Effects of irradiance and pH on growth, photophysiology, and high-value metabolites production in Chlamydomonas acidophila

Acidophilic microalgae represent a promising but underexplored source of bioactive metabolites with potential biotechnological applications. Herein, we evaluated the influence of pH and irradiance on the growth performance, photophysiology, and metabolite profiles of a strain of the acidophile Chlamydomonas acidophila . A factorial experimental design evaluated three pH levels (3, 5, and 7) and two irradiance intensities (250 and 500 µmol photons m –2 s –1 ), assessing growth, photosynthetic efficiency, fatty acid composition, carotenoid and phenolic profiles, and antioxidant activity. Growth responses differed markedly across treatments, cultures at pH 3 and 7 reached nearly 8.4 × 10 6 cells mL⁻ 1 , whereas at pH 5, regardless of irradiance, C. acidophila exhibited lower cell densities. Fatty acid profiles remained relatively stable among the treatments and were dominated by α-linolenic (~ 24.3%) and palmitic acids (~ 18.17%). In contrast, carotenoids and phenolics exhibited strong modulation by pH and irradiance: lutein accounted for more than half of total carotenoids and reached 1.56 ± 0.01 mg g⁻ 1 at pH 7 under 250 µmol photons m –2 s –1 irradiance, whereas vanillic acid reached its highest concentration (282 µg g –1 dry weight) at pH 3 under 500 µmol photons m –2 s –1 . Antioxidant activity was likewise condition-dependent, with methanolic extracts showing the strongest responses, particularly at pH 7 under low irradiance (44.98 ± 1.39% in the ABTS assay). Collectively, these results reveal striking physiological plasticity and metabolic reorganization in C. acidophila , positioning this acidophilic strain as a promising and unconventional chassis for the high-value production of vanillic acid, lutein, and antioxidant compounds under scalable cultivation conditions.