Green and red macroalgae extracts show antibacterial effects and induce innate immune responses in Nile tilapia and rainbow trout in vitro

•Marine sulphated polysaccharides (MSP) have direct antibacterial effects in vitro.•The antibacterial effect of MSP is dependent on seaweed species and extraction.•MSP-rich extracts modulate fish immune responses.•Effects of MSP-rich extracts on head kidney leukocytes appear fish species specific.•Differences exist between seaweed species in MSP-driven immune modulation.