Optimized continuous bioproduction of ectoine from carbon dioxide and industrial contaminants using Guyparkeria halophila

The production of high-value compounds such as ectoine in the pharmaceutical industry faces significant challenges, including high costs, resource intensity and reliance on refined sugars. This study presents a novel bioproduction platform converting carbon dioxide (CO 2 ) and the industrial contaminant thiosulfate (S 2 O 3 2- ) into ectoine using the halophilic strain Guyparkeria halophila . To overcome limitations in biomass accumulation and incomplete S 2 O 3 2- oxidation, cultivation and operational parameters, including S 2 O 3 2- loading rate, pH, and dilution rate, were systematically optimized in continuous stirred tank reactors. A S 2 O 3 2- loading rate of 5 g d -1 supported higher specific ectoine accumulation and promoted complete S 2 O 3 2- oxidation, while a moderate pH increase up to 7.6 further improved CO 2 assimilation. Additionally, implementing a prior semi-batch operation followed by a low dilution rate stage (0.10 d -1 ) effectively enhanced biomass and ectoine productivity. Under these optimized conditions, biomass accumulation increased significantly to 290.0 ± 20.2 mg L -1 , with specific ectoine contents of 387.3 ± 23.1 mg Ect g biomass -1 and productivities of 10.6 ± 0.6 g Ect m -3 d -1 . This work demonstrated a scalable, efficient and sustainable platform for ectoine biosynthesis that integrates CO 2 valorization and industrial by-product utilization, highlighting the potential of halophilic microbes for greener and economically viable pharmaceutical manufacturing. • First demonstration of continuous ectoine production from CO 2 and S 2 O 3 2- . • Specific ectoine content consistently reached 38.7% of biomass dry weight. • Optimized operational parameters overcame low biomass & incomplete S 2 O 3 2- oxidation. • Initial semi-batch operation with low dilution rates enhanced ectoine productivity. • Demonstrates a scalable, continuous bioprocess for CO 2 and S 2 O 3 2- valorization.