Effects of bioelectricity on CO₂ capture by the symbiosis between microalgae and soil bacteria

Carbon sequestration can be expected to mitigate climate warming in sustainable agriculture. The soil structure can also be improved to reduce the application of fertilizers in the global carbon trading market. Agricultural CO₂ emissions sources are characterized by high dispersion and mobility with low concentration. It is highly required to develop a series of suitable CO₂ capture approaches in modern agriculture. Correspondingly, the coexistence of microalgae and flora has been commonly used in agricultural scenes, such as aquaculture farms, biogas digesters, composting, and irrigation systems. If this symbiotic system can be developed to consume CO₂ emissions, great potential can be gained in agricultural carbon sequestration. This study aims to explore the effects of bioelectricity on CO₂ capture by the symbiotic system between the chlorella and soil bacteria. A series of tests were then constructed to analyze the CO₂ capture and conversion of the symbiotic system under 1% v/v CO₂ emission. Furthermore, the bioelectricity generated by the bacteria was then collected by electrodes and external circuits. A comparison was finally made on the variations of CO₂ sequestration and bacterial distributions after the stimulation of bioelectricity. The results indicate that the symbiotic system shared the highest efficiencies of CO₂ removal, whether the external circuit was closed or not. By contrast, the CO₂ removal efficiencies of the pure soil bacteria were the lowest, compared with the chlorella and symbiotic systems. The CO₂ removal efficiency increased from 52% by chlorella to 81% by the symbiotic system with the bioelectricity. Dissolved organic carbon (DOC) that was consumed by the bacteria also enabled the chlorella to convert more CO₂ into dissolved inorganic carbon (DIC) and biomass. Among them, the concentrations of DIC and the biomass of the bioelectric-treated symbiotic system reached 1.4 mmol/L and 2.7 g/L, respectively, after seven days. The CO₂ was in situ reproduced in the symbiotic system via the metabolism of bacteria. The random collision between the microalgae and the bacteria cells effectively shortened the mass transfer distance from the CO₂ molecules to the chlorella cells, thus improving the mass transfer efficiency of CO₂. There was a great increase in the local CO₂ concentration of the medium. Some chlorella cells were then avoided to suffer the CO₂ concentrating mechanism (CCM). More energy was also saved for the rest enzymatic reactions to promote their growth. The bioelectric generation and transmission were accelerated to adjust the pH and dissolved oxygen content of the medium after the consumption of DOC. Therefore, the growth environment of the microbes was optimized to further promote the CO₂ conversion. The output voltage of the symbiotic system fluctuated with the light-dark period. A 200mV output voltage was achieved over a 1000 Ω external resistance in the illumination period. The maximum power density also increased by 46% from the dark to the light. Moreover, the symbiotic system was powered only by light energy with a limited carbon source supply entirely from 1% v/v CO₂ inlet. Compared with the pure soil microbial community, the chlorella was invaded to form a symbiotic system and then replaced the Leuconostoc with the Azoarcus. Simultaneously, the proportion of Citrobacter and Raoultella plummeted, whereas, the Rikenella, Tyzzerella, and Trichococcus increased significantly, leading to an overall enhanced diversity. The dominant bacterial composition remained almost unchanged in the microalgae-bacteria symbiotic system under the bioelectric stimulation. However, the Raoultella also re-emerged and then replaced Rikenella and Tyzzerella. In addition, the main components of Citrobacter, Alcaligenes, and Bacteroides reshaped the microbial community in the symbiotic system. KEGG metabolic pathway indicated that the microbial community was primarily formed the synergistic interactions with the microalgae. The soluble organic substances were also decomposed during microalgae photosynthesis and proteins in algae cell debris. This finding can provide a strong reference for in-situ CO₂ capture in agriculture.