Filamentous fungi are promising microbial ingredients (MI) for use in aquaculture feeds due to their high protein content and bioactive components. Paecilomyces variotii (also known as PEKILO® mycoprotein) is a β-glucan- and nucleotide-rich MI with a crude protein content of about 60-70% (Bajpai, 2017). The objective of the current study was to evaluate the effects of P. variotii produced from sulfite stillage derived from forest by-products on pellet quality and growth performance, nutrient digestibility and utilization, and expression of immune-related biomarkers in the distal intestine of Atlantic salmon (Salmo salar) reared in freshwater.
Materials and methods
Four isonitrogenous, isolipidic, and isoenergetic diets were formulated. Diet 1 was a control diet formulated with fish meal, soy protein concentrate, and wheat gluten meal as protein ingredients. Diets 2, 3, and 4 were formulated so that P. variotii replaced 5, 10, and 20% of the crude protein content of the diets, respectively. Groups of 40 fish (initial average body weight of 24 g fish-1) were fed the experimental diets ad libitum in triplicate tanks for a period of 9 weeks. Daily feed intake in each tank was quantified by collection of uneaten feed using wedge wire screens. Fish were batch weighed at the start and end of the experimental period. Following the 9-week period, six fish per tank were randomly sampled and a small section of the distal intestine closest to the anus was collected and secured in RNAlater for gene expression analysis. Moreover, a pooled sampled of 20 whole fish at the start of the experiment and five fish per tank at the end of the experiment were randomly sampled for chemical analysis. The remaining fish in each tank were carefully stripped for fecal collection from the posterior intestine for determination of nutrient digestibility.
Results and discussion
Increasing levels of P. variotii in the feeds was associated with changes in physical pellet quality, including significant linear and/or quadratic decreases in pellet length, pellet width, expansion, and durability. Conversely, significant linear and quadratic increases in water activity, bulk density, sinking velocity, and water stability index were associated with increasing dietary levels of P. variotii. The changes in physical pellet quality may be attributable to the high total fiber and β-glucan content of P. variotii. There were no significant differences in weight gain, growth rate or feed intake among fish fed the experimental diets, but a significant linear improvement in feed conversion ratio with increasing dietary inclusion level of P. variotii was observed (Table 1). The apparent digestibility coefficients for crude protein and gross energy of the experimental diets decreased linearly with increasing dietary levels of P. variotii. Similar results were observed for the essential and non-essential amino acids. Nonetheless, linear increases in nitrogen, energy, and mineral retention efficiencies were observed with an increasing inclusion level of P. variotii. This has also been reported in salmon fed other MI including bacterial meal and yeast (Øverland et al., 2010; Øverland and Skrede, 2017). Interestingly, dietary inclusion of P. variotii resulted in significant upregulation in the expression of several cytokines (tnfa, ifng, il10, and tgfb), effector molecules (inos, arg1, sod), and transcription factors (irf4) in the distal intestine (Figure 1). These results suggest that P. variotii can induce the activation and control of the immune response through promoting pro- and anti-inflammatory responses, supporting immune homeostasis in the distal intestine, a mucosa-associated lymphoid tissue (MALT) (Morales-Lange et al., 2022).
Overall, replacement of up to 20% of the crude protein content of the feed with P. variotii improved feed conversion ratio and nutrient utilization efficiency of Atlantic salmon juveniles reared in freshwater. In addition, P. variotii also showed immunomodulatory effects in the distal intestine. P. variotii is a highly promising alternative ingredient for use in salmon feeds.
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