Evaluation of the influence of CO2 on hydrogen production by Caldicellulosiruptor saccharolyticus

Stripping gas is generally used to improve hydrogen yields in fermentations. Since CO2 is relatively easy to separate from hydrogen it could be an interesting stripping gas. However, a higher partial CO2 pressure is accompanied with an increased CO2 uptake in the liquid, where it hydrolyses and induces an increased requirement of NaOH to maintain the pH. This enhances the osmotic pressure in the culture by 30%, which inhibited the growth of Caldicellulosiruptor saccharolyticus. Indications for this conclusion are: i) Inhibition could almost completely be circumvented by reducing the bicarbonate through decreasing the pH (from 6.5 to 5.5), ii) Growth rates were reduced by 60 ± 10% at an osmotic pressure of 0.218 ± 0.005 osm/kg H2O independently of the stripping gas, iii) Increased extracellular DNA and protein concentrations were observed as a function of the osmotic pressure. In addition to growth inhibition, the increased sodium bicarbonate in the effluent will contribute to a negative environmental impact when applied at industrial scal

Water Management in Lignocellulosic Ethanol Production- a Case Study and Comparative Analysis from a Swedish Perspective

The project presented here has focused on studying the water balance in a wheat straw-based conceptual High Gravity (i.e. suspended solids in the bioreactors at above 20 %) lignocellulosic ethanol process using xylose-fermenting yeast, cultivated on the hydrolysate from the process. Based on an initial review of inhibitory substances in lignocellulosic ethanol production, different relevant inhibitors were selected to be included in the analysis of water flows in the process. Experimental analyses of compounds at different positions in the ethanol process were conducted, based on material extracted from the Biorefinery Demo Plant, in Ornskoldsvik, Sweden. The results from analyses were used in flowsheeting model development, which in turn was used in order to analyse the impact of recycling process streams in the conceptual ethanol process. The main result is a comparative analysis on energy efficiency and process economics between different recycling options for three different concepts (two High Gravity alternatives and one Low Gravity alternative with 10 % suspended solids). The results indicate the levels of inhibitory substances at different positions in the ethanol process, and connect this information with the opportunities for recycling and reducing water flow. It is shown that water is an important factor for the economic performance of the process, and that a higher solids content in the process gives better results due to lower investment costs. It is also shown that recycling process streams can have a strong effect on both energy performance (flash steam recycle) and economics (hydrolysate recycle)

Hydrogen Production by Thermophilic Fermentation

Of the many ways hydrogen can be produced, this chapter focuses on biological hydrogen production by thermophilic bacteria and archaea in dark fermentations. The thermophiles are held as promising candidates for a cost-effective fermentation process, because of their relatively high yields and broad substrate palette. Yet many challenges remain to be faced, including improving productivity, tolerance to high osmolality and growth inhibitors, and reactor configuration. This review consolidates current insights in the quest for high yields and productivities within thermophilic hydrogen production. Important is to understand how environmental parameters affect the redox- and energy metabolism of the microorganism(s) involved. This knowledge is required for designing an optimal bioreactor configuration and operation

Pyrophosphate as a central energy carrier in the hydrogen-producing extremely thermophilic Caldicellulosiruptor saccharolyticus

The role of inorganic pyrophosphate (PPi) as an energy carrier in the central metabolism of the extremely thermophilic bacterium Caldicellulosiruptor saccharolyticus was investigated. In agreement with its annotated genome sequence, cell extracts were shown to exhibit PPi-dependent phosphofructokinase and pyruvate phosphate dikinase activity. In addition, membrane-bound pyrophosphatase activity was demonstrated, while no significant cytosolic pyrophosphatase activity was detected. During the exponential growth phase, high PPi levels (approximately 4 ± 2 mM) and relatively low ATP levels (0.43 ± 0.07 mM) were found, and the PPi/ATP ratio decreased 13-fold when the cells entered the stationary phase. Pyruvate kinase activity appeared to be allosterically affected by PPi. Altogether, these findings suggest an important role for PPi in the central energy metabolism of C. saccharolyticu

Probing the redox metabolism in the strictly anaerobic, extremely thermophilic, hydrogen-producing Caldicellulosiruptor saccharolyticus using amperometry

Changes in the redox metabolism in the anaerobic, extremely thermophilic, hydrogen-forming bacterium Caldicellulosiruptor saccharolyticus were probed for the first time in vivo using mediated amperometry with ferricyanide as a thermotolerant external mediator. Clear differences in the intracellular electron flow were observed when cells were supplied with different carbon sources. A higher electrochemical response was detected when cells were supplied with xylose than with sucrose or glucose. Moreover, using the mediated electrochemical method, it was possible to detect differences in the electron flow between cells harvested in the exponential and stationary growth phases. The electron flow of C. saccharolyticus was dependent on the NADH- and reduced ferredoxin generation flux and the competitive behavior of cytosolic and membrane-associated oxidoreductases. Sodium oxamate was used to inhibit the NADH-dependent lactate dehydrogenase, upon which more NADH was directed to membrane-associated enzymes for ferricyanide reduction, leading to a higher electrochemical signal. The method is noninvasive and the results presented here demonstrate that this method can be used to accurately detect changes in the intracellular electron flow and to probe redox enzyme properties of a strictly anaerobic thermophile in vivo

Biological hydrogen production from lignocellulosic biomass

Biologically derived hydrogen (biohydrogen) from lignocellulosic biomass has the potential to be an ideal renewable fuel as its combustion does not produce carbon-based emissions and it can be derived from sources which do not affect food production. Moreover, the by-products of biohydrogen production can be fed to an anaerobic digester producing biogas. Thus, a two-step process involving biohydrogen production followed by biogas production is emerging as a viable option for conversion of lignocellulosic biomass. Essential aspects of designing a viable hydrogen production system such as biomass selection, inhibitory compounds in the biomass, removal and salvage of hydrogen and the desirable qualities in any hydrogen producing organism are discussed in this chapter