Biomass Production, Phosphorus Use Efficiency, and Fatty Acid Composition in Phaeodactylum tricornutum Cultivated in Wastewater from an Atlantic Salmon Recirculating Aquaculture System

Abstract Recirculating Aquaculture Systems (RAS) are becoming increasingly important as a sustainable method for fish production. However, further advancements are needed to enhance the sustainability of RAS, particularly in the management of waste-streams. Furthermore, the capital and operational expenditures of running a RAS is high, so solutions turning waste-streams into added value is a focus area for the industry moving forward. Wastewater is high in nitrogen, and phosphorus (P) is foreseen to be the limiting nutrient. Here we evaluated the growth dynamics, biomass production, nutrient uptake, phosphorus use efficiency (PUE), and fatty acid composition of the diatom Phaeodactylum tricornutum cultivated in aquaculture wastewater (AWW) from a land-based Atlantic salmon RAS. The diatom was cultivated in various treatments at pilot scale (10 L), including 100% AWW and AWW supplemented with silicon (Si) and P, and compared with a control treatment using Guillard’s F/2 medium. The highest dry weight biomass concentrations of ∼2.0 g L⁻¹ was achieved for the nutrient-supplemented AWW treatments, even outcompeting the control, demonstrating its viability as a growth medium when supplemented with Si and P. The control treatment achieved the highest specific growth rate of 0.422 d⁻¹. The “Si+P addition” treatment achieved the highest PUE, indicating that co-supplementation strategies can enhance phosphorus utilization. However, fatty acid yields, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), were lower in AWW-based treatments, likely due to micronutrient deficiencies or the effect of possible contaminants. Despite this, P. tricornutum cultivated in AWW offers a sustainable and possible cost-effective solution for producing biomass and valuable fatty acids. Competing Interest Statement The authors have declared no competing interest. Footnotes CRediT authorship contribution statement: SBS: conceptualization, methodology, investigation, formal analysis, visualization, and writing the original draft. TMG and KS: Contributed to the methodology and provided supervision. TMG, KS and VD participated in writing, reviewing and editing the manuscript. TMG: Funding acquisition. All authors reviewed and approved the final version for submission.