Acetone-butanol fermentation revisited.

A stable continuous method for producing ethanol from the anaerobic bacterial fermentation of a gaseous substrate containing at least one reducing gas involves culturing in a fermentation bioreactor anaerobic, acetogenic bacteria in a liquid nutrient medium; supplying the gaseous substrate to the bioreactor; and manipulating the bacteria in the bioreactor by reducing the redox potential, or increasing the NAD(P)H TO NAD(P) ratio, in the fermentation broth after the bacteria achieves a steady state and stable cell concentration in the bioreactor. The free acetic acid concentration in the bioreactor is maintained at less than 5 g/L free acid. This method allows ethanol to be produced in the fermentation broth in the bioreactor at a productivity of greater than 10 g/L per day. Both ethanol and acetate are produced in a ratio of ethanol to acetate ranging from 1:1 to 20:1.

Methods for increasing the production of ethanol from microbial fermentation

The effect of butanol challenge (0, 1.0, 1.5% [vol/vol]) and growth temperature (22, 37, 42°C) on the membrane composition and fluidity of Clostridium acetobutylicum ATCC 824 and a butanol-tolerant mutant, SA-2, was examined in chemically defined medium. Growth of strain ATCC 824 into the stationary phase coincided with a gradual increase in the percent saturated to percent unsaturated (SU) fatty acid ratio. When challenged with butanol at 22 and 37°C, ATCC 824 demonstrated an immediate (within 30 min) dose-response increase in the SU ratio. This strain showed little additional change over a 48-h fermentation. Compared with ATCC 824, growth of SA-2 into the late stationary phase at 22 or 37°C resulted in an overall greater increase in the SU ratio for both unchallenged and challenged cells. This effect was minimized when SA-2 was challenged at 42°C, probably due to the combination of the membrane fluidizing effect of butanol and the elevated temperature. Growth at 42°C resulted in an increase in longer acyl chain fatty acids at the expense of shorter acyl chains for both strains. The membrane fluidity exhibited by SA-2 remained essentially constant at various butanol challenge and temperature combinations, while that for the ATCC 824 strain increased with increasing butanol challenge. By synthesizing an increased amount of saturated fatty acids, the butanol-tolerant SA-2 strain has apparently developed a mechanism for maintaining a more stable membrane environment. Growth of the microorganism is necessary for butanol to fluidize the membrane. Incorporation of exogenous fatty acids (18:1) did not significantly improve the butanol tolerance of either strain. Since SA-2 was able to produce only trace amounts of either butanol or acetone, increased tolerance to butanol does not necessarily coincide with greater solvent yields in this strain.

https://aem.asm.org/content/aem/53/12/2854.full.pdf

Effect of butanol challenge and temperature on lipid composition and membrane fluidity of butanol-tolerant Clostridium acetobutylicum

Biology, ecology, and biotechnological applications of anaerobic bacteria adapted to environmental stresses in temperature, pH, salinity, or substrates.

The solvent-forming clostridia have attracted interest because of their ability to convert a range of carbohydrates to end-products such as acetone, butanol and ethanol. Polymeric substrates such as cellulose, hemicellulose and starch are degraded by extracellular enzymes. The majority of cellulolytic clostridia, typified by Clostridium thermocellum, produce a multi-enzyme cellulase complex in which the organization of components is critical for activity against the crystalline substrate. A variety of enzymes involved in degradation of hemicellulose and starch have been identified in different strains. The products of degradation, and other soluble substrates, are accumulated via membrane-bound transport systems which are generally poorly characterized. It is clear, however, that the phosphoenolpyruvate-dependent phosphotransferase system (PTS) plays a major role in solute uptake in several species. Accumulated substrates are converted by intracellular enzymes to end-products characteristic of the organism, with production of ATP to support growth. The metabolic pathways have been described, but understanding of mechanisms of regulation of metabolism is incomplete. Synthesis of extracellular enzymes and membrane-bound transport systems is commonly subject to catabolite repression in the presence of a readily metabolized source of carbon and energy. While many genes encoding cellulases, xylanases and amylases have been cloned and sequenced, little is known of control of their expression. Although the mechanism of catabolite repression in clostridia is not understood, some recent findings implicate a role for the PTS as in other low G-C Gram-positive bacteria. Emphasis has been placed on describing the mechanisms underlying the switch of C. acetobutylicum fermentations from acidogenic to solventogenic metabolism at the end of the growth phase. Factors involved include a lowered pH and accumulation of undissociated butyric acid, intracellular concentration of ATP and reduced pyridine nucleotides, nutrient limitation, and the interplay between pathways of carbon and electron flow. Genes encoding enzymes of solvent pathways have been cloned and sequenced, and their expression correlated with the pattern of end-product formation in fermentations. There is evidence that the initiation of solvent formation may be subject to control mechanisms similar to other stationary-phase phenomena, including sporulation. The application of recently developed techniques for genetic manipulation of the bacterium is improving understanding of the regulatory circuits, but a complete molecular description of the control of solvent formation remains elusive. Experimental manipulation of the pathways of electron flow in other species has been shown to influence the range and yield of fermentation end-products. Acid-forming clostridia can, under appropriate conditions, be induced to form atypical solvents as products. While the mechanisms of regulation of gene expression are not at all understood, the capacity to adapt in this way further illustrates the metabolic flexibility of clostridial strains.

Physiology of Carbohydrate to Solvent Conversion by Clostridia

The fermentation of glucose by Clostridium acetobutylicum on a synthetic medium is carried out with a conversion of carbon source into solvents of 32 %. The ratio of butanol, acetone and ethanol products is approximately 0.6 - 1.9 - 6. The synthetic medium supplemented with acetic acid at a concentration of 2 g/l increases acetone formation and the ratio of products is approximately 0.5 - 3 - 6 with a conversion of glucose into solvents of 34 %. The addition of 2 g/l butyric acid permits obtaining a distribution of 0.8 - 2.4 - 6, with a conversion of 35 %.

J. Ballongue

Papers

Effects of acetic and butyric acids on solvents production by Clostridium acetobutylicum