Aquaculture is the fastest food producing system of world which has grown tremendously during the past few decades to meet the growing demand for aquatic food. Approximately, 25% of the animal protein requirement of the human throughout the world is from aquatic organisms. Over the years, dramaticchanges inaquaculture practices have lead tothe significant 18% increase in production per year. Currently most developed countries are adopting intensiveand super intensive types of aquaculture practices with high stocking density which have been made to enhance the production. Such types of culture practices necessitate the artificial supplementation of nutrient enriched diets, growth promoters, vitamin and minerals alongwith several other feed additives to achieve maximum output. Therefore, in the modern days of aquaculture practices nutrient plays a crucial role in the growth and health of the fish. Recently much emphasis has been given to develop nutritionally balanced diet to achieve maximum growth of the cultured aquatic species.The success of such practices solely depends upon both macro- and micro components of nutrients.However, little information’s are available on the nutrient requirements especially on trace mineral form, requirement and bioavailability in wide range of aquatic organisms. Herein in this article, the role of selenium (Se) especially green synthesized selenium nanoparticles in fish has been discussed.Among various micronutrients, the importance of mineral nutrition especially selenium is significant to any organism. It is one of the essential dietary micronutrients in animals and aquatic species are no exception.
In past few decades, by virtue of it’s unique properties, selenium has attracted considerable attention of researchers particularly in the area of health management.Being a trace element, Se is essential for maintaining health, growth, immunity, biochemical and physiological functions of any living organism. Over the years the dietary requirement of this unique trace element has been optimized for several economically important species of fishes like Nile tilapia, Atlantic salmon, channel catfish, rainbow trout, gibel carp, common carp, Indian major carps like rohu, mrigal etc. Literatures indicate significant variation in the optimum dietary requirement of selenium among different cultured fish species (Table 1). The selenium requirement mostly varies from low concentration of 0.15 mg kg-1 diet to higher concentration of 1.8 mg kg-1 diet. For example, as per NRC recommendation, 0.25 mg selenium kg-1 feed is optimum for fish like Nile tilapia, channel catfish etc. The findings of various studies indicate that the optimum concentration of selenium in the form of NaSeO is less than 1.2 mg kg-1 diet for Atlantic salmon. Similarly for Channel catfish, selenium in the form of Na2SeO3 is recommended to be 0.12 mg kg-1 feed. While it varies from 0.15 to 0.38 mg kg-1 feed for Rainbow trout, it is 12 mg kg-1 feed for Beluga Sturgeon. Similarly, it is 0.7, 0.2, 1.18 mg kg-1 feed for grouper hybrid Striped Bass and gibel carp respectively. It’s dietary requirement for normal growth and health maintenance varies among the fish species. Certain fishes require high concentration of selenium and in fishes like Rainbow trout it’s requirement varies from 500 to 4000 mg kg-1 feed. Similarly, for African catfish it is 300 mg kg-1 but it is much less for fishes like Common carp (0.12 and 0.15 mg kg-1 feed) and Channel catfish (0.25 mg kg-1 feed). Therefore such variation may be due to the cultured species and the form of selenium used.
Selenium has been associated with several biological activities and contributes to anti-oxidative mechanism, anti-carcinogenic properties, thyroid metabolism, muscle development and functioning. It is a vital component of various metabolic pathways in animals. Since the discovery of selenium as a component of glutathione peroxidase in 1973, more than 30 selenium-containing proteins have been identified. But till date, the detailed biological functions of only 10 selenoproteins have been known. Among different selenoproteins, glutathione peroxidase (GSH-Px), one of the key selenoenzymes has been well known for its potency to protect cellular membranes from damage by converting hydrogen peroxide to water. H2O2 and several other intermediates of cellular reduction pathways can damage cellular membranes, disrupt cellular function and may negatively impact animal health.The tissue concentrations of selenium are associated with glutathione peroxidase activity and are directly related to dietary selenium intake.
Mg Kg-1 Feed)
|4||Channel catfish||Na2SeO3/ Se-Yeast||0.12|
|5||Rainbow trout||Na2SeO3||0.15 - 0.38|
|10||Indian major carp, Rohu||Organic Se||1.0|
|11||Indian major carp, Mrigal||Organic Se||2.5|
Further it has been identified as an essential part of normal reproductive and immune function. It stimulates the immunity of host especially humoral immunity mechanisms with anincrease in immunoglobulins level. Dietary supplementation of selenium increases the antibody production, enhances the phagocytic activity of neutrophils and macrophages. Nonetheless, it is indispensable in the production of the lymphocyte migration inhibition factorand interleukin 2, which accelerates the proliferation,maturation and activity of T lymphocytes.Like higher animals, selenium also acts asan essential trace element for all aquaticanimals anditplays important roles in antioxidant stress defenses, DNA and protein synthesis at normal concentration.It plays an essential role in physiological functions including reproduction, immune system, growth, and development of fish species. Supplementation of this micronutrient considerably improves the overall healthstatus of aquatic species in general and fish in particular.
However, several components regulate the availability of seleniumand hence affect its activity in host. Factors like composition of the supplemented diet, the form/type of selenium used (organic, inorganic and nano form), host factors (cultured species, age, size, and health status) and finally the soil and water parameters of the cultured pond. Amongst, soil can affect the selenium requirement of aquatic species. Aquaculture practices in areas with high selenium levels in the soil likeorganic carbon-rich soils may not require further dietary supplementation of selenium. The bioavailability of selenium is usually dependent upon various physico-chemical reactions and/or processes in soil such as redox-transformations and pH shifts, adsorption/desorption, precipitation/dissolution, formation of Se–ligand complex, methylation and geological parent material etc.
In country like India, considerable variation has been recorded in the soil selenium content with both Se deficient (0.025-0.71 mg kg-1) and seleniferous soils with higher selenium concentrations (1- 20 mg kg-1). The concentration of selenium in most normal soils is estimated to be 200 µg kg-1. The selenium levels in states like Punjab (224.7 µg kg-1to 288.0 µg kg-1) and Himachal Pradesh (274.8 to 343.8 µg kg-1) states are normal to low while it varies in state like Haryana (207.8 to 552.0 µgkg-1).Looking into the fact that Indian soil exhibits diversity in the selenium levels among different states, it is utmost important to analyze the bioavailable selenium content of the soil in a particular area so that adverse consequences to the cultured aquatic species can be prevented.
Nowadays feed containing desired amount of selenium are being prepared for specific aquatic species. However, it is very crucial to use the correct selenium form to achieve maximum beneficial effect.All the animals including aquatic species can utilize both inorganic as well as organic forms of selenium.Literatures indicate that organic forms of selenium have more advantages especially in reducing oxidative stress in comparison to inorganic forms and incorporate more into skeletal muscles, kidney, liver, and gastrointestinal mucosa proteins as selenomethionine and selenocysteine. Among the various organic forms,selenium-yeast which is produced from the fermentation of yeast, Saccharomyces cerevisiae is widely used in aquaculture. Selenium-yeast acts appreciably on the growth, reproduction and immune system of animals thereby improving the productivity of farm animals which is of great economic benefit. On the other hand, a very delicate difference between the optimum concentration and toxicity level has been observed in various aquatic species supplemented with inorganic form of selenium. While, the elemental form of selenium is usually insoluble and less toxic as compared to other forms of selenium, the inorganic form of selenium when used at optimum dose undernormal conditionscould be toxic to the host speciesduring stress conditions.Furthermore,Se in the form of selenate ion (SeO42−) has been reported to be more toxic than selenite (SeO32−) form.
Therefore, a very narrow margin has been found to exist between activity and toxicity for organic and inorganic forms of selenium. On the contrary, transformation of nutrients/compounds/substances to their nano-form can enhance their absorption into fish and increase the efficiency of the feed. In this regard, selenium nanoparticles arealso found to be better in terms of bioavailability, high bioactivity and most importantly less toxicity. As per reports, the bioavailability of selenium nanoparticles of 100 - 500 nm sizes is almost similar to that of other forms of selenium into plants, microbes, animals and human beings. Therefore, in recent times selenium nanoparticles have attracted remarkable attention to explore their ability to execute beneficial effects in different animals including aquatic animals.
Nowadays, selenium nanoparticles are successfully synthesized by several physical, chemical and biological methods. Out of these approaches, selenium nanoparticles are widely synthesized by chemical reduction, radiolysis reduction processes, sono-chemical process etc. However, such methods have many drawbacks like difficulty in handling of toxic and hazardous chemicals, ultrasound, radiation etc. Contrarily, biosynthesis is an eloquent, biocompatible, safe, eco-friendly and recyclable way of preparing any nanoparticle and selenium is no exception. Hence being ecofriendly, less toxic, cost effective and stable yield without agglomeration, selenium nanoparticles are now prepared using this method.Thesynthesis of selenium nanoparticles has been done by using different plant extracts and microorganisms like bacteria, fungi, yeasts and algae plants. Many bacteria, fungi and plants are reported to synthesize selenium nanoparticles. Although, reports on the green synthesis of selenium nanoparticles by using plant extracts are less, plants like Aloe vera, Blumea axillaris, Vitis vinifera, Allium sativum etc are found to synthesize these nanoparticles. Besides, microorganisms like Klebsiella pneumonia, Bacillus cereus can also synthesize selenium nanoparticles.
In the recent past, stable selenium nanoparticlesare successfully synthesized and explored for fish and shellfish health management.Supplementation of selenium nanoparticlescan improve the growth performances and antioxidant defense mechanisms of several fish species. Selenium nanoparticles helpin protecting fish body cells and cellular components from the oxidative damage through the activation of selenoenzymes and selenoproteins. Itis involved in the metabolism of thyroid hormones, which are important for normal growth and development. Supplementation ofselenium nanoparticlesin fish increases the thyroid hormone activation which contributes to better growth and feed conversion efficiency.
Further their supplementation can improve the flesh/meat qualityof fish. A linear relationship between selenium concentrationsin the fish muscle with selenium supplementation has been reported in various fish species like Atlantic salmon, rainbow trout, grouper, yellowtail kingfish, common carp, hybrid striped bass, Indian major carps etc. In fishes like common carp, Cyprinus carpio better growth performance, feed conversion (highest weight gain, feed conversion ratio), tissue composition, and high accumulation of selenium in muscle and liver, antioxidant defense responses and blood biochemical parameters (increase in GPx, SOD, CAT activities and decrease in AST, ALT, LDH activities) have been recorded by selenium nanoparticles (0.7 mg nano-Se kg-1 feed) supplemented feed as compared to its organic form (selenomethionine) and inorganic form(sodium selenite) of supplementation.
For commercial farmers, faster growth and reproductivity is an imperative aspect. While the role of selenium nanoparticles in increasing the growth of fishes has already been an established fact, their positive effect on reproductive maturity is also demonstrated. Similarly, selenium nanoparticles are effective in limiting oxidative stress. Fishes often produce reactive oxygen species (ROS) which cause several complications like rapid aging, retarded growth, etc. Administration of selenium nanoparticles reduces the adverse consequences of ROS by stimulating the production of various antioxidants enzymes like catalase, glutathione peroxidase, super oxide dismutase (SOD) etc. Using these selenium nanoparticles with feed can therefore be a good source of tweaking the growth performances along with reproductive maturity.
Furthermore, vitamins and sulfur-containing amino acids are found to enhance the effects of selenium. Selenium nanoparticles are found to act synergistically with vitamins like vitamin C, E etc. Enhancement of feeding weight and feed conversion ratio, growth, and physiological responses in fish has been reported by dietary supplementation of selenium with vitamin C and vitamin E. In fishes like rainbow trout, combined supplementation of vitamin E @ 50 ppm and selenium @ 0.35 ppm showed significant weight gain and feed conversion ratio. The synergistic effect of selenium with vitamin C supplementation has been demonstrated by increase in various haematological parameters like haemoglobin content, haematocrit value, erythrocytes count etc. in fishes. Nonetheless, their antioxidant property possibly helps to stabilize the red blood cells and thereby providing protection from cell lysis during normal metabolism or during ROS production. Furthermore, supplementation of fishes with selenium nanoparticles along with vitamin C leads to stimulation of immunity by the production of B lymphocytes, lysozyme content etc.
Apart from these activities selenium in general and Selenium nanoparticles are also reported to inhibit pathogens. Earlier studies indicate the antimicrobial effects of inorganic selenium (sodium selenite) supplementation on disease resistance of Macrobrachium rosenbergii against the pathogen Debaryomyces hansenii. Likewise the antimicrobial effect of organic selenium against Aeromonas hydrophila infection in fish has been reported. Several microbial diseases like fin rot, gill rot, other skin diseases etc., are found to affect the productivity and cause a great economic loss to aquaculture.Although, many antibiotics are currently available and used, the development of resistance to these drugs by the pathogen becomes an obstruction. Selenium nanoparticles can play a crucial role in minimising the use of chemotherapeutic agents to mitigate microbial infections in aquaculture. Selenium nanoparticles, especially green synthesized, exhibit significant antimicrobial property and hence could be a viable alternative to this hindrance. Nevertheless, its ability to inhibit biofilm formation is further an added benefit. Despite, limited attempts have been made to study the antimicrobial potential of selenium nanoparticles in general and green synthesized selenium nanoparticles in particular against aquatic pathogens.
Selenium is very essential for the good health condition of host. Though there are many beneficiary effects of selenium due to its involvement in several physiological functions, there is a fine partition between adequate dose, deficiency and toxicity. Selenium being a trace element is required in small quantity for fish and its permissible limit in aquafeed is 0.5 mg per kg feed in Europe but the dose may possibly vary among different countries. While it’s deficiency can lead to poor growth and moresusceptibility to mortality, highconcentrationof this trace element can have toxic effects resulting in retardation of growth, organ damage, reducedfeed efficiency, SGR, and FCR. The main problem regarding selenium toxicity is that the levels determined during stressful conditions are usually higher and the use of such higher levels of Selenium during normal conditions may become toxic to fish. Further, selenium toxicity was demonstrated when exposed for a longer duration. The symptoms of chronic toxicity include behavioral changes, lower haematological parameters, pathological changes in organs, altered reproductive performances, etc. Fish like rainbow trout fry exposed to selenite at concentration 47 μg per L for a period of 60 days lead to reduced length and mortality. Thus, the differentiation of selenium for different fish species should be based on different environmental conditions such as the normal and stressful conditions.
However, the role of selenium in general and selenium nanoparticles in particular needs to be explored to greater depth to validate claimed beneficial effects in different aquatic species. Further there is a need for more in-depth studies to optimize therequired dose of selenium nanoparticles with respect to individual species and also to compare the activities of green synthesized selenium nanoparticles with chemically and physically synthesized selenium nanoparticles so that proper enactment of selenium nanoparticles can be made to improve the aquaculture growth.
Dr S. K. Nayak (E. mail: email@example.com)
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