Clostridial whole cell and enzyme systems for hydrogen production: current state and perspectives

Hydrogen production pathways in Clostridia and their improvement by metabolic engineering

Biological production of hydrogen has a tremendous potential as an environmentally sustainable technology to generate a clean fuel. Among the different available methods to produce biohydrogen, dark fermentation features the highest productivity and can be used as a means to dispose of organic waste biomass. Within this approach, Clostridia have the highest theoretical H2 production yield. Nonetheless, most strains show actual yields far lower than the theoretical maximum: improving their efficiency becomes necessary for achieving cost-effective fermentation processes. This review aims at providing a survey of the metabolic network involved in H2 generation in Clostridia and strategies used to improve it through metabolic engineering. Together with current achievements, a number of future perspectives to implement these results will be illustrated.

Enhancing biohydrogen production from xylose at low temperature (20 °C) using natural FeS2 Ore: Thermodynamic analysis and mechanistic insights

This study investigates the efficacy of pyrite in enhancing biohydrogen production from xylose at low temperature (20 °C). Higher hydrogen yield rates (Rm) and reduced lag time (λ) were achieved across initial xylose concentrations ranging from 2-10 g/L. At an optimal xylose concentration of 5 g/L, pyrite reduced λ by 2.5 h and increased Rm from 1.3 to 2.7 mL h-1. These improvements are attributed to pyrite's ability to enhance the secretion of extracellular polymeric substance and flavins, facilitate NADH and NAD+ generation and transition, and favor biohydrogen production. Thermodynamic analyses and Gibbs free energy calculations further elucidated pyrite's role in the full reaction process and rate-limiting steps at low temperature. This study offers valuable insights into improving the efficiency of biohydrogen production at low temperature, with significant implications for energy conservation.

Using directed evolution to improve hydrogen production in chimeric hydrogenases from algal species

Algae generate hydrogen from sunlight and water utilizing high-energy electrons generated during photosynthesis. The amount of hydrogen produced in heterologous expression of the wild-type hydrogenase is currently insufficient for industrial applications. One approach to improve hydrogen yields is through directed evolution of the DNA of the native hydrogenase. Here, we created 113 chimeric algal hydrogenase gene variants derived from combining segments of three parent hydrogenases, two from Chlamydomonas reinhardtii (CrHydA1 and CrHydA2) and one from Scenedesmus obliquus (HydA1). To generate chimeras, there were seven segments into which each of the parent hydrogenase genes was divided and recombined in a variety of combinations. The chimeric and parental hydrogenase sequences were cloned for heterologous expression in Escherichia coli, and 40 of the resultant enzymes expressed were assayed for H2 production. Chimeric clones that resulted in equal or greater production obtained with the cloned CrHydA1 parent hydrogenase were those comprised of CrHydA1 sequence in segments #1, 2, 3, and/or 4. These best-performing chimeras all contained one common region, segment #2, the part of the sequence known to contain important amino acids involved in proton transfer or hydrogen cluster coordination. The amino acid sequence distances among all chimeric clones to that of the CrHydA1 parent were determined, and the relationship between sequence distances and experimentally-derived H2 production was evaluated. An additional model determined the correlation between electrostatic potential energy surface area ratios and H2 production. The model yielded several algal mutants with predicted hydrogen productions in a range of two to three times that of the wild-type hydrogenase. The mutant data and the model can now be used to predict which specific mutant sequences may result in even higher hydrogen yields. Overall, results provide more precise details in planning future directed evolution to functionally improve algal hydrogenases.

Renewable biohydrogen production from Clostridium sp. LQ25 using different forms of ferric as electron acceptor

Clostridium sp. LQ25 was cultured in different forms of ferric (ferric citrate and ferric hydroxide) as electron acceptors to investigate growth, ferric reduction, hydrogen production, fermentation products and fermentation process. The growth of the strain LQ25 detected by protein was 82.8 ± 2.1 mg/L and 73.5 ± 1.7 mg/L using ferric citrate and ferric hydroxide as electron acceptors, which was 33.3 % and 18.4 % higher than without ferric, respectively. The accumulation concentration of Fe(II) was 9.0 ± 0.6 mg/L and 5.0 ± 0.2 mg/L when using ferric citrate and ferric hydroxide as electron acceptors, and ferric citrate was 1.8-fold higher than ferric hydroxide, which indicated that the ability of ferric reduction was higher using ferric citrate as electron acceptor. The hydrogen production of strain LQ25 was 238.0 ± 1.0 mmol/mol glucose and 113.0 ± 1.3 mmol/mol glucose under condition of ferric citrate and ferric hydroxide as electron acceptors, which was 2.6 and 1.2-fold higher than without ferric, respectively. The growth and hydrogen production of strain LQ25 was promoted by using ferric as electron acceptor, while the fermentation type of strain did not change and was always butyrate type. The differential expression of the genes of strain LQ25 was significant when using ferric as electron acceptor, mainly in NADH and PFL pathway. This study provided preliminary evidence for hydrogen production by Clostridium sp. LQ25 in the presence of electron acceptor.

Accelerating anaerobic digestion process with novel single chamber microbial electrochemical systems with baffle

A newly designed microbial electrochemical system (MES) with the addition of a baffle between the electrodes was integrated with the anaerobic digestion (AD) process for biogas upgradation. Novel MES configuration attained an increased methane production rate of 292.6 mL/L∙d and methane yield of 0.36 ± 0.006 [Formula: see text] /g_COD, which were higher than the values (185.3 mL/L∙d and 0.33 ± 0.009 [Formula: see text] /g_COD) from the MES operation without baffle, respectively. Moreover, the MES with baffle operation resulted in increased substrate removal (88.4 ± 0.5%) and less volatile fatty acids accumulation with a high energy efficiency of 99.6 %. Microbial community analysis revealed that acids metabolizing bacteria, Firmicutes, and Methanothrix were highly enriched in the cathode biofilm of MES with baffle. This study suggests that the baffle addition into the single chamber MES is beneficial to further improve the methanogenesis process for practical applications in the scaled-up MES-AD process.

Debottlenecking the biological hydrogen production pathway of dark fermentation: insight into the impact of strain improvement

Confronted with the exhaustion of the earth’s fossil fuel reservoirs, bio-based process to produce renewable energy is receiving significant interest. Hydrogen is considered as an attractive energy carrier that can replace fossil fuels in the future mainly due to its high energy content, recyclability and environment-friendly nature. Biological hydrogen production from renewable biomass or waste materials by dark fermentation is a promising alternative to conventional routes since it is energy-saving and reduces environmental pollution. However, the current yield and evolution rate of fermentative hydrogen production are still low. Strain improvement of the microorganisms employed for hydrogen production is required to make the process competitive with traditional production methods. The present review summarizes recent progresses on the screening for highly efficient hydrogen-producing strains using various strategies. As the metabolic pathways for fermentative hydrogen production have been largely resolved, it is now possible to engineer the hydrogen-producing strains by rational design. The hydrogen yields and production rates by different genetically modified microorganisms are discussed. The key limitations and challenges faced in present studies are also proposed. We hope that this review can provide useful information for scientists in the field of fermentative hydrogen production.

Hydrogen can be generated by microorganisms.

Dark fermentation is efficient for biological hydrogen production.

Strain improvement is critical to enhancing hydrogen-producing ability.

Measurement of fasting breath hydrogen concentration as a simple diagnostic method for pancreatic exocrine insufficiency

Background

Pancreatic exocrine insufficiency (PEI) is associated with the outcome of pancreatic disease. However, there is no method for assessing PEI that can be used noninvasively and easily for outpatient. It has been reported that changes in intestinal bacteria caused by PEI may increase breath hydrogen concentration (BHC) levels during glucose or lactose loading. We have evaluated the usefulness of fasting breath hydrogen concentration (FBHC) measurement without glucose loading for the evaluation of PEI.

Methods

Sixty patients underwent FBHC measurement, BT-PABA testing, and microbiome analysis. They were classified into PEI group (PABA excretion rate < 73.4%, n = 30) and non-PEI group (n = 30). The FBHC of the two groups were compared, and the diagnostic ability of PEI by them was evaluated. The 16 s rRNA (V3–V4) from fecal samples was analyzed by MiSeq.

Results

FBHC levels was higher in the PEI group 15.70 (1.4 to 77.0) ppm than in the non-PEI group 2.80 (0.7 to 28.2) ppm (P < 0.0001). FBHC was negatively correlated with PABA excretion rate (r =  − 0.523, P < 0.001). The cutoff value of FBHC of 10.7 ppm (95% CI: 0.678–0.913, P < 0.001) showed a sensitivity of 73.3% and specificity of 83.3% for PEI diagnosis. In the PEI group, there was a significant increase of relative abundance of phylum Firmicutes (P < 0.05) and the genus Clostridium (P < 0.05).

Conclusion

FBHC shows good potential as a simple and repeatable test for the diagnosis of PEI. The elevated FBHC levels may be caused by hydrogen-producing bacteria such as Clostridium.