Fermentation and hydrogen metabolism affect uranium reduction by clostridia

Impact of microbial processes on the safety of deep geological repositories for radioactive waste

To date, the increasing production of radioactive waste due to the extensive use of nuclear power is becoming a global environmental concern for society. For this reason, many countries have been considering the use of deep geological repositories (DGRs) for the safe disposal of this waste in the near future. Several DGR designs have been chemically, physically, and geologically well characterized. However, less is known about the influence of microbial processes for the safety of these disposal systems. The existence of microorganisms in many materials selected for their use as barriers for DGRs, including clay, cementitious materials, or crystalline rocks (e.g., granites), has previously been reported. The role that microbial processes could play in the metal corrosion of canisters containing radioactive waste, the transformation of clay minerals, gas production, and the mobility of the radionuclides characteristic of such residues is well known. Among the radionuclides present in radioactive waste, selenium (Se), uranium (U), and curium (Cm) are of great interest. Se and Cm are common components of the spent nuclear fuel residues, mainly as 79Se isotope (half-life 3.27 × 105 years), 247Cm (half-life: 1.6 × 107 years) and 248Cm (half-life: 3.5 × 106 years) isotopes, respectively. This review presents an up-to-date overview about how microbes occurring in the surroundings of a DGR may influence their safety, with a particular focus on the radionuclide-microbial interactions. Consequently, this paper will provide an exhaustive understanding about the influence of microorganisms in the safety of planned radioactive waste repositories, which in turn might improve their implementation and efficiency.

Effects of UV stress on Shewanella azerbaijanica bioremediation response

Shewanella azerbaijanica roles as a live electrode, passing electrons from electron donors to electron acceptors, to gain energy from the extracellular electron transfer (EET) pathway. The present study, considered the quantitative expressions of the major EET reductase genes (mtr cluster), together with uranium removal, live-cell counting, and spectrophotometry in UV-C treated bacteria (0, 60, 120 and 180 s). The simultaneous decline in the uranium removal and cell counting, along with major mtr gene expression patterns (mtrABDEF), approved the negative effects of UV-C radiation on uranium bioreduction in S. azerbaijanica. Uranium removal and cell counting decreased to 25.49% and 0.45 × 109 cells/mL in the 180s UV-C treated sample, respectively at 2 mM uranium concentration, while no decline trend found in 0.5 mM for the counted cells and uranium removal tests. No considerable expression of omcA and omcB (mtrC) genes were traced due to spontaneous mutagenesis during the in vitro serial passages, proposing a novel alternative EET pathway in S. azerbaijanica during uranium bioreduction process. The results could pave the way for further researches to modify the bioremediation process through genetic manipulation.

Genomic comparison of Clostridium species with the potential of utilizing red algal biomass for biobutanol production

BACKGROUND

Sustainable biofuels, which are widely considered as an attractive alternative to fossil fuels, can be generated by utilizing various biomass from the environment. Marine biomass, such as red algal biomass, is regarded as one potential renewable substrate source for biofuels conversion due to its abundance of fermentable sugars (e.g., galactose). Previous studies focused on the enhancement of biofuels production from different Clostridium species; however, there has been limited investigation into their metabolic pathways, especially on the conversion of biofuels from galactose, via whole genomic comparison and evolutionary analysis.

RESULTS

Two galactose-utilizing Clostridial strains were examined and identified as Clostridium acetobutylicum strain WA and C. beijerinckii strain WB. Via the genomic sequencing of both strains, the comparison of the whole genome together with the relevant protein prediction of 33 other Clostridium species was established to reveal a clear genome profile based upon various genomic features. Among them, five representative strains, including C. beijerinckii NCIMB14988, C. diolis DSM 15410, C. pasteurianum BC1, strain WA and WB, were further discussed to demonstrate the main differences among their respective metabolic pathways, especially in their carbohydrate metabolism. The metabolic pathways involved in the generation of biofuels and other potential products (e.g., riboflavin) were also reconstructed based on the utilization of marine biomass. Finally, a batch fermentation process was performed to verify the fermentative products from strains WA and WB using 60 g/L of galactose, which is the main hydrolysate from algal biomass. It was observed that strain WA and WB could produce up to 16.98 and 12.47 g/L of biobutanol, together with 21,560 and 10,140 mL/L biohydrogen, respectively.

CONCLUSIONS

The determination of the production of various biofuels by both strains WA and WB and their genomic comparisons with other typical Clostridium species on the analysis of various metabolic pathways was presented. Through the identification of their metabolic pathways, which are involved in the conversion of galactose into various potential products, such as biobutanol, the obtained results extend the current insight into the potential capability of utilizing marine red algal biomass and provide a systematic investigation into the relationship between this genus and the generation of sustainable bioenergy.

Toxicity of ionic liquids to Clostridium sp. and effects on uranium biosorption

As green solvents ionic liquids (ILs) show high potential in nuclear industry for extraction and purification of actinides. However, to date relatively little information has been gained on ILs application in microbial processes, for example biosorption of radionuclides. We investigated the effects of three ILs, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), N-ethylpyridinium trifluoroacetate (EtPyCF3COO) and N-ethylpyridinium tetrafluoroborate (EtPyBF4) on the growth and biosorption of uranium by Clostridium sp. The ILs affected the growth of the bacterium as evidenced by decreases in optical density, total gas production, and organic acids production from glucose metabolism. The IC50-48h of three ILs decreased in the order of BMIMPF6 (8.26mM)>EtPyBF4 (7.04mM)>EtPyCF3COO (4.05mM). Uranium biosorption by the bacterial cells decreased by 75% in the presence of 1% (v/v) BMIMPF6 and by about 90% with 1% (v/v) EtPyBF4 or EtPyCF3COO, in comparison to the control without ILs. The diminished biosorption may be attributed to the membrane damages induced by EtPyBF4 and EtPyCF3COO, which can be visualized by Transmission Electron Microscope (TEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) analysis revealed the accumulation of uranium inside peripheral membrane of the cells exposed to uranium alone or with BMIMPF6, while little or no accumulation was observed in the presence of EtPyBF4 and EtPyCF3COO. These results imply that potential toxicity of ILs towards microorganisms is a particularly important issue in limiting its biotechnological applications.