Enhancement of microalgal carbon fixation via synergistic interactions with a facultative autotrophic bacterium Cytobacillus KQ-2.

Microalgal carbon fixation has emerged as a promising strategy for sustainable carbon capture and storage (CCS). However, enhancing microalgal CO2 fixation efficiency under high-CO2 conditions remains a significant challenge. In this study, we explored the enhancement of microalgal carbon fixation via synergistic interactions with a facultative autotrophic bacterium under industrially relevant CO2 concentrations. A novel strain, Cytobacillus KQ-2, capable of metabolizing both inorganic and organic carbon as its carbon sources, was enriched and characterized. Genome analysis revealed genes associated with both the Calvin cycle (Calvin-Benson-Bassham) and the reductive tricarboxylic acid (rTCA) cycle, indicating inherent carbon fixation potential. To enhance carbon assimilation, a co-cultivation strategy integrating microalgae with KQ-2 under 20 % CO2 conditions was developed, resulting in a 52.11 % increase in total CO2 fixation over 10 days compared to microalgal mono-culture. Mechanistic studies revealed that KQ-2 accelerated CO2 hydration through carbonic anhydrase, stimulating carbon-concentrating mechanisms (CCMs) in microalgae. Metabolic cross-feeding between two microbes was observed, alongside enhanced in the TCA cycle, Calvin cycle, and amino acid metabolism. Additionally, levels of antioxidant substances increased, including gallic acid and oxidized l-glutathione, while ROS levels were reduced, indicating an improvement in oxidative stress responses. Electrochemical analysis demonstrated increased current density and cyclic voltammetry (CV) responses, indicating enhanced electron transfer in the co-culture, which provided a robust platform for sustainable carbon capture. Collectively, these findings reveal a synergistic microbial platform for efficient CO2 capture under industrial gas conditions.