INTRODUCTION
Atlantic salmon (Salmo salar) aquaculture faces significant challenges associated with skin integrity loss, driven primarily by sea lice (Lepeophtheirus salmonis) infestation, but also mechanical handling and crowding stress. Skin lesions constitute major welfare and biosecurity concerns, functioning as entry points for opportunistic pathogens and are directly linked to reduced growth, increased mortality, and substantial economic losses. The wound microenvironment involves dynamic shifts in local microbial community composition alongside a transcriptional immune response both at the lesion site and in surrounding epithelial tissue, processes that have been increasingly characterized in recent years. Functional feeds represent a promising non-pharmacological strategy to actively support skin recovery following injury. Marine algae are of particular interest given their diverse bioactive compounds — including sulfated polysaccharides, phlorotannins, fucoxanthin, EPA, and β-glucans — with documented immunomodulatory, anti-inflammatory, and antimicrobial properties. Ascophyllum nodosum, Phaeodactylum tricornutum, and Chaetoceros muelleri have shown relevant biological activity in ex vivo assays and preliminary feeding trials, yet their therapeutic capacity to modulate wound healing at both the microbial community and transcriptional level in Atlantic salmon needs to be further explored. The present study evaluates the effect of dietary supplementation with these three algae, on the wound healing response in Atlantic salmon subjected to wound model, characterizing wound microbiota and local transcriptional profiles to provide an integrated assessment of host–microbiome dynamics during cutaneous recovery.
METHODS
Atlantic salmon were subjected to a two-step wound protocol, as an attempt to mimick the establishment of a sea lice-associated lesion, combining a standardized perifocal abrasion of mucus and scales followed by a central punch biopsy. Immediately following wounding, fish were assigned to one of four experimental diets administered throughout the 45-day recovery period: a control diet (CTRL) and three diets supplemented with Ascophyllum nodosum whole biomass (ASCOP), Phaeodactylum tricornutum hydrolysed biomass (PHAEO), or Chaetoceros muelleri broken biomass (CHAETO). Wound closure was monitored photographically over the full recovery period, encompassing the main healing stages: re-epithelization, inflammation, granulation, and maturation. At key time points corresponding to each stage, wound tissue samples were collected for microbiota community profiling and transcriptional analysis of stage-specific marker genes.
RESULTS
Photographic analysis of wound closure indicates a trend towards faster healing in fish fed PHAEO and CHAETO diets compared to CTRL and ASCOP. Transcriptional profiling across the four healing stages revealed differential expression of genes associated with immune activation (il1B, tnfa, mhcII), inflammatory regulation (il10, il6, cox2, ptgs2), and tissue remodeling (mmp9, mmp13, fn1, krt) between dietary groups. Microbiota community composition at the wound site differed among treatments, with further characterization currently underway. These findings suggest that therapeutic dietary inclusion of selected microalgae, particularly P. tricornutum and Chaetoceros muelleri, initiated immediately following injury, may actively support wound healing in Atlantic salmon. These results point to the potential of algae-based functional feeds as a post-injury nutritional intervention with practical welfare implications for salmon aquaculture. Further analysis will clarify the microbial and transcriptional mechanisms underlying the observed differences in healing dynamics.
Acknowledgment
This work is supported by the Horizon Europe project LOCALITY, funded by the European Union under Grant agreement ID 101112884. Views and options expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them. Bernardo Carvalho is the recipient of a Ph.D. scholarship supported by FCT (Ref. 2022.12615.BDANA).