Metabolic Responses of Newly Isolated Microalgal Strains Cultured in an Open Pond Simulating Reactor Under Balanced Conditions and Nutrient Limitation

Microalgal strains—Picochlorum costavermella VAS2.5, Picochlorum oklahomense PAT3.2B and SAG4.4, Microchloropsis gaditana VON5.3, and Nephroselmis pyriformis PAT2.7—were evaluated in an Open Pond Simulating Reactor (OPSR) under varied conditions to assess their biomass yield and high-value metabolite production. Overall, the strains produced 269.1–523.0 mg/L of biomass under balanced growth conditions in modified Artificial Seawater, continuous illumination, and pH 8.5. Phosphorus limitation notably enhanced yields for SAG4.4 and PAT2.7 (529.0 ± 52.2 mg/L and 452.2 ± 21.0 mg/L, respectively). Conversely, nitrogen limitation reduced productivity. In most strains the glycolipid plus sphingolipid fraction was dominant. Significant quantities of 20:5(n-3) were traced in the cultures of VAS2.5 and VON5.3, while the PAT3.2B and SAG4.4 strains produced considerable amounts of 18:3(n-3). In contrast, the most interesting fatty acid synthesized by PAT2.7 was 16:1(n-7), which was also detected in significant quantities in VAS2.5 and VON5.3. Polysaccharide content remained stable across conditions (10–15%), and protein levels reached 45–50% under control and phosphorus-limited environments. Pigment synthesis peaked at control conditions. Overall, the biochemical profiles of these strains revealed their potential for use primarily as feed additives in the aquaculture sector.