Marine heatwaves (MHWs) are an emerging challenge for aquaculture, increasing the frequency and severity of disease outbreaks in farmed fish, weakening immunocompetence and compromising overall health and survival. As climate change stressors intensify, there is an urgent need for cost-effective and environmentally friendly strategies to enhance fish resilience. This study investigated the efficacy of Laminaria digitata, a brown macroalga, included in aquafeeds as powder (0.3% and 1.5%) or extract (0.3%) in improving the stress response of juvenile gilthead seabream (Sparus aurata) co-exposed to Vibrio harveyi infection during a category III MHW event. Under control conditions, L. digitata supplementation maintained or improved fish growth performance and overall well-being. After MHW exposure, the 1.5% powdered and 0.3% extract diets were more effective in mitigating thermal stress, reducing liver oxidative stress and lowering plasma cortisol levels. In infected fish, these diets improved resistance to V. harveyi, with reduced cortisol and alanine aminotransferase concentrations indicating hepatoprotective properties, and lower lipid peroxidation and decreased antioxidant enzyme activities reflecting an amplified capacity to counteract oxidative stress induced by inflammation. Overall, L. digitata is a promising aquafeed supplement, with the 1.5% powdered form offering a cost-effective alternative to the extract without compromising efficacy.

Extreme weather events, particularly marine heatwaves (MHWs), increasingly threaten aquaculture systems worldwide by impairing animal physiology and economical sustainability. This showcases the need to develop nutritional approaches that enhance animal performance under sub-optimal conditions. This study evaluated the effects of dietary supplementation with the brown macroalga Laminaria digitata (whole dried powder or extract) on the antioxidant status, digestive activity, and lipid metabolism of juvenile Sparus aurata exposed to a simulated MHW. Fish were fed four diets (control, 0.3% extract, and 0.3% or 1.5% powder) for 30 days before being exposed to a category III Mediterranean MHW. Under optimal temperature, macroalgae supplementation reduced oxidative status (lower catalase activity). The powder-feeds decreased lipid peroxidation, while the extract-feed elicited the opposite. All supplemented diets reduced proteolytic activity, and the extract-feed also decreased amylase activity. The MHW impaired gastrointestinal antioxidant defenses and liver lipid metabolism, decreasing catalase and glutathione S-transferase activities, as well as ΣPUFA n-6, 16:1 n-7, and 18:2 n-6 levels. The 0.3% powder-feed mitigated MHW-induced reductions in antioxidant activity, while both 0.3%-diets prevented thermal stress-related alterations on fatty acid profile. Overall, L. digitata powder at 0.3% was most effective at enhancing thermal stress resilience, supporting its value as a functional aquafeed ingredient.

Extreme weather events, such as marine heatwaves (MHWs), pose serious threats to the aquaculture sector, facilitating the occurrence of disease outbreaks and compromising farmed animals’ welfare and survival. Hence, finding eco-innovative strategies to improve animal immunocompetence is essential to assure aquaculture’s sustainability and resilience in a rapidly changing ocean. This study evaluated the immunostimulatory potential of Laminaria digitata powder (0.3% and 1.5%) and extract (0.3%) in juvenile gilthead seabream (Sparus aurata) exposed to a Vibrio harveyi outbreak during a Category III MHW event (T = 25.7 °C). Overall, L. digitata supplementation did not significantly affect fish immunocompetence under optimal rearing conditions (T = 21.4 °C; no infection), nor did it induce any adverse effects. However, both the powder (1.5%) and extract (0.3%) forms of L. digitata supplementation effectively mitigated the negative impacts prompted by the MHW and Vibrio harveyi infection—evidenced by improvements in fish health indicators, hematological parameters, leukocyte viability, granulocyte proportions, and reductions in peroxidase activity and immunoglobulin M levels. From an economic standpoint, supplementation with 1.5% L. digitata powder emerged as the most promising strategy, offering a practical balance between effectiveness and affordability for large-scale applications. These findings highlight the potential of L. digitata as an immunostimulatory aquafeed supplement, with promising benefits for fish health and resilience under adverse rearing conditions.

Plastic pollution is a major environmental concern. In humans, ingestion through contaminated seafood is a recognized exposure route to microplastics, which may impact gut health. However, the extent to which microplastics interfere with digestion and nutrient absorption remains unclear. To this end, the present work aimed to assess, for the first time, the influence of microplastic particles (polyethylene terephthalate, PET, and polylactic acid, PLA) on the digestibility of three selected seafood species (gilthead seabream, Sparus aurata; Atlantic salmon, Salmo salar; and hard clam, Mercenaria mercenaria) using an in vitro human digestion model. Furthermore, this study evaluated the potential degradability of microplastics along the gastrointestinal tract and examined how particle type and exposure level (10 or 20 particles) may influence seafood digestibility. Protein digestibility in S. aurata and S. salar filets was ~86%, while in M. mercenaria it was ~73%, regardless of microplastic presence or quantity. PET and PLA integrity was affected differently by digestion, with PLA showing greater surface degradation. These findings provide preliminary insight into the mutual interactions between microplastics and the human digestive process, highlighting the importance for further research into how the leaching of plastics additives may or may not influence the bioaccessibility of essential nutrients.

Immunity is crucial for the physiological regulation of organisms, serving as the primary defense against pathogens and environmental stressors. The isolation and analysis of immune cells provide key insights into immune responses to external pressures. However, the lack of harmonized protocols for less studied species, such as marine fish, often leads to technical and analytical challenges that hamper data interpretation and a thorough understanding of species-specific immune responses. This study aimed to set up an optimized flow cytometry-based analytical procedure to characterize and determine the viability of leukocytes from the head kidney (the main hematopoietic organ in teleost fish) of juvenile gilthead seabream (Sparus aurata). The procedure began with the isolation of leukocytes through a homogenization process using Hanks' balanced salt solution, followed by an optimized Percoll density gradient centrifugation method to ensure high recovery rates of leukocytes with minimal erythrocyte contamination required for efficient subsequent flow cytometry analysis. Additionally, a novel technique using a cell-reactive dye (LIVE/DEAD Fixable Dead Cell Stain Kit) was employed to distinguish viable from dead cells based on their fluorescent staining patterns. Fixation was achieved with 3.7% formaldehyde, preserving cell morphology, viability, and staining efficiency. Flow cytometry analysis successfully identified three predominant leukocyte populations: lymphocytes, monocytes, and granulocytes. This method not only allowed viability tests but also the accurate differentiation of cell types. The improvement in flow cytometry protocols represents a step forward in fish immunology by increasing the accuracy and efficiency of immune cell analysis. Furthermore, by allowing the fixation of cells for later analysis, this protocol significantly reduces the time and effort required for immune assessments, making it a valuable tool for both research and practical applications in various fields of research.