Source: SEAMEO BIOTROP's Research Grant | 2020
Aquaculture is one of the leading sectors producing high-protein food sources. Data from FAO shows that the trend of aquaculture production for food purposes is increasing every year compared to production from the fishing sector which is experiencing stagnation due to changes in marine productivity and other anthropogenic factors (Barange et al. 2014; Brander 2007; Burge et al. 2014; Cochrane et al. 2014 al. 2009; FAO 2018; IPCC 2007). With the increasing consumption of fish per capita, increasing the productivity of the aquaculture sector is a priority to maintain the value of fish consumption per capita, which is also increasing yearly. One of the main challenges in the aquaculture sector today is the climate change phenomenon. In a publication by Cochrane et al. (2009), stated that the critical impacts of the climate change phenomenon on the aquaculture sector are (1) changes in the environment and water fertility (2) changes in the spread of disease. Both of these impacts must be anticipated because they can reduce aquaculture production. Publications by Burge et al. (2014); Harvell et al. (2002); Marcogliese (2008) show that there is not only evidence of the effect of climate change on the spread of disease in various aquatic animals ranging from fish to mollusks, but also an increase in intensity in the pathogen transmission rate in aquatic animals along with increasing air temperature. With the danger of increased pathogen transmission, engineering is necessitated to improve the health status of fish.
One effort that can be performed to improve the health status of fish is to provide feed containing functional ingredients. Functional materials are materials that have tertiary functions that are beneficial to the body's physiology as well as being a source of nutrients and energy sources (Herrero, Cifuentes, and Ibanez 2006). The concept of functional food/foodstuffs was first introduced in Japan (Arai 1996).
One component that has the potential to be developed as a functional ingredient is arginine. Arginine is one of the essential amino acids for fish that plays a role in the formation of various compounds ranging from ornithine, citrulline, nitric oxide, ammonia, proline, and polyamine. Based on previous research, arginine has potential as a growth promoter and can increase immunocompetence of fish by nitric oxide production (Barziza, Buentello, and Gatlin III 2000; JA Buentello and Gatlin III 2001; J. Buentello and Gatlin III 1999; Cheng Bu, Buentello, and Gatlin III 2011; Cheng, Gatlin III, and Buentello 2012). Moreover, labscale studies show the importance of the availability of arginine in the body to activate macrophage through nitric oxide production (Baydoun et al. 1994; Norris et al. 1995; 2 Wijnands et al. 2012, 2015). Fish have a significant difference in arginine metabolism compared to mammals. In mammals, arginine can be recycled through the ornithine-urea cycle because it has the carbamoyl phosphate synthase enzyme, which plays a role in converting ornithine to citrulline. Meanwhile, in most teleosts, the CPS enzyme is not found/does not have high activity; hence cannot convert ornithine to citrulline. Therefore, oral arginine supplementation at doses above the optimum dose will not increase the availability of arginine in the body of fish (Chiu, Austic, and Rumsey 1986; Fournier et al. 2003; Huggins, Skutsch, and Baldwin 1969; Wright, Felskie, and Anderson 1995 ).
One of the non-essential amino acids that can increase the availability of arginine in the body is citrulline. Previous research conducted on Rainbow trout fish shows that increasing citrulline in the body can also increase arginine concentration (Fauzi et al. 2019). Research on several terrestrial animals also shows an increase in the availability of arginine if citrulline is supplemented to the subject animals (Cohen and Hayano 1946; Curis et al. 2005; Deutz 2008; Wijnands et al. 2012). Citrulline is a non-essential amino acid that is found in Citrullus lanatus watermelon. Some reviews mention that citrulline has enormous potential in increasing the availability of arginine in the body even compared to supplementation with arginine itself (C. Breuillard, Cynober, and Moinard 2015; Osowska 2004).
Citrulline supplementation is also known to increase the production of nitric oxide, a compound that has a vital role in innate immune systems in laboratory tests using cell culture (Baydoun et al. 1994; Charlotte Breuillard et al. 2017; McKinley-Barnard et al. 2015; Rapovy et al. 2015 al. 2015; Su and Austic 1999; Vadgama and Evered 1992; Wijnands et al. 2012, 2015). Therefore, citrulline has a high potential to be applied as a supplementation in feed to increase the availability of arginine in the body and to improve the immune status of fish.
Of the various raw materials available, watermelons are known to contain high levels of citrulline even in the green skin pulp area, which is considered as agro-industrial waste (Rimando and Perkins-Veazie 2005). Research on the use of watermelon pulp as a source of citrulline in fish feed has never been done before, although previous studies using watermelon in humans showed the ability of this material in increasing the availability of arginine in the body (Collins et al. 2007; Wu et al. 2007).