Algal carbon assimilation by Senegalese sole (Solea senegalensis) from a compound diet including 13C-enriched Ulva ohnoi meal

The incorporation of macroalgae in formulated fish diets is an important goal for fish farming sustainability that is under current development. While several studies have evaluated the effects of algae on growth or functional aspects in fish, the fraction of algae biomass that is effectively incorporated into their tissues has not yet been assessed. This is a key issue to understand the potential that macroalgae may have to replace conventional food resources whose availability is limited. In present study, a compound feed that included the green seaweed Ulva ohnoi enriched in the stable 13C isotope (δ13C = 1235‰) was used to determine its nutritional contribution in a carnivorous fish species such as Senegalese sole (Solea senegalensis). Under isotopic equilibrium with the diet, the negative discrimination factor found for algal 13C, both under natural isotope abundance (Δ13C = −11.1167‰), and for algae that were isotopically enriched (Δ13C = −684.58‰) suggested that certain algal constituents were selectively digested. Fitting 13C uptake kinetics in sole tissues over time determined that 37.05 ± 2.01% and 36.35 ± 2.76% of the algal carbon was incorporated into muscle and liver, respectively. The metabolic contribution of 13C to isotope turnover was 21.7% in muscle and 64.9% in liver. The isotopic enrichment of the algae resulted in more stable C assimilation estimates compared to those obtained from food sources with natural isotope abundance. In general, variability of results was less affected by lipid extraction when 13C-enriched alga was used, thus achieving less dependence for a sample pre-treatment whose application is under debate. In the non-lipid fraction of muscle and liver, C assimilation was estimated at 24.1% and 29.0%, respectively. In the lipid fraction, assimilation estimates above 100% in both tissues were consistent with a strong lipid routing effect and also with metabolic processes that incorporate C of non-lipid origin into new lipid compounds. Results obtained in this study suggest that U. ohnoi represents a good dietary source of proteins and lipids for sole, while carbohydrates do not appear to be used. •First use of isotopically enriched alga in a flatfish diet.•Around 37% of algal carbon assimilated into Senegalese sole tissues.•Algal protein and lipid mainly contribute to assimilated carbon in fish.•Very poor nutritional use of algal carbohydrates.•High lipid routing effect from Ulva ohnoi to Solea senegalensis.