Abstract

Given the potential of algae as a new aquafeed ingredient, this study evaluated the effect of the tank-cultivated macroalgae Ulva rigida C. Agardh (Chlorophyta) on innate immune response, intestinal microbiota and their resistance against Photobacterium damselae subsp. piscicida in juvenile gilthead seabream (Sparus aurata L.) after a short pulse of administration. An algae-free diet was used as control, and an experimental diet was formulated containing 25% of U. rigida (UL-25). After 2, 7, 15 and 30 days biological samples were taken. No statistical significance (p > 0.05) was detected in respiratory burst activity. Diet UL-25 significantly enhanced alternative complement pathway activity respect to control diet from the beginning of the experiment (p 0.05). When fish were challenged against P. damselae subsp. piscicida, UL-25-fed fish were more resistant against the pathogen than the fish fed control diet, obtaining a relative survival percentage of 54.55%. The ability of UL-25 diet to modify the intestinal microbiota was also confirmed. In conclusion, the use of a short pulse of dietary administration of 25% U. rigida in S. aurata diet modulates the immune response, intestinal microbiota and reduced the mortality rate during P. damselae subsp. piscicida infection.