According to the results of this study, adding MPE to the diet improved the growth of the shrimps, and the highest specific growth factor, weight gain and protein efficiency were recorded for the group fed with MPE15. To the best of our knowledge, there was no study on the use of the macroalgae premix extract in shrimp diet prior to this research. However, the inclusion of algae and algae extracts in the diets of shrimps has garnered considerable attention due to its reported high levels of protein and many other growth stimulants. Hence, it can improve growth and appetite, stimulate immunity and reinforce resistance to pathogens (Schiener et al., 2014 ; Akbary and Aminikhoei, 2018a ; Akbary et al., 2021a). According to the report by Akbary and Aminikhoei (2018c), the best weight of L. vannamei shrimps (1g) was recorded for shrimps fed with a diet containing 1g P. australis polysaccharide extract kg− 1 feed, which is in line with the findings of this study and indicates that algal polysaccharide can improve the growth of beneficial bacteria and intestinal health and also stimulate shrimp growth. Moreover, the highest final weight, specific growth rate and protein efficiency were reported for L. vannamei shrimps fed with 1 and 1.5 g Iyengaria stellata algae extract and 1g Jania adhaerens algae extract kg− 1 feed (Akbary et al., 2020a ; Akbary et al., 2021a). The improvement in shrimp growth can be attributed to the presence of active substances such as flavonoids and tannins in the diet containing the aforesaid macroalgae, which increased feed consumption. One of the positive mechanisms of macroalgae that influences growth is the presence of compounds such as amino acids and essential fatty acids, which are substantially involved in the growth and vital functions of shrimps (Akbary et al., 2021a). The lowest levels of the FCR were observed in white shrimps fed with 1g P. australis algae extract kg− 1 feed (Akbary and Aminikhoei, 2018c) and 1.5 g J. adhaerens red algae extract kg− 1feed (Akbary et al., 2020a), which is in line with the findings from this research. The results regarding the protein efficiency rate after the inclusion of algae in white shrimp feed were positive (Cruz-SuÁRez et al., 2009 ; Akbary et al., 2020a), which complies with the results of the present study. The improvement in the consumption of protein may vary by algae species. For instance, the PER of shrimps fed with a diet containing Ulva was higher than shrimps on the diet containing Macrocystis and Ascophyllum algae (Cruz-SuÁRez et al., 2009). However, it is assumed that algae can increase the absorption and digestion of dietary proteins or regulate lipid metabolism (Akbary et al., 2020a).
According to the results of this study, the inclusion of MPE in the shrimps’ diet improved the carcass quality. The highest level of crude protein was recorded for the group fed with MPE10 and MPE15, while the highest level of fat and dry matter was recorded in the MPE15 group. Choi et al.(2015) revealed that adding 20g kg− 1 of red algae extract (P. yezoensis) significantly increased the crude fat content in the olive flounder as compared with the control group, but there was no significant difference between different concentrations of red algae extract, which is consistent with the results of this study. It could also be argued that protein in the metabolism process takes its primary path, i.e. the tissue synthesis path, due to the inclusion of algae extract in the feed and is stored in the form of protein (Shalaby et al., 2006). Besides, the use of algae extract in the diet plays a major role in fat synthesis and metabolism. Nakagawa et al. (1997) demonstrated that the use of Ulva algae powder changed the fat metabolism in the blackhead seabream, Acanthopagrus schlegelii and the use of algae powder led to the storage of body fat and reduced body weight loss during winter. Ergün et al. (2008) studied the effect of sea lettuce extract, Ulva on the body chemical composition of nile tilapia fish, Orechromis niloticus and showed that fish fed with 5% of algae extract contained less carcass fat than the control group, which is in line with the results of this study. It could be stated that the inconsistency of results originates from the difference in the method of using algae (algae powder and extract), the difference in the study fish species, the duration of algae use, the algae type, and the difference in the experiment environmental conditions.
The amount of PUFA in the groups fed with MPE was significantly higher than the control group, while the highest level of PUFA was observed in the MPE15 group, which complied with the study conducted by Choi et al. (2015), who indicated that saturated fatty acids are substantially involved in increasing the PUFA level in the muscles of fish fed with algae extract (Choi et al., 2015). Choi et al. (2014) showed that the inclusion of Hizikia fusiformis algae glycoprotein in the diet of olive flounder, Paralichthys olivaceus, led to changes in PUFA levels, including DHA, ARA, linoleic acid (LIA), and EPA (Choi et al., 2014). Choi et al. (2015) suggested that the use of 20 g kg− 1 feed of red algae Pyropia yezoensis increased DHA, ARA and LIA in the muscle of olive flounder, which is in line with the findings from this study. The main reason for the change in lipid metabolism following the addition of algae to the diet yet is still unknown, but the results of relevant research indicates that the addition of seaweed to diet leads to a positive change in the process of lipid metabolism, thereby increasing the amount of PUFA and the positive efficiency of stored lipids (Choi et al., 2014). Similar to these results, Akbary et al. (2021a) investigated the fatty acid composition in the muscle of L. vannamei fed with the extract of I. stellate brown algae and reported that the highest levels of PUFA and EPA were observed in shrimps fed with 1 and 1.5 g algae extract kg− 1feed. It could be stated that the premix extract of the study macroalgae is a great source of ω3 fatty acids, (e.g DHA, and EPA) and is capable of increasing the muscle lipid profile in proportion to the ratio of ω3 fatty acid concentration (Güroy et al., 2007 ; Akbary et al., 2020a) which reduces the 3-n/6-n ratio. This finding is in line with the study by Choi et al. (2015).
The amino acid compound in many macroalgae can be considered relatively complete regarding the essential amino acids. Many species of algae contain the majority of essential and non-essential amino acids (Ortiz et al., 2006 ; Gressler et al., 2010). Based on the results of this study, as the concentration of MPE added to the diet grows, the total essential amino acids, non-essential amino acids and total amino acids in the body of shrimps, L.vannamei, increased. Kalaiswlvi et al. (2018) reported that Artemia nauplii enriched with ether extract, acetone extract and especially ethanolic extract of Phyllanthus amarus significantly increased the total protein level and amino acid and lipid concentration of freshwater shrimp larvae ,Macrobrachium rosenbergii, as compared with non-enriched Artemia nauplii. Moreover, diets containing U. lactuca and G. vermiculophylla promoted amino acid digestibility in L. vannamei (Anaya-Rosas et al., 2019) which is consistent with the results of this study. The amino acid composition of algae has been investigated numerous times. In this study, glutamic acid accounted for 21.35% of total amino acids. In most brown algae, glutamic acid accounts for between 22 and 44% of total amino acids (Øverland et al., 2019), which complies with the findings from this research.
Sitostanol was the major sterol in shrimps fed with MPE, while cholesterol was the only free sterol in MPE0. Cholesterol forms the central core of steroids. Other researchers have specified in their reports that cholesterol is the main steroid in red algae, and its amount is significantly higher than that of brown algae (Saeidnia et al., 2012 ; Akbary et al., 2021b). However, this compound was observed in brown algae such as Padina australis and S.marginatum (Akbary et al., 2021b), which is in line with the results of the present study. It could be stated that the type of these compounds varies by species in environmental conditions. These compounds are among the secondary metabolites and the secondary metabolites of any living organism vary or transform into similar derivatives under environmental conditions (Desmond and Gribaldo, 2009). Given the considerable medicinal value of sterols, including campesterol, stigma sterols can serve as suitable sources in the pharmaceutical industry, where these sterols play a substantial role in preventing the growth of cancer cells and cardiovascular diseases (Fernandes and Cabral, 2007).
The antioxidant property grows with an increase in the content of the phenolic and flavonoid compounds. Flavonoids and tannins are highly capable of removing free radicals, and this capability is mainly determined by the number of aromatic rings and the nature of the moving hydroxyl groups (Akbary et al., 2021c).The results of this study mirrored the direct link between an increase in MPE concentration and the phenol and flavonoid content, while the phenol and flavonoid content in shrimps fed with MPE0 was zero. According to the results of this study, the phenol and flavonoid content of the premix aqueous extract of the studied brown macroalgae included 83.46 ± 7.07mg of GAE g− 1 extract and 10.01 ± 0.98mg QE g− 1 dry extract. After assessing the total phenol content and antioxidant properties in three macroalgae species from Chabahar coast in Iran, Akbary et al. (2021c) reported that the total phenol content in the aqueous extract of certain species of brown algae namely P. australis (69.66 ± 2.08 mg GAE g− 1extract), S. marginatum (72.33 ± 2.08 mg GAE g− 1extract) and Ahnfeltiopsis pygmaea (76 ± 2 mg GAE g− 1extract) indicated that the phenol content in the aqueous extract of the three macroalgae species was lower than the macroalgae premix as compared with the present study. It could be stated that different macroalgae species contain different amounts of phenolic and flavonoid compounds (Hongayo et al., 2012). Previous research is also indicative of the efficiency of brown algae for antioxidant activities. For instance, Tenorio-Rodriguez et al. (2017) reported that among the 17 large algae including green, red and brown algae, the brown macroalgae extract was found to have the highest antioxidant activity and sources of natural bioactive compounds, and the antioxidant activities of algae can be attributed to the presence of various secondary metabolites such as phenolic compounds and carotenoids (Hongayo et al., 2012). Besides, phenolic compounds of macroalgae are of importance as potential factors in improving the health and performance of aquatic animals (Naiel et al., 2020 ; Naiel et al., 2021).
According to the results of this study, the inclusion of MPE in the diet improved the antioxidant activity of the L. vannamei shrimp as compared with shrimps on the control diet. Akbary and Aminikhoei (2018a) reported that the inclusion of 1.5 g kg− 1 of U. rigida algae extract in the diet of shrimps significantly increased the activity of SOD and reduced glutathione. Akbary et al. (2021a) also indicated that the 1-gram concentration of I. stellate increased the level of SOD (19.32 U mg− 1 protein), GPX (249.06 U mg− 1 of protein), CAT (31.19 U mg− 1 of protein), and phenol oxidase (31.19 U mg− 1 of protein), which is seemingly in line with the results of this research. The phenolic compounds in MPE can reduce oxidative stress by removing free radicals (Shi et al., 2005). Concentration of MDA is indicative of toxic processes triggered by free radicals, and the level of MDA serves as a good indicator of the level of lipid peroxidation (Peixoto et al., 2016). Based on the results of this study, the lowest MDA level was observed in MPE15, which complies with the research by Peixoto et al. (2016) and Akbary et al. (2021a).