Antioxidant, antibacterial, and immunostimulatory potentials of terrestrial and marine extracts from by-products and low-value biomass: an ex vivo study in gilthead seabream (Sparus aurata) head kidney leukocytes

The expansion of aquaculture and the drive toward more sustainable ingredients have promoted the incorporation of alternative and novel raw materials as alternatives to traditional marine raw materials, which can provide bioactive functions in addition to fulfill fish nutritional requirements. In this context, agro-industrial by-products and low-value marine biomass emerge as promising sources of antibacterial, immunomodulatory, and antioxidant bioactive compounds. Valorizing these raw materials within a circular economy framework offers the dual benefits of reducing waste and improving fish resilience. This study evaluated nine natural extracts of terrestrial and marine origin as potential functional ingredients for aquaculture. Terrestrial by-product extracts (TE) included pomegranate peel (rich in punicalagin or ellagic acid), citrus fruits, and grape seeds, whereas marine included marine macro- and micro-algal extracts (ME) (Rhodomonas lens, Desmodesmus sp., Osmundea pinnatifida, Gracilaria sp., and Dictyota sp.). Extracts were characterized by determining their total phenolic and flavonoid contents. Antioxidant activity was evaluated using two methods: 2, 2′-Azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) and Ferric Reducing Antioxidant Power (FRAP) assays. Antibacterial activity was assessed against Vibrio anguillarum, V. harveyi, and Photobacterium damselae subsp. piscicida. To assess the effects of extracts at cellular level, ex vivo assays were performed on head kidney leukocytes from gilthead seabream (Sparus aurata), evaluating cytotoxicity, respiratory burst, phagocytic activity, and peroxidase activity. TE showed higher levels of both phenolic compounds and flavonoids than ME, which are usually related to higher antioxidant activity. In addition, TE showed stronger antibacterial effects against the three pathogenic bacteria tested. However, ME in general terms, presented higher immunomodulatory potential, causing respiratory burst activation or higher peroxidase activity in leukocytes. These findings highlight distinct bioactivities depending on extract origin, suggesting that future in vivo studies evaluating the combined use of terrestrial and marine extracts may be of interest to explore potential complementary effects in aquaculture species such as Sparus aurata.