<![CDATA[Microalgae utilize dissolved inorganic carbon species such as dissolved carbon dioxide (CO2), bicarbonate (HCO3-), and carbonate (CO32-) for photosynthesis, depending on medium pH. Carbonate salts have been investigated as an alternative inorganic carbon carrier in microalgal cultivation systems. This study evaluated the effects of different sodium carbonate (Na₂CO₃) concentrations on the growth behavior of Euglena gracilis under non-aerated cultivation conditions. Cultivation was conducted in F1 medium supplemented with Na₂CO₃ concentrations of 0, 20, 40, 60, 80, and 100 mg/L. Biomass was monitored using UV–Vis spectrophotometry at 680 nm and converted to dry weight (g/L). Rather than promoting growth, Na₂CO₃ supplementation generally accelerated biomass decline, which was analyzed using a first-order decay model. Higher Na₂CO₃ concentrations were associated with greater biomass decline, reaching up to approximately 49% reduction after 15 days. The decay rate constants were 2.2 × 10⁻³, 3.9 × 10⁻³, 2.5 × 10⁻², 3.79 × 10⁻², 4.26 × 10⁻², and 3.08 × 10⁻² day⁻¹ for 0, 20, 40, 60, 80, and 100 mg/L, respectively. The results indicate that Na₂CO₃ supplementation under acidic and non-aerated conditions did not improve E. gracilis growth within the tested concentration range. The observed biomass decline was likely influenced by multiple interacting factors, including altered ionic conditions and limited gas transfer. These findings provide an initial evaluation of Na₂CO₃ supplementation as an alternative inorganic carbon source for E. gracilis cultivation.]]>
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