Potential of Bioethanol‐Derived Filamentous Fungi as Alternative Aquafeed Ingredients: Impacts on Nutrient Digestibility and Gut Microbiota in Rainbow Trout ( Oncorhynchus mykiss )

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Abstract
As global aquaculture expands to meet the growing protein demand, the industry faces mounting pressure to reduce reliance on conventional protein sources. Identifying alternative protein sources that are environmentally sustainable, resource‐efficient and nutritionally viable is crucial for the future of aquafeed development. In this regard, filamentous fungi have emerged as a promising alternative protein source due to their favourable nutritional profile and sustainable production potential. Although filamentous fungi contain high protein content and can be efficiently cultivated on organic waste materials, they are not yet utilised in commercial aquafeeds. In this study, juvenile rainbow trout ( Oncorhynchus mykiss ) were fed an extruded commercial‐like reference (Ref) diet and four experimental diets where Ref was mixed with 30% biomass of Aspergillus oryzae (AO), Neurospora intermedia (NI), Rhizopus delemar (RD), or Rhizopus oligosporus (RO) grown on thin waste from ethanol production. The fungal diets exhibited nutritional profiles and growth performance comparable to the Ref diet, despite significantly higher feed conversion ratios (FCRs) observed in the AO, NI and RO groups. Dietary digestibility was generally lower in the fungal‐diet groups than in the Ref group, although the digestibility of some amino acids in the AO group did not differ significantly from Ref. Dietary inclusion of fungal biomass induced noticeable shifts in gut microbial composition compared to the Ref diet, with the RO diet resulting in a distinct beta diversity profile. Firmicutes phylum members, including Weissella , Lactobacillus and Bacillus , were significantly more abundant in fish fed the NI, RD and RO diets, respectively. Overall, the evaluated filamentous fungi produced species‐dependent nutritional and microbiota responses in rainbow trout, although further optimisation is required to improve their nutritional value at the tested inclusion level.
Funding Information
  • Vetenskapsrådet (2022-06725)
  • Svenska Forskningsrådet Formas (2020-01129)
  • Familjen Kamprads Stiftelse (20202297)

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