Brown algae such as Ascophyllum nodosum (AN) and Fucus vesiculosus (FV) are gaining considerable attention as functional feed additives due to their health-beneficial properties. This study evaluated the antioxidant potential of AN and FV extracts in intestinal epithelial cells and the in vivo model Caenorhabditis elegans (C. elegans). Aqueous AN and FV extracts were characterized for total phenolic content (TPC), antioxidant capacity (TEAC, FRAP), and phlorotannin composition using LC-HRMS/MS. Antioxidant effects were assessed in vitro, measuring AAPH-induced ROS production in Caco-2 and IPEC-J2 cells via H₂DCF-DA, and in vivo, evaluating the effects of paraquat-induced oxidative stress and AN or FV treatment on worm motility, GST-4::GFP reporter expression, and gene expression in C. elegans. FV exhibited higher total phenolic content, antioxidant capacity (TEAC, FRAP), and a broader phlorotannin profile (degree of polymerization [DP] 2–9) than AN (DP 2–7), as determined by LC-HRMS/MS. Both extracts attenuated AAPH-induced oxidative stress in epithelial cells, with FV showing greater efficacy. In C. elegans, pre-treatment with AN and FV significantly mitigated a paraquat-induced motility decline by 22% and 11%, respectively, compared to PQ-stressed controls. Under unstressed conditions, both extracts enhanced nematode healthspan, with significant effects observed at 400 µg/g for AN and starting at 100 µg/g for FV. Gene expression analysis indicated that both extracts modulated antioxidant pathways in unstressed worms. Under oxidative stress, pre-treatment with AN and FV significantly reduced GST-4::GFP expression. In the nematode, AN was more protective under acute stress, whereas FV better supported physiological function in the absence of stressors. These findings demonstrate that AN and FV counteract oxidative stress in intestinal epithelial cells and in C. elegans, highlighting their potential as stress-reducing agents in animal feed.

The restriction of antimicrobial growth promoters in livestock production has intensified the search for nutritional strategies that support intestinal health while modulating inflammatory processes. Chronic or dysregulated inflammation can impair gut function and animal performance, highlighting the need for functional feed additives. Brown macroalgae are rich in bioactive compounds with immunomodulatory properties, though their mechanisms remain incompletely understood. In this study, the anti-inflammatory and barrier-protective effects of aqueous extracts from Ascophyllum nodosum (AN) and Fucus vesiculosus (FV) were investigated using complementary in vitro and in vivo models. Extracts were prepared by aqueous solid–liquid extraction and tested in lipopolysaccharide (LPS)-stimulated RAW264.7 and THP-1 macrophages, HEK-Blue TLR4 reporter cells, and Drosophila melanogaster models of intestinal inflammation and infection. Both extracts significantly reduced LPS-induced nitric oxide production in RAW264.7 macrophages in a concentration-dependent manner. In THP-1 macrophages, AN and FV attenuated secretion of inflammatory mediators, including TNF-α, IL-6, IL-33, CXCL9, CXCL10, CXCL11, and CCL7. Reporter assays demonstrated selective inhibition of TLR4-dependent NF-κB activation. In Drosophila melanogaster, supplementation reduced intestinal barrier disruption, mortality, and infection-induced immune activation. Overall, AN and FV attenuate inflammatory signaling and protect intestinal integrity via TLR4-dependent NF-κB inhibition, supporting their potential as functional feed additives to enhance gut health and resilience in livestock.