Abstract

Chilean salmon farming still suffers from high mortality rates due to Salmonid Rickettsial Septicemia (SRS) caused by Piscirickettsia salmonis genogroups (LF-89 and EM-90). To modulate fish immunity against bacterial pathogens (without compromising host growth), we investigated the bioactive properties (e.g., iron-chelating or antioxidant activity) of sixteen peptides that were pre-selected from public databases and chemically synthesized following an Fmoc strategy. Then, prior to a four-week small-scale experiment with pre-smolts Atlantic salmon (Salmo salar), two synthetic peptides were chosen based on DPPH-radical scavenging activity, oxygen radical absorbance, antioxidant or iron-chelating function, as well as the modulation of immune-related biomarkers (e.g., ifny, nlrc3, il1b) in RTS11 cells or head kidney leukocytes (HKLs) from rainbow trout (Oncorhynchus mykiss). Peptides (p4426: antioxidant; p4429: iron-chelating) were microencapsulated and added (at 150 ?g kg?1) into a commercial-like diet (CD) to produce three other experimental feeds (AD: antioxidant, ID: iron-chelating, AID: p4426 and p4429 at equal concentration). After four weeks of feeding, data showed that fish's body weight and length increased with ID and AID diets (compared to CD), while ID also increased the number of red blood cells per mL. Furthermore, HKLs from fish fed synthetic peptides and stimulated with protein extracts of P. salmonis genogroups (LF-89, EM-90 or a mixture of both) modulated innate immune biomarkers such as chemokine il8 and effector molecules (i.e., arg1, sod1), compared to CD group. Therefore, the use of microencapsulated peptides with iron-chelating or antioxidant activities should be explored as potential feed additives to strengthen host immunity against P. salmonis. © 2025 The Authors