currently covers most of the world markets (Higuera-Ciapara et al. 2006;Seabra and Pedrosa 2010;Rahman et al. 2016).Also, the key role in the pigmentation of aquatic animals is played by astaxanthin (Rahman et al. 2016).Given its unique features, the use of astaxanthin has attracted considerable interest in the last years in aquatic animal rearing and is expected to be a feasible pathway to the sustainable development of aquaculture (Nakano and Wiegertjes 2020;Lu et al. 2021).
The AX can naturally synthesize from a wide variety of sources, such as bacteria, red yeast Phaffia rhodozyma, microalgae, Haematococcus pluvialis, Chlorella vulgaris, Chlorella zofingiensis, and Chlorococcum sp. which have biodegradable, no drug resistance, and an environment friendly in a variety of fish species.Also, AX can be obtained indirectly in our diet by consuming crustaceans (e.g., copepods, shrimp, and krill) and Salmonidae (e.g., salmon, rainbow trout) species, whose diets include natural sources of astaxanthin (Liu et al. 2016;Wang et al. 2018b;Lu et al. 2021).
Commercial synthesis of astaxanthin is currently the most cost-effective that dominated by synthetically derived astaxanthin in over 95% of the feed market (> 95%) (Lim et al. 2018;Stachowiak and Szulc 2021).According to Grand View Research, the global astaxanthin market value was estimated at USD 1.0 billion in 2019 and expecting to witness a compound annual growth rate of 16.2% from 2019 to 2027 to reach USD 3398.8 million by 2027, owing to only using natural astaxanthin in the pharmaceutical, cosmetic, and food industries and its multifunctional health benefits and safety (Silva et al. 2021;Stachowiak and Szulc 2021).
Astaxanthin takes part a crucial role in antioxidant, anti-inflammatory, immunity enhancement, and growth promotion (Jagruthi et al. 2014;Li et al. 2014;Lim et al. 2018).Previous studies have shown that dietary AX could reduce oxidative stress and also could improve the immune response, disease resistance, and growth performance of different fish and crustaceans (Jagruthi et al. 2014;Han et al. 2018;Lim et al. 2018;Li et al. 2020).Thus, the use of astaxanthin in aquaculture can boost the immunity of aquatic animals, decreasing mortality and preventing antibiotic abuse (Alishahi et al. 2015;Lu et al. 2021).
This article is to review the recent progress of available scientific works of literature regarding the most significant activities of astaxanthin as an economically valuable product in aquaculture, including its antioxidative, immunity response, and anti-inflammatory properties, its protective effects on reproduction, skin pigmentation, infectious diseases, and stress tolerance.

Chemical structure and biochemistry of astaxanthin
The major structure of carotenoids is composed of hydrocarbons of 40 carbon atoms, including two terminal ring systems linked by a chain of conjugated double bonds or polyene systems.In general, carotenoids can be singled out as two groups based on their structural chemical elements: carotenes, which are only composed of carbon and hydrogen, and xanthophylls, which contain oxygen derivatives.In the xanthophylls, oxygen is exhibited as hydroxyl (OH) groups, keto-moieties (C = O) groups, or as a combination of both, as displayed in astaxanthin (AX) (Fig. 1) (Britton 1995;Higuera-Ciapara et al. 2006;Seabra and Pedrosa 2010;Yuan et al. 2011;Lin et al. 2016).Structural features of (OH) and (C = O) groups in each ionone ring elucidate some of the astaxanthin properties, such as esterification ability, a more polar nature, and a major antioxidant capacity (Liu and Osawa 2007;Guerra et al. 2012).The presence of a polyene system provides the carotenoids with their distinctive molecular structure and chemical and light-absorbing characteristics.Every double polyene bond can be existed in two configurations, as cis or trans geometric isomers.Thermodynamically, transisomers are more stable than other cis-isomers.As known, most carotenoids, in specific astaxanthin, are found as trans isomers in nature; however, they could be isomerized to another form when exposed to light, heat, acid, or metal ions (Liu and Osawa 2007).Besides geometric isomers which contain two stereogenic carbon atoms at the C-3 and C-3′ in each molecule, astaxanthin exists in three stereoisomers: two enantiomers (3R, 3′R-and 3S, 3′S-astaxanthin) and an inactive meso form (3R, 3′S-astaxanthin) (Turujman et al. 1997;Ambati et al. 2014).Naturally, 3S, 3′S-astaxanthin is the most abundant stereoisomer as a variety of organisms produce astaxanthin in different stereoisomeric ratios (Grewe et al. 2007;Wang et al. 2008).In particular, crustaceans contain three types of optical isomers (Higuera-Ciapara et al. 2006).In the natural environment, astaxanthin is mainly esterified with fatty acids (monoesters and diesters) or conjugated with proteins, such as in crustacean exoskeleton and salmon muscle, giving molecular stability, whereas astaxanthin could also be found without esterification as a free-ester astaxanthin form (Turujman et al. 1997;Østerlie et al. 1999;Storebakken et al. 2004).On the other hand, synthetic astaxanthin includes a racemic mixture of stereoisomers and is not found as a free-ester astaxanthin form (Yuan et al. 1997;Higuera-Ciapara et al. 2006).Another substantial source of AX is the marine bacterium Agrobacterium aurantiacum (A.aurantiacum), which synthesized (3S,3′S)-astaxanthin and (3S,3′R)adonixanthin (4-ketozeaxanthin) from β-carotene through two hydroxylation steps at C-3 and 3′ and oxidation steps at C-4 and 4′ (Yokoyama et al. 1994(Yokoyama et al. , 1995;;Yokoyama and Miki 1995;Wang et al. 2017).
Astaxanthin has centrally located conjugated double bonds, hydroxyl, and keto-groups (Higuera-Ciapara et al. 2006;Ambati et al. 2014).These double bonds give its red coloration and robust antioxidant activity through donating the electrons and reacting with free radicals to convert them to more stabilized products in a wide variety of living organisms (Guerin et al. 2003;Seabra and Pedrosa 2010;Donoso et al. 2021).Indeed, AX has both lipophilic and hydrophilic properties, as it can usually link with the integral part (inside and outside) of the complex membrane structure (Fig. 2) and can be incorporated into liposome phospholipid bilayers; thus, AX exhibited the highest antioxidant efficiency when compared to other carotenoids, such as β-carotene, lycopene, and vitamin C (Miki 1991;Britton 1995Britton , 2008;;McNulty et al. 2008;Yamashita 2015).

Astaxanthin sources
Natural microorganisms, particularly algae, fungi, yeast, and bacteria, are constituted the primary natural sources of astaxanthin (Table 1).Besides, the astaxanthin-containing organisms that are consumed by aquatic animals for the acquisition of glamorous coloration pile astaxanthin in the tissues of aquatic organisms.For example, aquatic zooplanktons fed on marine algae (rich in beta-carotene, fucoxanthin, and diatoxanthin) are in turn converting β-carotene, which accumulated from marine algae to astaxanthin in their body; consequently, they are ingested by nautical fish (e.g., salmonids and trouts) and crustacean organisms (e.g., shrimp, crabs, crayfish, lobsters, and krill) at higher nutritive levels (Johnson and An 1991;Lim et al. 2018).Synthetic astaxanthin is commercially obtained by either chemical composition (Li et al. 2011;Cheng et al. 2016) or natural microbial resources, such as red yeast Xanthophyllomyces dendrorhous (formerly Phaffia rhodozyma) (Rodríguez-Sáiz et al. 2010;Hara et al. 2014;Dursun and Dalgıç, 2016) and green microalga Haematococcus pluvialis (H.pluvialis) (Cheng et al. 2016;Wang et al. 2016;Li et al. 2019).The leading role is undoubtedly achieved by H. pluvialis which is one of the most promising sources of natural astaxanthin, and so many studies have investigated the best conditions to synthesize and extract astaxanthin from H. pluvialis (Ambati et al. 2014;Shah et al. 2016;Zhao et al. 2019).Furthermore, other significant sources of astaxanthin include the flesh of wild and farmed marine fish, such as salmonids and trout.However, considerable variations were assayed in the muscle astaxanthin content among species.For instance, astaxanthin concentrations in wild Oncorhynchus species varied from 3 mg kg −1 flesh in chum salmon Oncorhynchus keta up to 38 mg kg −1 flesh in sockeye salmon Oncorhynchus nerka (EFSA 2005).As well, astaxanthin contents in wild and farmed Atlantic salmon Salmo salar were reported as 3-10 mg kg −1 flesh and 1-9 mg kg −1 flesh, respectively (EFSA 2005).Besides, astaxanthin concentrations were significantly higher in fresh salmon compared to pouch packaged and canned products (Sutliff et al. 2020).Therefore, salmonid fillets can be served as a good dietary source of natural astaxanthin.
Natural astaxanthin vs synthetic astaxanthin Many differences are documented between synthetic astaxanthin and natural astaxanthin.Firstly, synthetic astaxanthin is cheaper than natural microalgal astaxanthin since microalgal cultivation and harvesting are cost-consuming.Secondly, synthetic astaxanthin is mostly unesterified while microalgal astaxanthin is esterified (Ambati et al. 2014;Su et al. 2020).
Thirdly, synthetic astaxanthin and microalgal astaxanthin contain different geometrical and optical isomers (Su et al. 2020).Previous research assayed that microalgal astaxanthin could be better than synthetic astaxanthin in astaxanthin accumulation, safety, and potential nutritive quality of Chinese mitten crab (Eriocheir sinensis) (Yang et al. 2017;Su et al. 2020).
Besides, synthetic astaxanthin is markedly inferior to algal natural astaxanthin as an antioxidant (Capelli et al. 2013).Thus, natural astaxanthin from algae and aquatic animals has shown better benefits than synthetic astaxanthin.

Synthetic mechanisms of astaxanthin
Astaxanthin production has relied mostly on most microalgal strains, such as Chlorella zofingiensis, Haematococcus pluvialis, and Scenedesmus obliquus.
Generally, no obvious morphological changes are observed during the cultivation of green-colored microalgae (Ranjbar et al. 2008).Under adverse conditions, the microalgae cells are transformed into resting cysts, in which microalgae growth is prohibited but the survival efficiency of algal cells is intensified (Kobayashi 2003).In the resting stage, the bloodred color of microalgal cells and astaxanthin content originated in a harsh environment (Ranjbar et al. 2008).Therefore, astaxanthin synthesis in microalgal strains can be regarded as a self-protection mechanism, which enhances the survival of algal cells at the expense of microalgal biomass accumulation.
β-carotene forms a general precursor for astaxanthin from microalgal cells.The precursor is catalyzed by the enzymatic activity of β-carotene ketolase and hydroxylase, resulting in metabolic intermediates canthaxanthin and zeaxanthin, respectively (Rajesh et al. 2017).Therefore, the synthesis process of astaxanthin is performed through different pathways according to microalgal species and enzymatic activities of β-carotene (Fig. 4) (Li et al. 2008;Qin et al. 2008).
Besides, the natural contents of three geometric isomers (all-trans, 9-cis, and 13-cis-astaxanthin) of astaxanthin differ in microalgae.For instance, the content of all-trans astaxanthin is higher than that of cis astaxanthin in H. pluvialis and Chlorella zofingiensis; however, cis-astaxanthin has much higher antioxidative properties (Liu and Osawa 2007).In recent years, astaxanthin productivity can be improved by genetic modification technology (Lu et al. 2021).
Under the natural environment, the astaxanthin synthesis in microalgae is too low to encounter the market demand.Therefore, many studies reported production technologies to promote astaxanthin productivity and alleviate the conflict between produced microalgal biomass and astaxanthin synthesis.Many external conditions could be modified to impact the trophic stages of microalgae and so promote the formation of the resting cyst for improving astaxanthin cultivation.As previously reported, astaxanthin synthesis in algal cells is favored by the supply of glucose, salt, nitrogen deficiency, high-intensity illumination, and the addition of trace elements (Fábregas et al. 1998;Janchot et al. 2019;Han et al. 2020;Lu et al. 2021).Under industrial conditions, one-stage and two-stage processes are regarded in the production of astaxanthin.In the one-stage process, some essential nutrients, such as carbon, phosphorus, and nitrogen, are provided into culture media to promote microalgae growth.After that, a harsh environment, such as nutrient depletion and pH change, could dramatically modify the water environment for microalgae cells to transform into resting cysts and the astaxanthin synthesis pathway is activated.Moreover, some inducers (e.g., nitrogen and glucose) are implemented together with microalgae inoculation to enhance its growth.The lower production cost of astaxanthin is one of the disadvantages of the onestage process.Besides, some inducers would negatively impact algae growth, such as observed in a previous study that nitrogen would hinder the protein synthesis in algal cells and further decrease microalgae growth (Del Río et al. 2008;Mao et al. 2018;Han et al. 2020).As well, environmental changes may adversely impact glucose concentration, which is favorable to astaxanthin synthesis in algal cells (Li et al. 2008).
Two-stage cultivation is employed to overcome the disadvantages of the one-stage process as it mainly consists of a growth stage and an induction stage.The growth of microalgae cells is provided via sufficient nutrients, whereas the induction stage requires specific conditions to promote the transformation of algal cells to resting cysts (Fábregas et al. 2001;Zhu et al. 2021).Therefore, a two-stage process is a more effective strategy to improve astaxanthin production in microalgae (Affenzeller et al. 2009;Lu et al. 2021).As previously reported, the astaxanthin yield reached 4.0% of dry weight in the two-stage cultivation model, while the astaxanthin yield was only 0.8% of dry weight in the one-stage cultivation model (Aflalo et al. 2007).
Genetic modification (GM) has become one of the recent technologies for improving astaxanthin productivity as the genes regulating astaxanthin synthesis have been characterized (Huang et al. 2013;Lu et al. 2021).Previous reports have transferred the genes related to astaxanthin synthesis into tobacco and tomato by using GM technology, obtaining novel plants and fruits enriched with astaxanthin (Hasunuma et al. 2008;Huang et al. 2013).Genetic improvement of H. pluvialis strains was previously performed to produce astaxanthin using classical mutagenesis, resulting in production of various mutants of H. pluvialis that have higher astaxanthin accumulation capacity (Hu et al. 2008;Hong et al. 2012;Gómez et al. 2013).Besides, genetic engineering using transformations of H. pluvialis chloroplast and its nuclear genomes was recently achieved by vector transformation of transgenes into the nuclear genome (5′ or 3′ end) of the endogenous dominant selection marker, phytoene desaturase (pds) variant (Gutierrez et al. 2012;Sharon-Gojman et al. 2015;Shah et al. 2016).However, up to now, the application of these technologies in astaxanthin synthesis is mainly conducted in lab research.Therefore, the astaxanthin synthetized by genetic engineering technology has not been widely used in aquaculture.

Challenges and opportunities in astaxanthin scale-up
To our knowledge, the high market price of astaxanthin is about 2000 USD per kilogram, which is mainly attributed to the high production cost of microalgae biomass.Thus, its price still may not be affordable to many aquaculture factories, however microalgal astaxanthin is much cheaper than astaxanthin from ocean fisheries (Onorato and Rösch 2020;Sui et al. 2020).Besides, the low content of astaxanthin in biomass is another item, causing the high production cost.Previous documents elucidated that astaxanthin contents in dry biomass of H. pluvialis and Ch.zofingiensis estimated at 25-35 mg g −1 and 1-2 mg g −1 , respectively (Kim et al. 2006;Chen et al. 2009).So, a high amount of astaxanthin produced for aquaculture could result in high consumption of microalgae biomass, electricity, water, chemicals, and labor work.Contamination of algal biomass is another important factor in challenges facing microalgae production.Microalgae cultivation may accumulate many heavy metals in biomass, further threatening the safety of the cultured fish diet.This pollution originated from using of wastewater in the cultivation of microalgae to decrease the cost of producing astaxanthin (Kang et al. 2006;Suresh Kumar et al. 2015;Ledda et al. 2016).As well, industrialization is the cause of heavy metal pollution in rivers, lakes, and underground water, highlighting the hazards of heavy metal pollution in microalgae growth.Owing to this pollution, the physical characteristics of the microalgae cells are negatively affected (Suresh Kumar et al. 2015;Lu et al. 2020).
Furthermore, oxidation occurring during storage of feed containing astaxanthin has deleterious issues, owing to the high sensitivity of astaxanthin to oxidation from the presence of a highly unsaturated molecular structure.Hence, astaxanthin depletion will further boost during long-term storage (Takeungwongtrakul and Benjakul 2016;Kasprzak et al. 2020).Besides, deleterious compounds (e.g., heptanal, heptane, nonanal, hexanal, and 2-butenal) are produced from the oxidation of lipid-soluble astaxanthin in animal feed, resulting in reducing of lipid quality during storage, which will negatively cause the nutrients loss of fish feeds (Kasprzak et al. 2020;Lu et al. 2021).

Functional benefits of astaxanthin in aquaculture
The biological activities of astaxanthin and its health benefits on cultured fish are displayed in Fig. 5.

Astaxanthin is a potential carotenoid in different aquatic animals
It is recognized that the coloration of fish is a crucial quality criterion required by consumers to evaluate the health status, nutritive value, degree of freshness, and taste of farmed fish and crustaceans.In parallel, the marketing value of marine fish (e.g., salmon, red porgy, or red sea bream) depends on the color of the fish as well as the exoskeleton and muscular epithelium of shrimps, lobster, and other crustacean carapaces and molluscan gonads (EFSA 2014;Lim et al. 2018).Therefore, the external characteristics should be managed, giving paramount value to cultured aquatic animals (De Carvalho and Caramujo 2017).Besides, the quality of marine fish markedly depends on skin and muscle pigmentation in the market, thus providing humans with high-quality fish meat products.Astaxanthin is a widely distributed and highly valued carotenoid that is responsible for the pink-red color in the fins, skin, muscle, and gonads in aquatic animals, including zooplankton, marine fish, and crustaceans (Maoka 2011).Most aquatic animals (e.g., salmon, red sea bream, rainbow trout, ornamental fish, crayfish, lobster, and shrimp) cannot biosynthesize astaxanthin de novo; thus, the entrance point of astaxanthin is through dietary ingestion of higher heterotrophic zooplanktons or acquired in formulated artificial diet (Rajasingh et al. 2006).Curiously, astaxanthin retention in fish is significantly promoted in the presence of dietary lipids; this may lead to higher astaxanthin concentrations in fish muscle.This results from the fact that astaxanthin is esterified with fatty acids to intensify the storage of lipids; meanwhile, free astaxanthin may be incorporated into cell membranes, preventing lipid Fig. 5 Potential health benefits of astaxanthin on aquatic animals peroxidation and protecting membrane structures (Sommer et al. 2006;McNulty et al. 2007).In addition, dietary astaxanthin and lipids may enhance the digestion, absorption, and transport mechanisms of the digestive systems of fish (Castenmiller and West 1998).When rainbow trout were fed natural astaxanthin from H. pluvialis algae and synthetic astaxanthin (75 mg kg −1 of feed) using fish oil or olive oil for 6 weeks, higher serum concentrations of astaxanthin, as well as muscle astaxanthin levels, were higher in fish supplemented with synthetic carotenoid and olive oil (Choubert et al. 2006).Previous studies have recommended that dietary natural astaxanthin from microorganisms is better than synthetic one because of its superior bio-accumulation in teleost skin and muscle (Sigurgisladottir et al. 1994;Kurnia et al. 2007;Lu et al. 2021).
Given its unique properties, the progressive expansion of natural astaxanthin application as a feed additive in the aquaculture industry has been acknowledged as an insatiable instance.Numerous previous studies determined the effect of dietary natural or/and artificial astaxanthin on the skin and muscle pigmentation of various aquatic animals that have been categorized in Table 2.

Benefits of astaxanthin on the reproduction of aquatic animals
A fundamental role of astaxanthin in reproductive performance, including egg production and quality, has been evidenced in many aquatic animals (Fig. 6) (Tizkar et al. 2013(Tizkar et al. , 2015;;Palma et al. 2017).Supplemented carotenoids consumed by aquatics are accumulated in the liver and crustaceans' hepatopancreas which are then transferred to the ovaries in the late stages of maturity (Harrison 1990(Harrison , 1997;;Tizkar et al. 2013).
Besides, most of the reproductive activities of aquatic organisms are mediated by astaxanthin via its accumulation within reproductive organs, such as observed in salmonid fish during sexual maturation, pooling from the muscle and liver into the gonads that concentrated in the unesterified form in the mature eggs, which its coloration is considered an indicator about egg quality (Blount et al. 2000;Rajasingh et al. 2006;Nie et al. 2011;Wade et al. 2015a).
One of the most coordinated effects of astaxanthin is its function as a primary source of vitamin A precursors (retinol and 3,4-didehydroretinol) (Torrissen 1990; Moren et al. 2002;Blomhoff and Blomhoff 2006), which they have a substantial role in cell signaling during the shaping of developing vertebrate embryos (Duester 2008;Kam et al. 2012).For this reason, it plays a crucial role in the increment of vitamin A concentration in feeding fry of Atlantic salmon, Salmo salar (Christiansen et al. 1994), as well as these investigations have been observed in the intestines of rainbow trout and Atlantic salmon (White et al. 2003).The impact of dietary astaxanthin supplementation on the breeding behavior of a variety of fish species has been documented in many previous studies.Dietary inclusion of astaxanthin (0.07, 12.46, 33.33, 65.06, and 92.91 mg kg −1 ) for six months has been regarded as an indispensable supplement for effective reproductive traits as this carotenoid has enhanced the oocyte maturation in the broodstock rainbow trout, O. mykiss, and fertilization rate in different maturity stages (Ahmadi et al. 2006;Bazyar Lakeh et al. 2010).Moreover, adding astaxanthin to broodstock diets had positive effects on the egg quality of yellowtail (Seriola quinqueradiata) (Verakunpiriya et al. 1997), striped jack (Pseudocaranx dentex) (Vassallo-Agius et al. 2001), and sea urchin (Lytechinus variegatus) (George et al. 2001).
The role of astaxanthin in enhancing the efficacy of the propagation processes and embryonic developmental stages for goldfish, which is the most precious ornamental fish in the world, has been taken great attention (Tizkar et al. 2013(Tizkar et al. , 2015)).Dietary synthetic astaxanthin intake (50, 100, and 150 mg kg −1 ) for 120 days has promoted the quality features of sperm (e.g.concentration, motility, osmolality) and fertility of goldfish Carassius auratus (Tizkar et al. 2015).In addition, the diameter and the number of fertilized eggs in the goldfish Carassius auratus were greater in groups fed 150 mg kg −1 of astaxanthin for four months, which correlated to higher egg survival rates in the incubation period (Tizkar et al. 2013).Recent investigation on the effects of astaxanthin supplementary diets (150 mg kg −1 feed) on egg quality and larvae quality parameters of clownfish (Amphiprion ocellaris) has observed a higher hatching rate of egg and survival rate with a lower malformed rate of larvae (Hue et al. 2020).It has been also claimed

Benefits of astaxanthin on the antioxidant capacity, immunity, and disease resistance
The rise of infectious illnesses in intensive farming, particularly in the early phases of production, constitutes a substantial drawback or leading threat that has a considerable influence on the global economy.Significant scientific efforts have been made throughout the years to improve the immune systems and antioxidant capacity of numerous fish and crustaceans by consuming astaxanthin (AX).We have demonstrated some previous reports on the impacts of Ax on enhancing antioxidant capacity, immunity, and disease resistance against infection in different aquatic animals (Table 3).Extensive research has demonstrated that AX has many advantages for crustaceans, in addition to the typical pigmentation properties, antioxidant activity, immune system support, resistance to disease, and resistance to environmental stressors like temperature, pH, and ammonia.But since it is generally known that crustaceans cannot produce AX on their own, several species of cultured crustaceans need dietary supplements of AX (Tejera et al. 2007;Wade et al. 2017).
The availability and synchronization of intrinsic and extrinsic antioxidants are necessary for fishes' antioxidant defenses to scavenge free radicals such as reactive oxygen species (ROS) (Ju et al. 2011).Catalase, superoxide dismutase (SOD), glutathione (GSH), glutathione reductase (GR), glutathione S-transferase (GST), and glutathione peroxidase (GPX) are examples of intrinsic or endogenous antioxidant defenses (Sommer et al. 1991).With reducing and nucleophilic characteristics, GSH is the main cellular thiol that is not a protein (Pisoschi and Pop 2015).The antioxidant enzyme GPx, which may destroy the lipid peroxide (LPO, MDA) produced within cells, is known to use GSH as a substrate (Livingstone 2001).Both environmental stressors and infection can increase ROS and MDA production and reduce antioxidant molecules such as GSH, SOD, and catalase (Pan et al. 2011;Ameur et al. 2012;Ho et al. 2013;Srikanth et al. 2013;Regoli and Giuliani 2014;Dawood et al. 2020b).The outstanding antioxidant properties of synthetic and algal AX are a strong pro-oxidant due to its exceptionally high unsaturation and oxidative potential in the chemical structure (Ismail et al. 2016).Indeed, AX has 100 times more antioxidative activity than tocopherol and beta-carotene, respectively (Mularczyk et al. 2020).AX possesses keto (= O) and hydroxyl (-OH) groups, conjugated carbon-carbon double bonds, and both lipophilic (hydrophobic, non-polar) and hydrophilic (polar) characteristics.Polar functional groups of AX are orientated outside the membrane, and the hydroxyl groups are attached across membranes (Kishimoto et al. 2016;Britton 2020).The AX backbones could act as molecular wire to strengthen the membrane's mechanical properties (Pashkow et al. 2008;Skibsted 2012).When compared to control fish fed a non-AX diet, fish fed AX showed a marked reduction in MDA levels in many tissues (Kurnia et al. 2007).MDA is a good marker of tissue oxidative stress-related injury in the body (Kaneda and Miyazawa 1987).Additionally, fish and invertebrates have been shown to produce more antioxidative enzymes like SOD, catalase, and GPx as well as cellular endogenous antioxidants like GSH when carotenoids like AX are administered (Pan et al. 2011;Sahin et al. 2014;Al-Amin et al. 2015;Gammone et al. 2015;Lim et al. 2018;Dawood et al. 2020a).As well, AX supplementation could increase serum TAC levels in different aquatic animals (Kurnia et al. 2007).
Additionally, it has been discovered that the addition of carotenoids increases the expression of HSP in cultured cells and the tissue of several animal species (Müller et al. 2013;Cheng et al. 2018;Fleischmann et al. 2020;Tan et al. 2020).Carotenoids demonstrated antioxidant action may work not only by directly scavenging ROS but also by regulating the expression of proteins involved in stress and antioxidant defenses.The three following reactions, electron transfer (oxidation and reduction), hydrogen abstraction (allylic hydrogen atom abstraction), and radical addition (adduct formation), are assumed to be the three processes through which carotenoids interact with free radicals (Pashkow et al. 2008;Sahin et al. 2014;Gammone et al. 2015;Nishino et al. 2016).Carotenoids' mechanisms of action in the body have been divided into the following four groups: Antioxidative and pro-oxidative actions, reduction of NF-kB signaling translation, activation of the nuclear factor erythroid 2-related factor 2 (Nrf2), and interaction with other transcription factors are all examples of these effects (Kaulmann and Bohn 2014;Niu et al. 2018;Rodriguez-Concepcion et al. 2018;Dawood et al. 2020a).It is well recognized that transcription factors like NF-kB and Nrf2 are connected to immunological response, inflammation, and oxidative stress responses.Inflammatory substances like tumor necrosis factor (TNF) and cytokines, as well as oxidative stress, activate the NF-kB pathway.On the other hand, it is known that the Nrf2 pathway plays a significant role in cells' defense against ROSinduced oxidative stress (Kaulmann and Bohn 2014).The inducible genes Nrf2, heme oxygenase 1 (HO-1), and iNOS, which control inflammatory responses and oxidative stress, are transcribed by the protein NF-kB, which is inhibited by AX in aquatic and different animals- (Kaulmann and Bohn 2014;Xie et al. 2018;Le Goff et al. 2019;Jiang et al. 2020).Moreover, AX supplementation in aquaculture can suppress inflammatory cytokines and apoptotic markers such as caspase 3, caspase 9, and IL-15, IL-1β and TNF-α levels (Song et al. 2017).Together, these findings imply that dietary carotenoids can enhance the body's natural antioxidant defenses, such as antioxidant enzymes, cellular endogenous antioxidants, and HSP, as well as increase resistance to oxidative stress and suppress inflammatory and apoptotic cascade.
Enhanced phagocytic activity in fish has been documented after treatment with diverse AX against different infections (Harikrishnan and Balasundaram 2005;Sahu et al. 2007;Harikrishnan et al. 2010bHarikrishnan et al. , 2011)).Since O 2 − is the first product released during the respiratory burst, O 2 − concentration has been accepted as an accurate parameter to quantify the intensity of a respiratory burst (Secombes 1990).The overall proportion of NBT-positive blood cells remained stable between increasing in rainbow trout following immunostimulant therapy (Jeney and Anderson 1993).These cells could be neutrophils that are still capable of the creation of reactive oxygen species.With all doses of the pathogen-specific supplemental diets, the serum lysozyme activity was significantly increased.Fish treated with herbal remedies containing AX for various illnesses have shown increased serum lysozyme activity (Martins et al. 2002;Harikrishnan and Balasundaram 2008;Harikrishnan et al. 2009, Vol:. (1234567890) 2010a).Increasing trends in serum lysozyme activity may have contributed to the improvement in non-specific defensive mechanisms described in several fish against pathogens (Sahu et al. 2007).Common carp, Cyprinus carpio, treated with 50 and 100 mg kg −1 of AX supplemental diet against pathogens showed a considerable increase in serum phagocytic, respiratory burst, lysozyme, and bactericidal activities that led to a decrease in the proportion of death during the first 30 days after Aeromonas hydrophila infection (Ju et al. 2011).Many previous reports reported that AX directly or indirectly confers antioxidant activity and enhances innate, cell-mediated, and humoral immune responses (Jyonouchi et al. 2000;Kurihara et al. 2002;Park et al. 2011).The impacts of AX on innate immunity may be correlated to its ability to trigger further antimicrobial effects processes, such as lysosomal enzyme release, complementary elements, cationic peptides, and the synthesis of oxygen reactive species (Chew et al. 2011;Smith et al. 2013).

Benefits of astaxanthin on growth and stress tolerance
Astaxanthin has great attention nowadays due to its numerous physiological actions in aquatic animals (Lim et al. 2018;Lu et al. 2021).Carotenoids may promote great nutrient use, resulting in improved growth performance in many aquatic species (Amar et al. 2001).For instance, adding astaxanthin to the diet may improve the growth of certain species, such as Micropterus salmoides (Xie et al. 2020), Trachinotus ovatus (Fang et al. 2021b), Marsupenaeus japonicas (Wang et al. 2018a), and Paralithodes camtschaticus (Daly et al. 2013).Additionally, adding carotene to the food improved the growth abilities of Oreochromis niloticus (Hu et al. 2006), Piaractus mesopotamicus (Bacchetta et al. 2019), and Penaeus monodon (Niu et al. 2014).There were three basic explanations for how carotenoid pigments could enhance crustacean growth.Firstly, the carotenoid pigment may control aquatic animals' metabolisms via increased digestive enzyme activity, which in turn promoted nutritional digestion, absorption, and utilization, leading to increased feed intake (Baron et al. 2008;Zhang et al. 2013).Secondly, the carotenoid pigment may also shorten the time between molt cycles in crustaceans and regulate the NADPH metabolism, both of which reduce energy consumption and improve growth performance (Hertrampf and Piedad-Pascual 2003;Mao et al. 2017).The last hypothesis is the ability of astaxanthin to enhance intestinal flora to break down indigestible components to extract more nutrients (Vasudevan et al. 2006).A previous report confirmed that red porgy (Pagrus pagrus) fed diets containing AX had significantly lower lipid percentages, which in turn improved lipid utilization and supplied extra energy to improve growth performance (Kalinowski et al. 2011).
The immoderate stress contributes to bodily physiological malfunction, growth rate decrease, immunological suppression, susceptibility to pathogenic invasions, and even mortality (Ndong et al. 2007;Nikoo et al. 2010;Liu et al. 2016).Therefore, it is crucial in aquaculture research to alleviate adverse conditions that could lead to significant stress and impair the host organism.AX has been reported to increase growth and resistance against stressors in many aquatic animals (Table 4).The ability of AX in crustaceans and fish diets to enhance stress resistance is attributed to its increased antioxidant capacity and immune response (Lim et al. 2018).However, under hypoxic conditions, shrimp-fed diets containing AX or beta-carotene had higher levels of HIF-1 mRNA expression, suggesting that dietary AX or beta-carotene may help partially reduce the hypoxia stress response by improving the effectiveness or utility of the oxygen transportation (Niu et al. 2014).It would have been possible for excessively high numbers of oxygen radicals to form under thermal, salinity, and osmotic stresses (Lim et al. 2018).AX may scavenge oxygen radicals in cells and lessen cellular damage and boost resistance because it has a lengthy conjugated doublebond structure and relatively unstable electron orbitals (Chien and Shiau 2005).

Pharmacokinetic properties of astaxanthin on human health concerning its bioavailability
Like most marine carotenoids, astaxanthin is generally absorbed by the organisms along with fatty acids via passive diffusion into the intestinal epithelium, where astaxanthin mixes with bile acid and assembles into micelles.Then, these micelles are partly absorbed by intestinal mucosal cells, which are combined with chylomicrons to be delivered to the liver.After that, astaxanthin is assimilated with lipoproteins and transported to a variety of tissues (OKADA et al. 2009).Furthermore, astaxanthin can maintain the functional integrity of cell membranes by placing itself in the lipid bilayers, which can protect cells, lipids, and membrane lipoproteins against oxidative damage.Once degraded, carotenoids are stored in the liver and re-secreted, either as very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), or highdensity lipoproteins (HDL), eventually being transported to the tissues via circulation (OKADA et al. 2009;Zuluaga et al. 2018).
The bioavailability of carotenoids relies on their chemical structures as polar-free astaxanthin has higher bioavailability than apolar β-carotene and lycopene (Bohn 2008;Yuan et al. 2011).Astaxanthin from H. pluvialis has also shown better bioavailability than β-carotene from Spirulina platensis and lutein from Botryococcus braunii (Ranga Rao et al. 2010).Furthermore, cis-astaxanthins have more preferential accumulation in the blood plasma compared with the trans-form in respect of evident shorter chain lengths (Bohn 2008;Yuan et al. 2011).However, there was a scientific controversy study that declared that xanthophyll esters such as astaxanthin were hydrolyzed in the small intestine for absorptive mechanism in humans as the enzymatic esterification of astaxanthin in the intestinal cells has a lower activity (Sugawara et al. 2009).Subsequently, the esterified astaxanthins were incorporated into the lipid core in chylomicron and loaded into a variety of human tissues.It was important to clarify that astaxanthin esters from H. pluvialis increased the bioavailability of astaxanthin as polar xanthophylls were preferable for esterification in intestinal cells to get more acknowledge about the absorption, metabolism, and biological function of polar carotenoids (Sugawara et al. 2009;Ranga Rao et al. 2010).
Many clinical studies have reported that even higher doses of natural or synthetic astaxanthin during the treatment trial period revealed harmless and non-toxic effects (Kupcinskas et al. 2008;Yuan et al. 2011;Donoso et al. 2021).Previous studies documented astaxanthin bioavailability in human plasma after administration of natural and synthetic sources of astaxanthin (Østerlie et al. 2000;OKADA et al. 2009).Astaxanthin absorption in humans was also promoted by lipid-based formulations as carotenes can solubilize into the oil phase of the food matrix with greater bioavailability and absorption capability (Olson 2004).
Given its unique health benefits, astaxanthin has widely associated with neuroprotective, cardioprotective, and antitumoral properties, proposing its ameliorative potential for the prevention of diseases like dementia, Alzheimer, Parkinson, and cardiovascular diseases.Besides, promising results would be applied to skin and eye health, highlighting its perspective on the prevention of skin photo-aging and the eye diseases like glaucoma, cataracts, and uveitis (Dhankhar et al. 2012;Ambati et al. 2014;Donoso et al. 2021).Astaxanthin exerts beneficial effects on pancreatic B-cell function related to antidiabetic activity via diminishing hyperglycemia in these cells and improving glucose tolerance and serum insulin levels (Uchiyama et al. 2002).Some data on human trials regarding the role of astaxanthin in the immune response were also conducted (Andersen et al. 2007;Macpherson et al. 2008;Park et al. 2010).

Conclusion
The roles of astaxanthin are emerging into the limelight owing to its great advantages in the aspects of the aquaculture industry for fish and crustacean rearing.All available data, i.e., use of astaxanthin as a feed supplement in cultured aquatic species, supports the conclusion that astaxanthin reveals no contraindication for aquatic animals' nutrition.The reported data have been devoted to the screening of astaxanthin in pigmentation and the weight gain, immunity enhancement, inflammatory response, and disease resistance of economic fish and crustaceans.Besides, the previous findings discussed in this review suggest that astaxanthin may be a promising candidate for the prevention of several diseases associated with oxidative stress in aquatic animals.It is expected that with astaxanthin, aforementioned benefits of astaxanthin will be a promising and safe bio-product for the sustainable development of aquaculture in the prospective future.However, the production of astaxanthin for aquaculture feeds estimated in the range of tons must face several challenges that include the application of highly rated standards in quality and safety issues and production costs.

Fig. 1
Fig. 1 Chemical planner structure of astaxanthin configurational isomers and a geometric Cis isomer

Fig. 2
Fig. 2 Schematic view of the astaxanthin molecule position at the cell membrane

Fig. 3
Fig. 3 Astaxanthin concentrations from natural sources are approximately presented from Wikipedia web site

Fig. 6
Fig. 6 Summary of benefits of astaxanthin supplementation on different maturity stages of salmon and rainbow trout species during reproduction

Table 1
Different natural sources of astaxanthin

Table 2
The impact of dietary astaxanthin on the pigmentation of aquatic animals

Table 3
The impact of dietary astaxanthin on the antioxidant capacity, immunity, and disease resistance

Table 4
The impact of dietary astaxanthin on growth and stress tolerance Vol:. (1234567890)