Evaluation of the Feasibility of Using CO₂ from Molasses-Based Ethanol Fermentation as a Carbon Source for Arthrospira platensis Cultivation in a Closed Tubular Photobioreactor

Abstract

Background: Carbon dioxide generated during ethanol fermentation is a biogenic carbon source that may support cyanobacterial cultivation, although its direct use in closed photobioreactors may be constrained by gas-liquid mass transfer, pH variation and reduced bicarbonate buffering. Aims: This study evaluated the feasibility of using off-gas from molasses-based ethanol fermentation as a supplemental carbon source for Arthrospira platensis cultivation in a closed tubular photobioreactor. Study Design: A. platensis was cultivated under three carbon-supply conditions: standard Zarrouk medium containing NaHCO₃ with ambient-air aeration; bicarbonate-free modified Zarrouk medium with ambient-air aeration; and bicarbonate-free modified Zarrouk medium supplied with fermentation off-gas. Place and Duration of Study: The experiment was conducted at the Institute of Life Sciences, Vietnam Academy of Science and Technology, Ho Chi Minh City, Vietnam, between March and September 2025. Methodology: Biomass concentration, medium pH, biochemical composition, inlet and outlet CO₂ levels and an apparent CO₂ retention indicator were evaluated. Results: The fermentation off-gas treatment reached a maximum biomass concentration of 0.79 ± 0.03 g/L, which was higher than that in the bicarbonate-free treatment with ambient air (0.66 ± 0.06 g/L) but markedly lower than that in standard Zarrouk medium containing NaHCO₃ (3.49 ± 0.02 g/L). The fermentation off-gas treatment produced biomass containing 52.43 ± 0.03% crude protein, 11.65 ± 0.05% carbohydrate, 6.14 ± 0.05% lipid, 7.12 ± 0.08 mg/g chlorophyll, 65.62 ± 0.11 mg/g phycocyanin and 1.29 ± 0.05 mg/g carotenoid. The apparent CO₂ retention indicator was approximately 1.8%, suggesting limited CO₂ retention under the current configuration. Conclusion: Fermentation-derived CO₂ can support A. platensis growth, but it did not replace bicarbonate completely under the tested operating conditions.