Role of electron donor in CO2 fixation of chemoautotrophic bacteria and its preconditions: Verification in Alcaligenes hydrogenophilus

Organic manure rather than chemical fertilization improved dark CO2 fixation by regulating associated microbial functional traits in upland red soils

Dark microbial fixation of CO2 is an indispensable process for soil carbon sequestration. However, the whole genetic information involved in dark CO2 fixation and its influence on dark CO2 fixation rates under diversified fertilization regimes were largely unclear. Here, revealed by 13C-CO2 labeling, dark CO2 fixation rates in upland red soils ranged from 0.029 mg kg-1 d-1 to 0.092 mg kg-1 d-1, and it was 75.49 % higher (P < 0.05) in organic manure (OM) soil but 44.2 % decline (P < 0.05) in chemical nitrogen fertilizer (N) soil compared to unfertilized (CK) soil. In addition, the normalized abundance and Chao1 index of dark CO2 fixation genes (KO level) were significantly different between OM and N soils, showing the highest and lowest, respectively. And they were positively (P < 0.05) correlated with dark CO2 fixation rate. Besides, among the identified CO2 fixation pathways in this study, the DC/4-HB cycle (M00374) was enriched in OM soil, yet the 3-HP cycle (M00376) was enriched in N soil, and their relative abundances were positively and negatively correlated (P < 0.05) with dark CO2 fixation rate, respectively. The PLS-SEM analysis revealed that dark CO2 fixation-related functional traits (i.e. normalized abundance, Chao1 index and gene composition) were directly and positively associated with dark CO2 fixation rate, and organic manure could exert a positive effect on soil dark CO2 fixation rate through enhancing soil properties (e.g., pH and soil organic carbon) and further altering associated microbial functional traits. These results have implications for explaining and predicting the soil CO2 fixation process from the perspective of microbial functional potential.

An Altered Microbiota in the Lower and Upper Female Reproductive Tract of Women with Recurrent Spontaneous Abortion

Recurrent spontaneous abortion (RSA) is a complex multifactorial disease. Recently, the microbiota of the female reproductive tract, as an emerging factor in RSA, has gradually attracted the attention of many clinical researchers. Here, we reported that the microbiota of the lower and upper female reproductive tracts from patients with RSA showed no significant differences in alpha diversity compared to that of controls. Beta diversity was significantly higher in the RSA group than in the control group in the vaginal microbiota (P = 0.036), cervical microbiota (P = 0.010) and microbiota from uterine lavage fluid (P = 0.001). In addition, dramatic decreases in gamma interferon and interleukin-6 cytokine levels were observed in the RSA group. In conclusion, our data suggested altered microbial biodiversity in the vagina, cervix and uterine lavage fluid in the RSA group. Alterations in the microbiota in the uterine cavity could be associated with altered cytokine levels, which might be a risk factor for RSA pathogenesis. Moreover, the microbiota composition differed markedly from the lower genital tract to the uterine cavity, and the microbiota in the uterine cavity also distinctly varied between endometrial tissue and uterine lavage fluid in the RSA group. Hence, sampling with these two methods simultaneously allowed a more comprehensive perspective of microbial colonization in the uterine cavity. IMPORTANCE As an obstacle to pregnancy, recurrent spontaneous abortion (RSA) can be caused by a variety of factors, and a current understanding of the etiology of RSA is still lacking; half of cases have an unknown cause. A substantial fraction of patients show no improvement after treatment. Since the microbiota of the female reproductive tract has been proposed as an emerging factor in RSA patients, further investigation is needed to provide guidance for clinical therapy. In general, this is the first report describing the distinct alterations of the vaginal, cervical, and uterine microbiota in RSA, not just that in the vagina. Furthermore, another major strength of this study derived from the further in-depth investigation and analysis of the characteristics of the microbiota colonizing the upper female genital tract in RSA, which provided a more comprehensive view for investigating the uterine microbiota.

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

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.

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

Since the free organic carbon (FOC) generated by chemoautotrophic bacterium self has a feedback inhibition effect on its growth and carbon fixation, a continuous flow membrane bio-reactor was designed to remove extracellular FOC (EFOC) and release its inhibition effect. The promotion effect of membrane reactor on growth and carbon fixation of typical chemoautotrophic bacterium and its mechanism were studied. The accumulated apparent carbon fixation yield in membrane reactor was 3.24 times that in the control reactor. The EFOC per unit bacteria and cbb gene transcription level in membrane reactor were about 0.41 times and 11.18 times that in control reactor in late stage, respectively. Membrane reactor separated out EFOC, especially the small molecules, which facilitated the release of intracellular FOC, thereby releasing the inhibition of FOC on cbb gene transcription, thus promoting growth and carbon fixation of the typical chemoautotrophic bacterium. This study lays a foundation for enhancing carbon fixation by chemoautotrophic bacteria and expands the application field of membrane reactor.