The Effects of Red–Blue Light on the Growth and Astaxanthin Production of a Haematococcus pluvialis Strain Isolated from Southern Thailand

Light is a crucial factor that influences algal production. Red and blue light have been widely utilized in two-stage cultivation approaches due to their significant roles in promoting algal growth and pigment accumulation. To investigate the effects of light on the growth of an isolated wild strain of Haematococcus pluvialis (H. pluvialis) and its astaxanthin production, white light (30 μmol photons m−2 s−1) mixed with various light intensities of red or blue light were applied to the algal culture during the stationary phase. The results indicate that white light combined with low-intensity red light (5 μmol photons m−2 s−1) significantly enhanced algal growth, achieving a maximum biomass of 0.43 g/L. The pH values in cultures exposed to all treatments involving red-light intensities were lower than those under solely white light conditions. Furthermore, the combination of white light and low-intensity red light improved photosynthetic efficiency, carbonic anhydrase (CA) activity, and the rate of CO2 fixation. In contrast, the mixture of white light with blue light at 15 μmol photons m−2 s−1 facilitated astaxanthin production, resulting in a maximum astaxanthin content of 6.75 mg/L. Blue light was found to increase reactive oxygen species levels, leading to elevated malondialdehyde (MDA) concentrations and enhanced catalase (CAT) activity. These findings suggest that red light plays a crucial role in activating CA activity and promoting cellular CO2 fixation, which helps maintain the stability of the culture medium pH and ultimately supports algal growth. Conversely, blue light contributes to photoprotective processes by accumulating protective pigments and enhancing the activity of protective enzymes; together, these mechanisms mitigate reactive oxygen species generated by blue light exposure. Our experimental results provide valuable insights for optimizing two-stage cultivation practices for isolated strains of H. pluvialis.