Growth parameters evaluation of H. pluvialis biofilm for astaxanthin production

Microalgae are a diverse group of photosynthetic microorganisms with significant ecological and biotechnological relevance. Due to their rapid growth and high biomass productivity, they are increasingly explored for sustainable applications in biofuels, wastewater treatment, pharmaceuticals, and carbon capture. In this study, Haematococcus pluvialis was selected as it is the primary natural producer of astaxanthin, a carotenoid known for its powerful antioxidant properties and wide range of applications in cosmetics, dietary supplements, pharmaceuticals, and aquaculture. Currently, approximately 95% of astaxanthin is produced synthetically. However, due to the presence of isomers (3R,3’R and 3R,3’S), synthetic astaxanthin is unsuitable for human consumption and is limited to aquaculture use. In contrast, natural astaxanthin is safe for human use, with applications in health-related industries, and can reach prices up to $7000/kg. Its market is projected to nearly double by 2030, increasing the need for more efficient and scalable cultivation methods for H. pluvialis. The aim of this study is to evaluate the growth parameters of H. pluvialis cultured as a biofilm on a rotating system, using light intensity as the sole stress factor. Growth was assessed both experimentally via physiological measurements and theoretically, by modeling growth using the Han model, with parameters extrapolated from experimental data. H. pluvialis have been cultivated in suspension with the aim of extrapolating the necessary parameters and applying them on the rotating system. The measurements are focused on the green motile stage of the algae and not the red non-motile one, where the algae produces astaxanthin. This is done because the focus is on the growth, which takes place during the exponential phase in the green stage. The following parameters were analyzed: optical density (OD), cell count (CC), astaxanthin content, chlorophyll content, dry weight, photosynthetic rate (Pn), dark respiration rate (Rd), non-photochemical quenching (NPQ), quantum yield (Qy), and relative electron transport rate (rETR). The results were biologically sound and consistent with existing literature. A stable and reproducible cultivation protocol was established, ensuring reliable and consistent experimental outcomes. Three light intensities were initially tested: 20, 50, and 150 μmol/m²·s. The growth rate vs. light intensity curve suggests that at 150 μmol/m²·s, growth reaches a plateau, indicating light saturation and that optimal growth conditions are around this value. However, the highest photosynthesis rate was observed at 50 μmol/m²·s, while the lowest occurred at 150 μmol/m²·s, suggesting potential photo-inhibition at higher intensities. Despite this, NPQ, rETR, and Qy measurements indicate that the cultures were not under physiological stress, implying that the observed effects are likely due to different metabolic energy allocation rather than stress. Further experiments are underway at light intensities of 35, 100, and 175 μmol/m²·s to complete the μ vs. I curve, refine the Han model fitting, and gain deeper insight into the physiology and metabolic behavior of H. pluvialis under varying light conditions.