A continuous flow membrane bio-reactor releases the feedback inhibition of self-generated free organic carbon on cbb gene transcription of a typical chemoautotrophic bacterium to improve its CO2 fixation efficiency

Protocol to analyze the inhibitory effect of free organic carbon generated by chemoautotrophic bacteria on their CO2 fixation

Here, we present a protocol to analyze the inhibition of self-generated extracellular free organic carbon (EFOC) on CO₂ fixation by chemoautotrophic bacteria. We detail the construction and operation of membrane reactor, followed by a simulation experiment to verify the inhibition of EFOC on CO₂ fixation. We further describe the analysis of main inhibitory components in EFOC and measurement of abundance and transcription level of ribulose bisphosphate carboxylase/oxygenase (RuBisCO) gene to clarify the mechanism of the main inhibition components on CO₂ fixation. For complete details on the use and execution of this protocol, please refer to Zhang et al. (2022).¹.

Continuous-flow membrane bioreactor enhances enrichment and culture of autotrophic nitrifying bacteria by removing extracellular free organic carbon

An activated sludge system can be inoculated with enriched nitrifying bacteria to enhance NH₄⁺-N removal, or enriched nitrifying bacteria can be added directly to a river to remove NH₄⁺-N. However, the enrichment culture is still generally inefficient and the technical bottleneck has not been clarified. Previous studies have shown that extracellular free organic carbon (EFOC) inhibits the growth of some autotrophic bacteria, and separating EFOC during culture with a membrane bioreactor (MBR) promotes the continuous growth of autotrophic bacteria and CO₂ fixation. However, whether a membrane bioreactor can also be used to enrich and culture autotrophic nitrifying bacteria by separating EFOC has not been verified. In this study, an MBR was constructed to separate EFOC during the culture of nitrifying bacteria in activated sludge to confirm that the MBR better enriches and cultures nitrifying bacteria than a sequencing batch reactor (SBR). Our results showed that after culture for 34 days, the rate of NH₄⁺-N removal and the nitrification rate by nitrifying bacteria in the MBR were 2.20-fold and 1.42-fold higher than in the SBR, respectively. The abundance of Nitrospira in the MBR was also 7.23-fold greater than in the SBR at the end of the experimental period. After 34 days, the average concentration of EFOC and the average EFOC/bacterial organic carbon ratio in the MBR were only 53% and 37% of those in the SBR, respectively. A correlation analysis suggested that the timely removal by the MBR of the EFOC generated during the culture process may be an important factor in promoting the growth of autotrophic nitrifying bacteria. The possible mechanism by which the MBR separates EFOC to the growth of promote autotrophic nitrifying bacteria is discussed from the perspective of the inhibitory effect of EFOC on cbb gene transcription. Our experimental results suggest a new approach to enhancing the enrichment of autotrophic nitrifying bacteria and extending the application of MBRs.

Bicarbonate uptake rates and diversity of RuBisCO genes in saline lake sediments

There is limited knowledge of microbial carbon fixation rate, and carbon-fixing microbial abundance and diversity in saline lakes. In this study, the inorganic carbon uptake rates and carbon-fixing microbial populations were investigated in the surface sediments of lakes with a full range of salinity from freshwater to salt saturation. The results showed that in the studied lakes light-dependent bicarbonate uptake contributed substantially (>70%) to total bicarbonate uptake, while the contribution of dark bicarbonate uptake (1.35-25.17%) cannot be ignored. The light-dependent bicarbonate uptake rates were significantly correlated with pH and turbidity, while dark bicarbonate uptake rates were significantly influenced by dissolved inorganic carbon, pH, temperature and salinity. Carbon-fixing microbial populations using the Calvin-Benson-Bassham pathway were widespread in the studied lakes, and they were dominated by the cbbL and cbbM gene types affiliated with Cyanobacteria and Proteobacteria, respectively. The cbbL and cbbM gene abundance and population structures were significantly affected by different environmental variables, with the cbbL and cbbM genes being negatively correlated with salinity and organic carbon concentration, respectively. In summary, this study improves our knowledge of the abundance, diversity and function of carbon-fixing microbial populations in the lakes with a full range of salinity.