Valorization of macroalgal biomass and agroindustrial by-products as sources of nutritional and functional ingredients for aquafeeds

Abstract

In current aquaculture, increased attention is paid in searching for new unconventional ingredients that can be used in the development of sustainable feeds able to provide essential nutrients or bioactive compounds with positive effects on the health and welfare of fish. In this sense, two categories of these ingredients, such as macroalgae biomass and agro-industrial by-products, present a great potential, not only for their nutritional value, but also for their contents in compounds with biological activity, mainly polyphenols. However, many of these ingredients have significant nutritional limitations, linked to the presence content of different antinutritive factors such as non-starch polysaccharides (NSPs), phytic acid, and in some cases inhibitors of digestive proteases. However, to counteract the potential negative effects of such compounds, different treatments (physical, chemical and enzymatic) can be used to improve the bioaccessibility of nutrients. Considering this, this Doctoral Thesis includes different experiments aimed to evaluate the potential of several unconventional ingredients that may be included in diets for different aquaculture species from a double perspective: i) as sources of nutrients, improving their nutritional value through enzymatic treatments, and ii) as a supply of compounds with possible beneficial effects on metabolism. Chapter 1 deals with the use of a macroalgae meal (the chlorophyte Ulva ohnoi) when included in feeds for two relevant species in European aquaculture (Dicentrarchus labrax and Sparus aurata), focusing on both improving the bioavailability of nutrients through a pretreatment with carbohydrases and on its possible supply of bioactives. The results show the possibility of using up to 5 % of U. ohnoi meal in the feed without compromising the zootechnical parameters, being the enzymatic pretreatment ineffective to improve its nutritional value. However, the inclusion of U. ohnoi determined an immunostimulatory effect, evidenced by an increase in the lysozyme activity of the mucus in the two mentioned species. Chapter 2 was oriented towards assessing the effect of an enzyme pretreatment with a multienzymatic complex (glucanases + phytase) of the vegetable ingredients used in two feeds for mullet (Mugil cephalus) on the digestive use of proteins and phosphorus. Assays were performed both in vitro and in vivo. The results of the in vitro test confirmed that in both feeds the enzymatic treatment significantly modified the potential bioavailability of some nutrients. The in vivo trial showed an improvement in protein digestibility in one of the feeds, but also a reduction in the net phosphorus digestibility in both, probably because the hydrolysis produced by phytase increased the amount of this nutrient in the feed digestive system above the intestinal absorption capacity of such nutrient. Chapter 3 evaluated the effect of enzymatic treatment of the same plant by-products used in the previous trial (brewer’s spent grain and rice bran), carried out by solid state hydrolysis (SSH) on growth and feed efficiency when they were included in a feed for M. cephalus. In a first test on a laboratory scale, fish that received the enzyme-treated feed showed a significant improvement in zootechnical parameters compared to those fed on the untreated feed, although not comparable to those of the commercial feed used as control. However, when carrying a test under real production conditions, values of growth and feed efficiency showed an equivalence between the enzyme treated and the commercial feeds and confirmed that the enzymatic pretreatment of plant by-products by SSH improves their nutritional value. Chapter 4 was proposed as a preliminary study aimed at evaluating the most determining factors affecting the digestive bioavailability of phenolic compounds present in bagasse and wine lees by means of in vitro digestive simulation models of two species with different feeding habits (S. aurata and M. cephalus). The results indicated that the presence of the feed matrix and the type of wine by-product have a significant effect on the digestive release of total and specific polyphenols, while the species had a considerable influence only for some specific polyphenols. Chapter 5 focused on the determination of the potential effects of the inclusion of two wine by-products (grape pomace and lees) in the feed on the growth, general metabolism, oxidative and immunological status of juvenile mullets (Liza aurata). The results showed a significant positive effect of grape pomace on feed efficiency, as well as on different indicators of the metabolic and immunological status of the fish, but also a dramatic change in the composition of the intestinal microbiota associated with lees consumption. In addition, a general improvement was found in the productive efficiency, physiological and immunological status of the fish when the feed was supplemented with any of the two by-products, both under normal production conditions and also after challenging subjecting the fish with a moderate acute hypoxia, largely due to its content in phenolic compounds.