The continuous increase in carbon dioxide concentration, as one of the main drivers of climate change, has highlighted the growing need for the development of sustainable strategies for reducing greenhouse gas emissions an‎d producing value-added biomass. The present study was conducted to investigate the biological fixation of carbon dioxide an‎d its conversion into green biomass an‎d subsequently carotenoid-rich red biomass, particularly astaxanthin, using the microalga Chromochloris zofingiensis. Cultivations were performed during the autotrophic growth phase under controlled conditions at two aeration rates (0.5 an‎d 2vvm) an‎d various carbon dioxide concentrations in both enriched an‎d control conditions. After reaching maximum biomass, the cultures were subjected to a shock phase consisting of nitrogen limitation an‎d high light intensity for a period of 40 to 70 days. Throughout all experiments, growth indicators, specific growth rate, carbon dioxide fixation rate, an‎d volumetric productivity were measured an‎d eva‎luated to determine optimal operational conditions. After selec‎ting the optimal operational conditions, the elemental composition of the biomass (CHNS), total lipid content, total protein content, an‎d specific growth rate at the end of the shock phase were determined for both green an‎d red biomass produced under optimal cultivation conditions. In addition, total carotenoid analysis was performed on red biomass powder obtained from cultures aerated at 0.5vvm under four different carbon dioxide concentrations in both control an‎d enriched conditions. The results showed that controlled carbon dioxide injection, compared with the control condition, led to increased green biomass production an‎d improved growth indicators. Cultivation at a carbon dioxide concentration of 0.2% an‎d an aeration rate of 0.5vvm was identified as the optimal condition in terms of growth performance, pH stability, green biomass dry weight (1.97 ± 0.05 g.L⁻¹), carbon dioxide fixation rate (0.25 g CO₂ .L⁻¹.day⁻¹), an‎d economic feasibility. Reducing the aeration rate while maintaining high biomass productivity resulted in effective mixing, reduced carbon dioxide loss, an‎d energy savings, which are significant from both economic an‎d environmental perspectives. Although the total carotenoid content per unit dry weight decreased in higher biomass densities due to the substantial increase in biomass concentration an‎d cellular self-shading, this reduction does not indicate a decline in the metabolic capacity of the microalga. Nevertheless, the obtained values (ranging from 0.77 ± 0.11% to 1.47 ± 0.10%) are considered noteworthy for autotrophic cultivation. Biomass produced under optimal growth conditions demonstrated a high capacity for effective conversion into carotenoid-rich red biomass after entering the shock phase. Chemical composition analysis of the biomass, including protein, lipid, ash, nitrogen, an‎d carbon content, fully explained the transition from green to red biomass as a result of carotenoid activity an‎d the applied shock conditions. Cultivation at the scale of a 70 L photobioreactor with a working volume of 40 L also confirmed the feasibility of industrial production an‎d the scalability of laboratory findings to a larger scale, providing relatively stable an‎d effective conditions in terms of microalgal growth, photosynthesis, an‎d biomass production, with a dry biomass concentration of 0.83 ± 0.13 g.L⁻¹. This study demonstrates that determining the optimal carbon dioxide concentration an‎d an appropriate aeration rate, together with controlled operational conditions, leads to the production of high-quality green an‎d red biomass, effective carbon dioxide fixation, economic efficiency, an‎d sustainable resource utilization, an‎d provides practical guidance for the development of economical an‎d industrial microalgae cultivation systems.

محمد هادي جزيني

حميد زيلوئي , محمدامين بوجاري