Mixed electron donors synergistically enhance CO2 fixation of non-photosynthetic microorganism communities through optimizing community structure to promote cbb gene transcription

Abstract

Studies have shown that mixed electron donors (MEDs) can enhance the CO2-fixing efficiency of non-photosynthetic microbial communities (NPMCs), even up to the level of fixation observed when H2 is used as an electron donor. However, this promotion effect is not stable because its mechanism remains unclear. To elucidate the mechanisms involved, allowing further regulation and optimization of the MED system for improving the CO2-fixing efficiency of NPMCs consistently, cbb gene transcription level and efficiency, extracellular free organic carbon (EFOC) content as well as microbial structure of NPMCs under MED and other electron donor systems were investigated. MEDs synergistically promoted CO2 fixation efficiency of NPMCs, even producing levels seen when H2 was used as the electron donor. Subsequent experiments revealed that the cbb gene abundance and transcription level in the MED system were high compared with those in other single-electron donor systems; the concentration of EFOC per unit cell was relatively lower than that in any other electron donor system; and the system developed a large number of dominant heterotrophic bacteria such as Enterobacteriaceae and Vibrionaceae. Data analysis revealed a high negative correlation between EFOC concentration per unit cell and cbb gene abundance as well as gene transcription level. These results implied that MEDs can promote a complex microbial community structure enriched with high-efficiency heterotrophic bacteria, which can effectively reduce excessive EFOC generated by NPMCs in the CO2 fixation process, promoting overall cbb gene abundance and transcription level within the NPMC and thus enhancing CO2 fixation.