Effect of tungsten and selenium on C1 gas bioconversion by an enriched anaerobic sludge and microbial community analysis

Chemical structure of hydrocarbons significantly affects removal performance and microbial responses in gas biotrickling filters

The control of emissions of short-chain hydrocarbons with different structures is critical for the petrochemical industry. Herein, three two-carbon-containing (C2) hydrocarbons, ethane, ethylene, and acetylene, were chosen as pollutants to study the effects of chemical structure of hydrocarbons on removal performance and microbial responses in biotrickling filters. Results showed that the removal efficiency (RE) of C2 hydrocarbons followed the sequence of acetylene > ethane > ethylene. When the inlet loading rate was 30 g/(m3·h) and the empty bed residence time was 60 s, the RE of ethane, ethylene, and acetylene was 57 ± 4.0 %, 49 ± 1.0 %, and 84 ± 2.7 %, respectively. The high water solubility resulted in the high removal of C2 hydrocarbons, while a low surface tension enhanced the removal of C2 hydrocarbons. Additionally, the microbial community, enzyme activity, and extracellular properties of microorganisms also contributed to the difference in C2 hydrocarbon removal. These results could be referred for the effective control of light hydrocarbon emissions.

Unlocking the potential of one-carbon gases (CO2, CO) for concomitant bioproduction of β-carotene and lipids

This study investigates the use of a Yarrowia lipolytica strain for the bioconversion of syngas-derived acetic acid into β-carotene and lipids. A two-stage process was employed, starting with the acetogenic fermentation of syngas by Clostridium aceticum, metabolising CO, CO2, H2, to produce acetic acid, which is then utilized by Y. lipolytica for simultaneous lipid and β-carotene synthesis. The research demonstrates that acetic acid concentration plays a pivotal role in modulating lipid profiles and enhancing β-carotene production, with increased acetic acid consumption leading to higher yields of these compounds. This approach showcases the potential of using one-carbon gases as substrates in bioprocesses for generating valuable bioproducts, providing a sustainable and cost-effective alternative to more conventional feedstocks and substrates, such as sugars.

Production of biofuels from C1 -gases with Clostridium and related bacteria-Recent advances

Clostridium spp. are suitable for the bioconversion of C₁ -gases (e.g., CO₂ , CO and syngas) into different bioproducts. These products can be used as biofuels and are reviewed here, focusing on ethanol, butanol and hexanol, mainly. The production of higher alcohols (e.g., butanol and hexanol) has hardly been reviewed. Parameters affecting the optimization of the bioconversion process and bioreactor performance are addressed as well as the pathways involved in these bioconversions. New aspects, such as mixotrophy and sugar versus gas fermentation, are also reviewed. In addition, Clostridia can also produce higher alcohols from the integration of the Wood-Ljungdahl pathway and the reverse ß-oxidation pathway, which has also not yet been comprehensively reviewed. In the latter process, the acetogen uses the reducing power of CO/syngas to reduce C₄ or C₆ fatty acids, previously produced by a chain elongating microorganism (commonly Clostridium kluyveri), into the corresponding bioalcohol.

Enrichment of homoacetogens converting H2/CO2 into acids and ethanol and simultaneous methane production

An anaerobic granular sludge was enriched to utilize H₂/CO₂ in a continuous gas-fed up-flow anaerobic sludge reactor by applying operating conditions expected to produce acetic acid, butyric acid, and ethanol. Three stages of fermentation were found: Stage I with acetic acid accumulation with the highest concentration of 35 mM along with a pH decrease from initial 6 to 4.5. In Stage II, H₂/CO₂ was replaced by 100% H₂ to induce solventogenesis, whereas butyric acid was produced with the highest concentration of 2.5 mM. At stage III with 10 µM tungsten (W) addition, iso-valeric acid, valeric acid, and caproic acid were produced at pH 4.5-5.0. In the batch tests inoculated with the enriched sludge taken from the bioreactor (day 70), however, methane production occurred at pH 6. Exogenous 15 mM acetate addition enhanced both the H₂ and CO₂ consumption rate compared to exogenous 10, 30, and 45 mM acetate by the enriched sludge. Exogenous acetate was failed to be converted to ethanol using H₂ as electron donor by the enriched acetogens.

Effect of selenium and tungsten on cell growth and metabolite production in syngas fermentation using "Clostridium autoethanogenum"

The effect of tungsten and selenium on cell growth and production of metabolites such as acetic acid and ethanol when fermenting syngas using "Clostridium autoethanogenum" was investigated to improve the process efficiency. General concentrations of selenium and tungsten in the medium are 0.01µM during acetogenic syngas fermentation. We conducted culture experiments at concentrations of 0, 0.001, 0.01 and 0.1µM for each heavy metal. The effect of selenium on cell growth and total metabolite production was greater than that of tungsten as the effect of selenium on formate dehydrogenase, an important enzyme of the Wood-Ljungdahl pathway, is greater than that of tungsten. Although an increase in tungsten had a marginal effect on total metabolite production, the ethanol/acetic acid production ratio increased significantly due to a decrease in acetic acid and an increase in ethanol production. Thus, tungsten plays a key role in activating aldehyde:ferredoxin oxidoreductase, a key enzyme in the reduction of acetate to ethanol. A specific ethanol productivity of 0.462g ethanol/g DCW∙d was obtained in a culture using 0.01µM selenium and 0.1µM tungsten, which was 2.18 times higher than when using 0.01µM of both selenium and tungsten.

Enhanced Removal of Hydrophobic Short-Chain n-Alkanes from Gas Streams in Biotrickling Filters in Presence of Surfactant

Emissions of n-alkanes are facing increasingly stringent management challenges. Biotrickling filtration in the presence of surfactants is a competitive alternative for the enhanced removal of n-alkanes. Herein, sodium dodecyl benzene sulfonate (SDBS) was added into the liquid phase feeding a biotrickling filter (BTF) to enhance the removal of various short-chain n-alkanes from n-hexane (C6) to methane (C1). The removal performance of C6-C1 and microbial response mechanisms were explored. The results showed that the removal efficiency (RE) of n-alkanes decreased from 77 ± 1.3 to 35 ± 5.6% as the carbon chain number of n-alkanes decreased from C6 to C1, under the conditions of an n-alkane inlet load of 58 ± 3.0 g/m³·h and EBCT of 30 s. The removal performance of n-alkanes was enhanced significantly by the introduction of 15 mg/L SDBS, as the RE of C6 reached 99 ± 0.7% and the RE of C1 reached 74 ± 3.3%. The strengthening mechanisms were that the apparent Henry's law coefficient of n-alkanes decreased by 11 ± 1.4-30 ± 0.3%, and the cell surface hydrophobicity of microorganisms improved from 71 ± 5.6 to 87 ± 4.0% with the existence of SDBS. Moreover, the presence of SDBS promoted the succession and activity of the microbial community. The activities of alkane hydroxylase and alcohol dehydrogenase were 5.8 and 5.9 times higher than those without SDBS, and the concentration of the cytochrome P450 gene was improved 2.2 times. Therefore, the addition of SDBS is an effective strategy that makes BTF suitable for the removal of various n-alkanes from waste gas streams.

Advances in cathode designs and reactor configurations of microbial electrosynthesis systems to facilitate gas electro-fermentation

In gas fermentation, a range of chemolithoautotrophs fix single-carbon (C1) gases (CO₂ and CO) when H₂ or other reductants are available. Microbial electrosynthesis (MES) enables CO₂ reduction by generating H₂ or reducing equivalents with the sole input of renewable electricity. A combined approach as gas electro-fermentation is attractive for the sustainable production of biofuels and biochemicals utilizing C1 gases. Various platform compounds such as acetate, butyrate, caproate, ethanol, butanol and bioplastics can be produced. However, technological challenges pertaining to the microbe-material interactions such as poor gas-liquid mass transfer, low biomass and biofilm coverage on cathode, low productivities still exist. We are presenting a review on latest developments in MES focusing on the configuration and design of cathodes that can address the challenges and support the gas electro-fermentation. Overall, the opportunities for advancing CO and CO₂-based biochemicals and biofuels production in MES with suitable cathode/reactor design are prospected.

Bioethanol Production From H2/CO2 by Solventogenesis Using Anaerobic Granular Sludge: Effect of Process Parameters

CO₂ fermentation by biocatalysis is a promising route for the sustainable production of valuable chemicals and fuels, such as acetic acid and ethanol. Considering the important role of environmental parameters on fermentation processes, granular sludge from an industrial anaerobic wastewater treatment system was tested as inoculum for ethanol production from H₂/CO₂ at psychrophilic (18°C), submesophilic (25°C), and mesophilic (30°C) temperatures. The highest acetic acid and ethanol production was obtained at 25°C with a final concentration of 29.7 and 8.8 mM, respectively. The presence of bicarbonate enhanced acetic acid production 3.0 ∼ 4.1-fold, while inhibiting ethanol production. The addition of 0.3 g/L glucose induced butyric acid production (3.7 mM), while 5.7 mM ethanol was produced at the end of the incubation at pH 4 with glucose. The addition of 10 μM W enhanced ethanol production up to 3.8 and 7.0-fold compared to, respectively, 2 μM W addition and the control. The addition of 2 μM Mo enhanced ethanol production up to 8.1- and 5.4-fold compared to, respectively, 10 μM Mo and the control. This study showed that ethanol production from H₂/CO₂ conversion using granular sludge as the inoculum can be optimized by selecting the operational temperature and by trace metal addition.

Combination of Trace Metal to Improve Solventogenesis of Clostridium carboxidivorans P7 in Syngas Fermentation

Higher alcohols such as butanol (C4 alcohol) and hexanol (C6 alcohol) are superior biofuels compared to ethanol. Clostridium carboxidivorans P7 is a typical acetogen capable of producing C4 and C6 alcohols natively. In this study, the composition of trace metals in culture medium was adjusted, and the effects of these adjustments on artificial syngas fermentation by C. carboxidivorans P7 were investigated. Nickel and ferrous ions were essential for growth and metabolite synthesis during syngas fermentation by P7. However, a decreased dose of molybdate improved alcohol fermentation performance by stimulating carbon fixation and solventogenesis. In response to the modified trace metal composition, cells grew to a maximum OD_{600 nm} of 1.6 and accumulated ethanol and butanol to maximum concentrations of 2.0 and 1.0 g/L, respectively, in serum bottles. These yields were ten-fold higher than the yields generated using the original composition of trace metals. Furthermore, 0.5 g/L of hexanol was detected at the end of fermentation. The results from gene expression experiments examining genes related to carbon fixation and organic acid and solvent synthesis pathways revealed a dramatic up-regulation of the Wood-Ljungdahl pathway (WLP) gene cluster, the bcs gene cluster, and a putative CoA transferase and butanol dehydrogenase, thereby indicating that both de novo synthesis and acid re-assimilation contributed to the significantly elevated accumulation of higher alcohols. The bdh35 gene was speculated to be the key target for butanol synthesis during solventogenesis.