Dual function of ammonium acetate in acetone-butanol-ethanol fermentation by Clostridium acetobutylicum

An Alternative Approach to Improve the Butanol Production Efficiency from Sweet Sorghum Stem Juice Using Immobilized Cells Combined with an In Situ Gas Stripping System

The effects of the nitrogen source and buffers used in butanol production with Clostridium beijerinckii TISTR 1461 from sweet sorghum stem juice (SSJ) containing 60 g/L of total sugar were first studied in this paper. Among the various nitrogen sources (dried spent yeast, urea, ammonium acetate, ammonium sulfate), urea was found to be the most suitable for butanol production. SSJ supplemented with urea (0.64 g/L) and cocktail buffers (KH2PO4, 0.5 g/L; K2HPO4, 0.5 g/L; ammonium acetate, 2.2 g/L) gave the highest butanol concentration (PB, 10.13 g/L). Then, the capability of immobilized C. beijerinckii TISTR 1461 cells for butanol fermentation was investigated. Two residual waste materials were examined as immobilized cell carriers. Bamboo chopstick pieces were more appropriate as carriers for cell immobilization than cigarette filter tips. The PB value of the immobilized cells on the bamboo chopstick pieces was ~13% higher than that on the cigarette filter tips. Using the response surface methodology (RSM), 1.9 cm bamboo chopstick pieces with a carrier loading of 1:32 (w/v) were the optimum conditions for cell immobilization for butanol production. Under these conditions, the PB value was 11.62 g/L. To improve the butanol production efficiency, a gas stripping system (GS) was connected to the fermenter. It was found that the PB (14.02 g/L) and butanol productivity (QB, 0.29 g/L·h) values improved by ~21% compared to butanol fermentation using no gas stripping.

Butanol Production by a Novel Efficient Method Using Mixed Cultures of Clostridium beijerinckii and Arthrobacter sp. in Stirred-Tank and Gas-Lift Bioreactors

Arthrobacter sp. BCC 72131, an obligate aerobic bacterium, was used to create anaerobic conditions, and Clostridium beijerinckii TISTR 1461 was used as a butanol producer in an acetone-butanol–ethanol (ABE) fermentation. Sweet sorghum juice (SSJ) medium containing 60 g/L of total sugar supplemented with 1.27 g/L of (NH4)2SO4 was used as a butanol production (BP) medium. Arthrobacter sp. was inoculated into the BP medium in 1-L screw-capped bottles. After 2, 4, 6 and 12 h of Arthrobacter sp. cultivation at 30 °C, C. beijerinckii was transferred into the BP medium to start butanol production at 37 °C. The results showed that C. beijerinckii inoculation after 6 h of Arthrobacter sp. cultivation gave the highest butanol titer (PB) at 12.56 g/L, with a butanol yield (YB/S) and volumetric butanol productivity (QB) of 0.34 g/g and 0.23 g/L·h, respectively. These values are approximately 10–27% higher than those of the control experiment using a single culture of C. beijerinckii TISTR 1461 and oxygen-free nitrogen (OFN) gas flushing to create anaerobic conditions. Field emission scanning electron microscopic (FE-SEM) images of Clostridium cells, as well as protein and free amino nitrogen concentrations in the broth during butanol fermentation were also studied to confirm the results. The butanol fermentation was then carried out in a 5.6-L stirred-tank and a 1.2-L low-cost gas-lift bioreactor by the mixed cultures using the optimal time of Clostridium inoculation. The PB, YB/S and QB values obtained were not significantly different from those in the 1-L screw-capped bottles. Hence, Arthrobacter sp. can be used as a novel method to create anaerobic conditions instead of a traditional method employing OFN gas flushing. Using mixed cultures of Arthrobacter sp. BCC 72131 and C. beijerinckii TISTR 1461 is a practical method to produce butanol on a large-scale, both in complex and low-cost bioreactors.

Characterization and evaluation of a natural derived bacterial consortium for efficient lignocellulosic biomass valorization

A consortium (HPP) with improved ability in biomass conversion was achieved by adjusting the proportion of Pseudoxanthomonas taiwanensis in a natural consortium (HP), but the mechanism behind was unknown. Herein, the diversities of microbial community structure and gene functions of the consortia were analyzed first, and found that HPP had a more balanced microbial structure with enriched gene pathways related to cellular processes, environmental information processing and metabolism. Then, key genes responsible for biomass conversion were further analyzed, finding that their abundance and distribution contributed to HPP's efficient biomass conversion. Finally, consolidated bioprocessing of agricultural wastes by HPP was carried out to verify its enhanced ability, and ethanol with the highest yield that was ever reported was achieved at 0.28 g/g. This is the first study which reported the underlying mechanisms for synergistic effects of microbial consortia, and will guide the artificial construction of complex microbial consortium for specific purpose.

Effect of fed-batch and chemostat cultivation processes of C. glutamicum CP for L-leucine production

Most of the current industrial processes for L-leucine production are based on fermentation, usually in fed-batch operation mode. Although the culture technology has advanced in recent decades, the process still has significant drawbacks. To solve these problems, we investigated the effects of chemostat culture conditions on the production of L-leucine by Corynebacterium glutamicum CP. The dilution rate, the nitrogen source, and the carbon–nitrogen ratio of the medium were optimized. With the addition of ammonium acetate to the chemostat medium, the initial C/N ratio was adjusted to 57.6, and the L-leucine titer reached the highest level at the optimal dilution rate of 0.04 h⁻¹. Compared with fed-batch culture, the L-leucine titer was reduced (from 53.0 to 24.8 g L⁻¹), but the yield from glucose was increased by 10.0% (from 0.30 to 0.33 mol mol⁻¹) and productivity was increased by 58.3% (from 1.2 to 1.9 g L⁻¹ h⁻¹). Moreover, the titer of the by-product L-alanine was significantly reduced (from 8.9 to 0.8 g L⁻¹). In addition, gene expression levels and activity of key enzymes in the synthesis of L-leucine and L-alanine were analyzed to explain the difference of production performance between chemostat culture and fed-batch culture. The results indicate that chemostat culture has great potential to increase the industrial production of L-leucine compared to current fed-batch approaches.

Novel Methods Using an Arthrobacter sp. to Create Anaerobic Conditions for Biobutanol Production from Sweet Sorghum Juice by Clostridium beijerinckii

Biobutanol can be produced by Clostridia via an acetone–butanol–ethanol (ABE) fermentation under strictly anaerobic conditions. Oxygen-free nitrogen (OFN) gas is typically used to create anaerobic conditions for ABE fermentations. However, this method is not appropriate for large-scale fermentations as it is quite costly. The aim of this work was to study the feasibility of butanol production from sweet sorghum juice (SSJ) by Clostridium beijerinckii TISTR 1461 using various methods to create anaerobic conditions, i.e., growth of a strictly aerobic bacterium, an Arthrobacter sp., under different conditions and a chemical method using sodium dithionite (SDTN) to consume residual oxygen. SSJ containing 60 g/L of total sugar supplemented with 1.27 g/L of (NH4)2SO4 was used as a substrate for butanol production. The results showed that 0.25 mM SDTN could create anaerobic conditions, but in this case, C.beijerinckii TISTR 1461 could produce butanol at a concentration (PB) of only 8.51 g/L with a butanol productivity (QB) of 0.10 g/L·h. Arthrobacter sp. BCC 72131 could also be used to create anaerobic conditions. Mixed cultures of C.beijerinckii TISTR 1461 and Arthrobacter sp. BCC 72131 created anaerobic conditions by inoculating the C.beijerinckii 4 h after Arthrobacter. This gave a PB of 10.39 g/L with a QB of 0.20 g/L·h. Comparing butanol production with the control treatment (using OFN gas to create anaerobic conditions, yielding a PB of 9.88 g/L and QB of 0.21 g/L·h) indicated that using Arthrobacter sp. BCC 72131 was an appropriate procedure for creating anaerobic conditions for high levels of butanol production by C. beijerinckii TISTR 1461 from a SSJ medium.

Pathway dissection, regulation, engineering and application: lessons learned from biobutanol production by solventogenic clostridia

The global energy crisis and limited supply of petroleum fuels have rekindled the interest in utilizing a sustainable biomass to produce biofuel. Butanol, an advanced biofuel, is a superior renewable resource as it has a high energy content and is less hygroscopic than other candidates. At present, the biobutanol route, employing acetone-butanol-ethanol (ABE) fermentation in Clostridium species, is not economically competitive due to the high cost of feedstocks, low butanol titer, and product inhibition. Based on an analysis of the physiological characteristics of solventogenic clostridia, current advances that enhance ABE fermentation from strain improvement to product separation were systematically reviewed, focusing on: (1) elucidating the metabolic pathway and regulation mechanism of butanol synthesis; (2) enhancing cellular performance and robustness through metabolic engineering, and (3) optimizing the process of ABE fermentation. Finally, perspectives on engineering and exploiting clostridia as cell factories to efficiently produce various chemicals and materials are also discussed.

Enhanced oxytetracycline removal coupling with increased power generation using a self-sustained photo-bioelectrochemical fuel cell

Photo-bioelectrochemical fuel cell (PBFC) represents a promising technology for enhancing removal of antibiotic pollutants while simultaneously sustainable transformation of organic wastes and solar energy into electricity. In this study, simultaneous antibiotic removal and bioelectricity generation were investigated in a PBFC with daily light/dark cycle using oxytetracycline (OTC) as a model compound of antibiotic. The specific OTC removal rate increased by 61% at an external resistance of 50 Ω compared to that in the open-circuit control, which was attributed to bioelectrochemically enhanced co-metabolic degradation in the presence of the bioanode. The OTC removal was obviously accelerated during illumination of cathode in contrast with a dark cathode due to the higher driving force for anodic bioelectrochemical reaction by using photosynthetic oxygen as cathodic electron acceptor during illumination than that using nitrate in dark. The bioelectrocatalytic activity of anodic biofilm was continuously enhanced even at an initial OTC concentration of up to 50 mg L^-1. The degradation products of OTC can function as mediators to facilitate the electron transfer from bacteria to the anode, resulting in 1.2, 1.76 and 1.8 fold increase in maximum power output when 10, 30 and 50 mg L^-1 OTC was fed to the bioanode, compared to the OTC-free bioanode, respectively. The OTC feeding selective enriched OTC-tolerant bacterial community capable of degrading complex organic compounds and producing electricity. The occurrence of ARGs during bioelectrochemical degradation of OTC was affected more greatly by the succession of the anodic bacterial community than the initial OTC concentration.