Introduction: The transition toward sustainable food systems requires strategies that simultaneously reduce bioprocess residues and enhance nutritional quality. Microalgal biorefineries generate substantial polar side-streams after lipid extraction, which remain largely underutilized despite their high nutrient content. Methods: Here, we evaluated the polar side-stream generated during lipid extraction of Results: Within a non-inhibiting range, the side-stream significantly stimulated duckweed growth, with 0.2 g L Discussion: The absence of carotenoid overaccumulation and major shifts in antioxidant activity indicates nutrient-enabled protein enrichment without detectable stress-associated metabolic reprogramming. These findings demonstrate that microalgal lipid extraction side-streams can function as nutrient-regime modulators capable of redirecting carbon-nitrogen allocation toward protein-rich biomass, coupling circular bioeconomy valorization with nutritional upgrading of a fast-growing aquatic crop. This work provides a proof-of-concept for integrating biorefinery side-streams into sustainable micro-scale photosynthetic food systems.