Dietary improves the European seabass () inflammatory response against .

Introduction: Modern fish farming faces challenges in sourcing feed ingredients, most related with their prices, 21 availability, and specifically for plant protein sources, competition for the limited cultivation space for 22 vegetable crops. In that sense, halophytes have the added value of being rich in valuable bioactive compounds and salt tolerant. This study assessed the inclusion of non-food fractions of in European seabass diets.

Methods: Different levels (2.5%, 5%, and 10%) were incorporated into seabass diets, replacing wheat meal (diets ST2.5, ST5, and ST10) or without inclusion (CTRL). Experimental diets were administered to seabass juveniles (8.62 ± 0.63 g) for 34 and 62 days and subsequent inflammatory responses to a heat-inactivated subsp. () were evaluated in a time-course manner (4, 24, 48, and 72 h after the challenge). At each sampling point, seabass haematological profile, plasma immune parameters, and head-kidney immune-related gene expression were evaluated.

Results: After both feeding periods, most parameters remained unaltered by inclusion; nonetheless, seabass fed ST10 showed an upregulation of macrophage colony-stimulating factor 1 receptor 1 ( and cluster of differentiation 8 ( compared with those fed CTRL after 62 days of feeding. Regarding the inflammatory response, seabass fed ST10 showed lower plasma lysozyme levels than their counterparts fed ST2.5 and ST5 at 24 h following injection, while 4 h after the inflammatory stimulus, seabass fed ST10 presented higher numbers of peritoneal leucocytes than fish fed CTRL. Moreover, at 4 h, fish fed ST2.5, ST5, and ST10 showed a higher expression of interleukin 1β (), while fish fed ST5 showed higher levels of ornithine decarboxylase ( than those fed CTRL. An upregulation of macrophage colony-stimulating factor 1 receptor 1 () and glutathione peroxidase () was also observed at 72 h in fish fed ST10 or ST5 and ST10 compared with CTRL, respectively.

Discussion: In conclusion, incorporating up to 10% of the non-food fraction in feed did not compromise seabass growth or immune status after 62 days, aligning with circular economy principles. However, inclusion improved the leucocyte response and upregulated key immune-related genes in seabass challenged with an inactivated pathogen.