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 C-13 isotope (delta C-13 = 1235%o) 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 (& UDelta;C-13 = 11.1167%o), and for algae that were isotopically enriched (& UDelta;C-13 =-684.58%o) suggested that certain algal constituents were selectively digested. Fitting C-13 uptake kinetics in sole tissues over time determined that 37.05 & PLUSMN; 2.01% and 36.35 & PLUSMN; 2.76% of the algal carbon was incorporated into muscle and liver, respectively. The metabolic contribution of C-13 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 C-13-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.

Automated summary

This study evaluates the nutritional contribution of the green seaweed Ulva ohnoi in the diet of Senegalese sole. The results show that the seaweed is a good source of proteins and lipids for the fish, with high carbon assimilation rates in muscle and liver. The study also suggests that lipid extraction has minimal impact on the results.