Comparative Effects of Marine Microalgae-Supplemented Diets on Growth Performance, Survival and Immune-Related Gene Expression in Nile Tilapia (Oreochromis niloticus) Challenged with Streptococcus agalactiae

This study evaluated the effects of dietary marine microalgae supplementation on growth performance, growth-and immune-related transcriptional responses, bacterial load and post-challenge survival in Nile tilapia (Oreochromis niloticus).Fish were fed a basal diet or diets supplemented with 5% (w/w) Chaetoceros muelleri, Tetraselmis chuii, or Thalassiosira weissflogii for 30 days.Growth performance and mRNA abundance of growth-related genes (GH, IGF1 and MSTN) were assessed at the end of feeding.Fish were subsequently challenged with Streptococcus agalactiae (110 5 CFU fish -1 ).Immune-related genes (hepcidin, IgM, IL-1β, MHC-II, TLR5 and TLR9) were quantified before challenge and at 1, 3 and 7 days post-infection (dpi) and bacterial load was determined at same post-challenge time points.Survival was monitored for 10 days.Fish fed C. muelleri and T. chuii-supplemented diets has significantly greater final body weight than controls, whereas average daily gain, feed conversion ratio and pre-challenge survival were unaffected.T. chuii increased GH and IGF1 mRNA abundance in liver and muscle, while MSTN expression decreased in all supplemented groups.Following challenge, C. muelleri and T. weissflogii produced greater transcriptional responses in several immune-related genes, including IgM, MHC-II, hepcidin, IL-1β and TLR9.Bacterial load was significantly affected by dietary treatment and sampling time, with lower loads in microalgaesupplemented groups at 3 and 7 dpi.Post-challenge survival was significantly higher only in the T. weissflogii group.Overall, the three microalgae species induced distinct physiological, transcriptional and protective responses, supporting their further evaluation as targeted functional feed ingredients for Nile tilapia.