Introduction
Fish health management in aquaculture is a fundamental axis for the sector’s performance and sustainability. Fish are frequently exposed to aggressions inherent to the culture conditions jeopardizing their homeostasis reducing growth and defensive performance. The gut and associated microbiota comprise a pivotal organ with key role in digestion, nutrient acquisition, metabolism, immune defense and also endocrine and neuronal regulation that are highly responsive to environmental changes. These changes often result in higher epithelial permeability, inflammatory responses and dysbiosis associated with reduction of functionality. Thus, gut health management is crucial for fish health and nutraceutical strategies can be applied to support gut health and function.
Algae are one of the most abundant marine resources and their nutritional properties have made them highly valued ingredients for human and animal nutrition. In addition, their diverse composition in bioactive substances such as polysaccharides (e.g. fucoidans, alginic acid, ulvans), polyphenols (e.g. phlorotannins) and pigments (e.g. fucoxanthin, zeoxanthin) makes them targets for human medicine and cosmetics due to their anti-inflammatory, antioxidant, antimicrobial properties, and their ability to stimulate cell and tissue repair. In aquaculture their potential has been highlighted as alternative protein and fatty acids source, however their prebiotic and functional properties indicate a possible relevant application in fish nutraceuticals for gut health management and performance .
The aim of this study was to assess the potential of algae biomass as gut health promoters for gilthead seabream ( Sparus aurata), using a gut explant model.
Material and methods
Gilthead seabream (210±10 g) were used to obtain the explants cultures. Intestines were dissected in aseptic conditions, separated in anterior and posterior sections, and were cut in small portions. These small sections were open to expose the lumen and were placed in plastic plates with lumen facing up. Medium was added to the wells and tissues were rested for a short period to acclimate. Several algae biomasses were tested, and these were the macroalgae Gracilaria gracilis , Fucus sp. , and Ulva sp. , and the microalgae Nannochloropsis sp. , Phaeodactylum tricornutum , Skeletonema sp., Isochrysis galbana and Tetraselmis sp..
Algae were used in their commercial form in dried biomass, and their protective and therapeutic effects were tested in the explants by application of suspensions in culture medium . Gut explants were stimulated with a commercial lipopolysaccharide (LPS) to promote an inflammatory response, and algae protective effect was tested with previous incubation of the tissue with an algae suspension (6h), whereas the therapeutic potential included incubation of the tissue only after LPS exposure (2h). Explant’s stability was assessed by measuring the lactate dehydrogenase activity in culture medium, and tissue response was assessed by measuring the expression of a panel of genes. These included genes related with inflammatory response (e.g. IL1b , COX2 ), adaptive immune response (e.g. MHCI , mIgM ), epithelium integrity ( e.g. OCL , CLD) and antioxidant response (e.g. CAT , GPx ). Multivariate integrative statistical approach was used to discriminate and rank responses to infer the potential of the different biomasses.
Results and Discussion
Seabream explants were responsive to commercial LPS, and after 6h an engaged inflammatory response was observed with alterations in tissue epithelial integrity at molecular level . When gut tissues were incubated with algae suspensions a modest immune modulation was observed with some species (mostly microalgae) . The protective effect of algae was reflected in an overall attenuated response to the LPS exposure, although not always significant. Algae suspensions homogeneity are relevant for tissue stimulation, and especially in the case of macroalgae, the biomass fragments size variability might influence gut explant response. However, when applied as therapeutic strategy after inflammatory cascade was triggered, gut response was soothed indicating a potential regulatory action exerted by algae.
This model allows a fast screening and ranking algae potential as gut performance enhancers, reducing the number of animals used and still accounting for the complexity of the intestinal multiple cell/bacteria interactions and arrangements. Algae biomass have shown a strong potential as functional ingredients for fish gut health management , and it is relevant to further explore the optimization of their functional properties by, for instance, increasing the availability of their bioactive compounds with cell disruption or other cost-efficient methods.