Management of growth and astaxanthin accumulation in the green microalgae Haematococcus pluvialis: effect of photosynthesis-related nutrients and light stress.

Haematococcus pluvialis is mainly known by its high astaxanthin content. This carotenoid, with strong antioxidant properties, is naturally accumulated in microalgal cells under stressful conditions (light intensity variation, nutrient deprivation, osmotic stress, etc.). Nevertheless, the cost-effectiveness of such methods at a large scale is controversial, in addition to their negative impact on cell growth. We examined the effectiveness of major nutrients (nitrogen, phosphorus, copper, and iron) modification in the culture medium as well as light intensity. This study is the first to report continuous monitoring of the red stage over a 60-day culture period on final astaxanthin yield, through both biomass change and pigment accumulation. Reduced light intensity prolonged the green growth stage, while elevated light intensity (150 µmol m-2 s-1) induced astaxanthin accumulation by over 140 mg/g. Nevertheless, growth was stopped early. Under nutrient starvation or depletion (N, P, Fe, Cu), growth decreased proportionally to their concentration, and even astaxanthin accumulation in zoospores. The fourfold reduction of P and N concentration (P/4, N/4) in the medium resulted in the highest productivity at 30 days, reaching 9.91 and 9.09 g/L of astaxanthin, respectively. Although yields declined after 40 days in most treatments, N/4 and N-limitation (N0) maintained the best results, indicating their efficiency in sustaining both astaxanthin accumulation and overall biomass. These changes in nutrient availability influenced the cells' transition to different forms, and the zoosporic form became a key stage for astaxanthin accumulation as the cells adapted to stress conditions.