First insight into the practical dietary application of Torula yeast single cell protein in gilthead sea bream

The growing global demand for sustainable and alternative protein sources in aquaculture has driven research into non-conventional feed ingredients, such as single cell proteins (SCP), as substitutes for fish meal (FM) and vegetable protein sources. The effects of dietary SCP from Torula yeast on growth, plasma biochemistry, and gut health were investigated for the first time in gilthead sea bream juveniles. Fish (initial weight: 24.7 ± 0.04 g) were fed for 76 days with four experimental diets containing increasing levels of SCP meal (0 % CTRL, 5 % SCP5, 7.5 % SCP7.5, and 10 % SCP10) as a substitute for FM. After the growth experiment (T1), fish were exposed to sub-optimal rearing conditions (temperature: 30 °C ± 0.4 °C; oxygen: 4.79 ± 0.51 mg L−1) for 8 days (T2). At T1, SCP7.5 exhibited growth and feed utilization more comparable to the CTRL group, whereas both SCP5 and SCP10 induced a drop. Additionally, SCP diets reduced AST, ALT, CK, and LDH plasma levels, particularly in the SCP10 group. Regarding the immune response, SCP7.5 enhanced local immunity, demonstrating the activation of both pro-inflammatory and anti-inflammatory pathways before and after exposure to high temperature-low oxygen conditions. The gut microbiome composition was influenced by dietary SCP, showing an increase in Bacillus sp. at both T1 and T2. According to PICRUSt2 analysis of gut microbiota functionality, the SCP5 and SCP10 diets promoted histidine metabolism. In conclusion, the findings indicated that 7.5 % SCP from Torula yeast can effectively replace FM. Additionally, Torula yeast exhibited functional properties like hepatic antioxidant function, and activating pro-inflammatory and anti-inflammatory pathways at the local level.