WWW.WA S .ORG • WORLD AQUACULTURE • DECEMBER 2022 61 mannoprotein. The chemical composition of the cell wall varies from one yeast species to another. The yeast cell wall consists of the innermost layer of plasma membrane followed by the mannoprotein layer, β glucan-chitin layer, β glucan layer, mannoprotein layer and outermost fibrillar layer (Fig. 2). Yeasts such as Phaffla rhodozyma are natural sources of astaxanthin pigment and have been used as a feed ingredient for salmon and ornamental fish culture. Yeast incorporated in fish feed enhances feed intake, fish body composition, ash content, gut microbial balance and immunity, thus increasing survival. Saccharomyces cerevisiae yeast as a feed additive in Nile tilapia diets increases growth and nutrient utilization (Goda et al. 2012). Compared to control diets, baker’s yeast boosts feed conversion ratio, dietary protein and energy consumption. Several other yeast species are also good feed supplements in aquaculture. Enrichment of Live Foods Artemia nauplii is a commonly used live food in aquaculture; however, it is an incomplete diet to many fish larvae due to the lack of essential polyunsaturated fatty acids, mainly EPA and DHA. To overcome this hurdle Artemia and zooplankton can be enriched with yeast. Digestion of yeast by Artemia can be improved by cell wall removal (mainly the mannoprotein layer) by enzymatic pretreatment. Artemia enriched with baker’s yeast improves growth performance and stress resistance in the post-larvae of Penaeus vannamei (Ahmadi et al. 2017). Yeast enrichment of live foods improves overall health status, digestibility, production of enzymes as well as provision of nucleotides in fishes and crustaceans. Immunostimulant Immunostimulants have been used in aquaculture as an alternative to combat disease, mainly enhancing non-specific immune responses. The yeast cell wall contains β-glucan and mannan-rich substances that positively affect the fish immune The success of aquaculture relies on the growth and survival of cultured species. Diseases are one of the major concerns faced by aquafarmers. Unregulated use of antibiotics has led to antibiotic resistance, which is a global crisis of concern. Recently, many biological approaches have been explored to improve the health of aquatic species. Microorganisms are widely used as a source of single-cell protein in feed and disease control agents such as probiotics and immunostimulants in aquaculture. Yeast, a single-celled eukaryote, is considered an excellent organism for application in aquaculture as a growth promoter, nutrient source, immunostimulant, probiotic and antimicrobial. (Fig. 1 and Table 1). Source of Nutrients and Growth Promoter There is a high demand for fishmeal by aquaculture. The increasing cost and dwindling resources are of major concern. Therefore, several unconventional sources have been investigated as protein sources in aquaculture. Different species of bacteria, yeast and microalgae are good sources of protein. Yeasts are the first and best studied microorganisms as an animal feed supplement (GamboaDelgado and Márquez-Reyes 2018). The main yeast strain used in producing compressed yeast is Saccharomyces cerevisiae (baker’s yeast). The protein content of yeast ranges from 25 percent (Debaryomyces hansenii) to 37 percent (Candida utilis) dry weight. Yeast is rich in carbohydrate, 21 percent (Debaromyces hansenii) to 39 percent (Saccharomyces cerevisiae) of dry weight. Yeast contains only a low amount of lipid, between 4-6 percent of dry weight. Ash ranges from 4.7-13 percent. RNA content is 4-7 percent and the DNA content is 0.05-0.32 percent of dry weight. Among the amino acids in yeast, aspartate and glutamate are the highest (8.1-20 percent), whereas cystine, methionine, tryptophan and histidine are the lowest (0.4-3.6 percent). The cell wall of yeast contains 85-90 percent polysaccharides and 10-15 percent protein (Sanderson and Jolly 1994). The main polysaccharides are glucan and mannan, with chitin in small amounts. Mannan polysaccharides with cell wall protein form Yeast as an All-Rounder in Aquaculture Elina Jose Vettom, Susan Shibu and Rejish Kumar V.J ( C O N T I N U E D O N P A G E 6 2 ) FIGURE 1. Applications of yeast in aquaculture. FIGURE 2. Composition of yeast cell wall.
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