Effects of Chlorella fusca-supplemented diet on intestinal microbiota and gene expression related to metabolism, stress, and immune response in Chelon labrosus

Microalgae are emerging as promising alternatives to traditional fishmeal and crop ingredients in aquaculture diets. In this study, we aimed to characterize the intestinal microbiota and evaluate the transcription of genes related to metabolism, stress, and the immune system in various tissues of thick-lipped grey mullet (Chelon labrosus) fed a diet containing 15 % of microalga Chlorella fusca against a microalgae-free control diet for 90 days. After this period, the fish were challenged with Aeromonas hydrophila and polyinosinic-polycytidylic acid (poly I: C) to assess their immune response during 72 h. Microbiota analysis revealed that dietary inclusion of microalgae did not produce significant differences in intestine diversity compared to the control group. The dominant genera differed between the groups, with Pseudomonas and Mycoplasma most abundant in the fish fed with C. fusca, and Brevinema, Cetobacterium, and Pseudomonas predominant in the control group. Gene expression analysis indicated significant changes in hif3 alpha, mhcII, ferritin, and mx genes in different organs of fish fed the microalgae-supplemented diet. Gene expression in the head kidney showed variations after A. hydrophila or poly I:C challenges, and higher peak values were observed in poly I:C-injected fish. Furthermore, c3 mRNA levels were significantly up-regulated in the fish fed with C. fusca at 24 h following A. hydrophila challenge. In conclusion, the inclusion of C. fusca in the fish diet modifies the composition of microbial species in the intestine but does not seem to affect its functionality. Regarding the influence on gene expression, the microalgae-supplemented diet effectively modulates the transcription of stress and immune-related genes, which may potentially enhance the fish resistance to stress and infections.