High irradiance compensated with CO₂ enhances the efficiency of Haematococcus lacustris growth

Haematococcus lacustris (H. lacustris), a promising source for astaxanthin production, is a light-sensitive microalga that is prone to sluggish growth when subjected to high levels of irradiance. A challenge in H. lacustris culture is to find a way to efficiently use illumination to maintain vigorous growth and harvest dense biomass, which is essential for further exploiting the potential for astaxanthin production. Previous studies have shown that in addition to illumination, carbon supply in culture is a key limitation for algae growth. Here, we investigated a combined culture approach involving high light intensity (110 μmol m^-2s^-1) and injection of a 1% (v/v) CO₂ air-gas mixture which provided an effective method for H. lacustris culture to achieve both a high growth rate and high cell density. The cell number in the group with high light exposure combined with CO₂ enrichment was increased almost four-fold compared with a high light group (110 μmol m^-2s^-1 without CO₂ injection). Additional experiments suggested a possible mechanism in which elevated CO₂ increases the electron sink capacity, thus alleviating photoinhibition and oxidative damage. The scaled-up photobioreactor demonstrated much better performance, with growth rates improved by 50-350 %, providing further evidence that this new method can improve algal cell production. Overall, our work provides an efficient way for H. lacustris culture and manufacture, with potential industrial applications.