Designing an SSCF process for bioethanol production from lignocellulosic substrates by co-fermentation of xylose and glucose

Different lignocellulosic materials, from e.g. forest industry and agriculture, are potentially interesting feedstocks for ethanol production. To meet desired overall yields during ethanol production from lignocellulosic materials, it is important to use both hexoses and pentoses. Saccharomyces cerevisiae, the most commonly used organism for industrial ethanol production, does not naturally ferment pentoses. Currently, genetically modified Saccharomyces cerevisiae strains are becoming available for xylose fermentation. However, simultaneous fermentation of xylose and glucose in genetically modified Saccharomyces cerevisiae requires a favorable ratio between these sugars. In simultaneous saccharification and fermentation (SSF) of spruce, a potential feedstock in the northern hemisphere, the proportion of xylose is relatively low. Still, conversion of all xylose would increase the ethanol yield by as much as 7-8%. However, the low ratio between xylose and glucose makes this conversion challenging. Hence, a suitable process design for simultaneous saccharification and co-fermentation (SSCF) is needed. In xylose rich materials, such as wheat straw, there is a more favorable ratio of xylose and glucose. On the other hand, there are indications that the residual xylose after fermentation is higher for these feedstocks. Consequently, there is a need for improved process design also in this case. In the present work studies of enzyme kinetics and consumption rates were carried out in order to create a model for sugar release and sugar uptake. An improved SSCF process was subsequently designed on the basis of this model and experimentally verified

A short review on SSF – an interesting process option for ethanol production from lignocellulosic feedstocks

Simultaneous saccharification and fermentation (SSF) is one process option for production of ethanol from lignocellulose. The principal benefits of performing the enzymatic hydrolysis together with the fermentation, instead of in a separate step after the hydrolysis, are the reduced end-product inhibition of the enzymatic hydrolysis, and the reduced investment costs. The principal drawbacks, on the other hand, are the need to find favorable conditions (e.g. temperature and pH) for both the enzymatic hydrolysis and the fermentation and the difficulty to recycle the fermenting organism and the enzymes. To satisfy the first requirement, the temperature is normally kept below 37°C, whereas the difficulty to recycle the yeast makes it beneficial to operate with a low yeast concentration and at a high solid loading. In this review, we make a brief overview of recent experimental work and development of SSF using lignocellulosic feedstocks. Significant progress has been made with respect to increasing the substrate loading, decreasing the yeast concentration and co-fermentation of both hexoses and pentoses during SSF. Presently, an SSF process for e.g. wheat straw hydrolyzate can be expected to give final ethanol concentrations close to 40 g L-1 with a yield based on total hexoses and pentoses higher than 70%

Prefermentation improves xylose utilization in simultaneous saccharification and co-fermentation of pretreated spruce

<p>Abstract</p> <p>Background</p> <p>Simultaneous saccharification and fermentation (SSF) is a promising process option for ethanol production from lignocellulosic materials. However, both the overall ethanol yield and the final ethanol concentration in the fermentation broth must be high. Hence, almost complete conversion of both hexoses and pentoses must be achieved in SSF at a high solid content. A principal difficulty is to obtain an efficient pentose uptake in the presence of high glucose and inhibitor concentrations. Initial glucose present in pretreated spruce decreases the xylose utilization by yeast, due to competitive inhibition of sugar transport. In the current work, prefermentation was studied as a possible means to overcome the problem of competitive inhibition. The free hexoses, initially present in the slurry, were in these experiments fermented before adding the enzymes, thereby lowering the glucose concentration.</p> <p>Results</p> <p>This work shows that a high degree of xylose conversion and high ethanol yields can be achieved in SSF of pretreated spruce with a xylose fermenting strain of <it>Saccharomyces cerevisiae </it>(TMB3400) at 7% and 10% water insoluble solids (WIS). Prefermentation and fed-batch operation, both separately and in combination, improved xylose utilization. Up to 77% xylose utilization and 85% of theoretical ethanol yield (based on total sugars), giving a final ethanol concentration of 45 g L^-1, were obtained in fed-batch SSF at 10% WIS when prefermentation was applied.</p> <p>Conclusion</p> <p>Clearly, the mode of fermentation has a high impact on the xylose conversion by yeast in SSF. Prefermentation enhances xylose uptake most likely because of the reduced transport inhibition, in both batch and fed-batch operation. The process significance of this will be even greater for xylose-rich feedstocks.</p

A mutated xylose reductase increases bioethanol production more than a glucose/xylose facilitator in simultaneous fermentation and co-fermentation of wheat straw

Genetically engineered Saccharomyces cerevisiae strains are able to ferment xylose present in lignocellulosic biomass. However, better xylose fermenting strains are required to reach complete xylose uptake in simultaneous saccharification and co-fermentation (SSCF) of lignocellulosic hydrolyzates. In the current study, haploid Saccharomyces cerevisiae strains expressing a heterologous xylose pathway including either the native xylose reductase (XR) from P. stipitis, a mutated variant of XR (mXR) with altered co-factor preference, a glucose/xylose facilitator (Gxf1) from Candida intermedia or both mXR and Gxf1 were assessed in SSCF of acid-pretreated non-detoxified wheat straw. The xylose conversion in SSCF was doubled with the S. cerevisiae strain expressing mXR compared to the isogenic strain expressing the native XR, converting 76% and 38%, respectively. The xylitol yield was less than half using mXR in comparison with the native variant. As a result of this, the ethanol yield increased from 0.33 to 0.39 g g-1 when the native XR was replaced by mXR. In contrast, the expression of Gxf1 only slightly increased the xylose uptake, and did not increase the ethanol production. The results suggest that ethanolic xylose fermentation under SSCF conditions is controlled primarily by the XR activity and to a much lesser extent by xylose transport

Temporal escape-adaptation to eutrophication by Skeletonema marinoi

Diatoms commonly set off the spring-bloom in temperate coastal environments. However, their temporal offset may change in regions subject to nutrient enrichment, and by peaking earlier, such populations can maintain their position in the vernal plankton succession. We tested whether the marine keystone diatom Skeletonema marinoi can accomplish this through thermal evolutionary adaptation. Eight geographically separated subpopulations, representing hydromorphologically and climatologically similar inlets displaying a range of trophic states, were compared in a common-garden experiment. At early-spring temperatures, both doubling times and variation coefficients thereof, correlated negatively with the trophic state of the environment of origin, indicating selection for fast growth due to eutrophication. At mid-spring temperatures, the relationships were reversed, indicating selection in the opposite direction. At late-spring temperatures, no significant relationships were detected, suggesting relaxed selection. Subsequent field observations reflected these findings, where blooming temperatures decreased with trophic state. Natural selection thus moves along with eutrophication towards colder temperatures earlier in the spring, favouring genotypes with the capacity to grow fast. The thermal niche shift demonstrated herein may be an evolutionary mechanism essentially leading to trophic changes in the local ecosystem.Peer reviewe

Temporal escape-adaptation to eutrophication by Skeletonema marinoi

Diatoms commonly set off the spring-bloom in temperate coastal environments. However, their temporal offset may change in regions subject to nutrient enrichment, and by peaking earlier, such populations can maintain their position in the vernal plankton succession. We tested whether the marine keystone diatom Skeletonema marinoi can accomplish this through thermal evolutionary adaptation. Eight geographically separated subpopulations, representing hydromorphologically and climatologically similar inlets displaying a range of trophic states, were compared in a common-garden experiment. At early-spring temperatures, both doubling times and variation coefficients thereof, correlated negatively with the trophic state of the environment of origin, indicating selection for fast growth due to eutrophication. At mid-spring temperatures, the relationships were reversed, indicating selection in the opposite direction. At late-spring temperatures, no significant relationships were detected, suggesting relaxed selection. Subsequent field observations reflected these findings, where blooming temperatures decreased with trophic state. Natural selection thus moves along with eutrophication towards colder temperatures earlier in the spring, favouring genotypes with the capacity to grow fast. The thermal niche shift demonstrated herein may be an evolutionary mechanism essentially leading to trophic changes in the local ecosystem.Peer reviewe

Circumstellar molecular composition of the oxygen-rich AGB star IK Tau: I. Observations and LTE chemical abundance analysis

The aim of this paper is to study the molecular composition in the circumstellar envelope around the oxygen-rich star IK Tau. We observed IK Tau in several (sub)millimeter bands using the APEX telescope during three observing periods. To determine the spatial distribution of the $\mathrm{^{12}CO(3-2)}$ emission, mapping observations were performed. To constrain the physical conditions in the circumstellar envelope, multiple rotational CO emission lines were modeled using a non local thermodynamic equilibrium radiative transfer code. The rotational temperatures and the abundances of the other molecules were obtained assuming local thermodynamic equilibrium. An oxygen-rich Asymptotic Giant Branch star has been surveyed in the submillimeter wavelength range. Thirty four transitions of twelve molecular species, including maser lines, were detected. The kinetic temperature of the envelope was determined and the molecular abundance fractions of the molecules were estimated. The deduced molecular abundances were compared with observations and modeling from the literature and agree within a factor of 10, except for SO$_2$, which is found to be almost a factor 100 stronger than predicted by chemical models. From this study, we found that IK Tau is a good laboratory to study the conditions in circumstellar envelopes around oxygen-rich stars with (sub)millimeter-wavelength molecular lines. We could also expect from this study that the molecules in the circumstellar envelope can be explained more faithful by non-LTE analysis with lower and higher transition lines than by simple LTE analysis with only lower transition lines. In particular, the observed CO line profiles could be well reproduced by a simple expanding envelope model with a power law structure.Comment: 13 pages, 14 figures, 8 tables *Accepted for publication in Astronomy and Astrophysic

Evolved star water maser cloud size determined by star size

Cool, evolved stars undergo copious mass loss but the details of how the matter is returned to the ISM are still under debate. We investigated the structure and evolution of the wind at 5 to 50 stellar radii from Asymptotic Giant Branch and Red Supergiant stars. 22-GHz water masers around seven evolved stars were imaged using MERLIN, at sub-AU resolution. Each source was observed at between 2 and 7 epochs (several stellar periods). We compared our results with long-term Pushchino single dish monitoring. The 22-GHz emission is located in ~spherical, thick, unevenly filled shells. The outflow velocity doubles between the inner and outer shell limits. Water maser clumps could be matched at successive epochs separated by <2 years for AGB stars, or at least 5 years for RSG. This is much shorter than the decades taken for the wind to cross the maser shell, and comparison with spectral monitoring shows that some features fade and reappear. In 5 sources, most of the matched features brighten or dim in concert from one epoch to the next. One cloud in W Hya was caught in the act of passing in front of a background cloud leading to 50-fold, transient amplification. The masing clouds are 1-2 orders of magnitude denser than the wind average and contain a substantial fraction of the mass loss in this region, with a filling factor <1%. The RSG clouds are ~10x bigger than those round the AGB stars. Proper motions are dominated by expansion, with no systematic rotation. The maser clouds survive for decades (the shell crossing time) but the masers are not always beamed in our direction. Radiative effects cause changes in flux density throughout the maser shells on short timescales. Cloud size is proportional to parent star size; clouds have a similar radius to the star in the 22-GHz maser shell. Stellar properties such as convection cells must determine the clumping scale.Comment: Accepted by A&A 2012 July 10 Main text 29 pages, 62 figures Appendix 44 pages, 23 figure

Helicobacter pylori Membrane Vesicles Stimulate Innate Pro- and Anti-Inflammatory Responses and Induce Apoptosis in Jurkat T Cells

Persistent Helicobacter pylori infection induces chronic inflammation in the human gastric mucosa, which is associated with development of peptic ulceration, gastric atrophy, and gastric adenocarcinoma. It has been postulated that secretion of immunomodulatory molecules by H. pylori facilitates bacterial persistence, and membrane vesicles (MV), which have the potential to cross the gastric epithelial barrier, may mediate delivery of these molecules to host immune cells. However, bacterial MV effects on human immune cells remain largely uncharacterized to date. In the present study, we investigated the immunomodulatory effects of H. pylori MV with and without the vacuolating cytotoxin, VacA, which inhibits human T cell activity. We show a high degree of variability in the toxin content of vesicles between two H. pylori strains (SS1 and 60190). Vesicles from the more toxigenic 60190 strain contain more VacA (s1i1 type) than vesicles from the SS1 strain (s2i2 VacA), but engineering the SS1 strain to produce s1i1 VacA did not increase the toxin content of its vesicles. Vesicles from all strains tested, including a 60190 isogenic mutant null for VacA, strongly induced interleukin-10 (IL-10) and IL-6 production by human peripheral blood mononuclear cells independently of the infection status of the donor. Finally, we show that H. pylori MV induce T cell apoptosis and that this is enhanced by, but not completely dependent on, the carriage of VacA. Together, these findings suggest a role for H. pylori MV in the stimulation of innate pro- and anti-inflammatory responses and in the suppression of T cell immunity

Improving simultaneous saccharification and co-fermentation of pretreated wheat straw using both enzyme and substrate feeding

<p>Abstract</p> <p>Background</p> <p>Simultaneous saccharification and co-fermentation (SSCF) has been recognized as a feasible option for ethanol production from xylose-rich lignocellulosic materials. To reach high ethanol concentration in the broth, a high content of water-insoluble solids (WIS) is needed, which creates mixing problems and, furthermore, may decrease xylose uptake. Feeding of substrate has already been proven to give a higher xylose conversion than a batch SSCF. In the current work, enzyme feeding, in addition to substrate feeding, was investigated as a means of enabling a higher WIS content with a high xylose conversion in SSCF of a xylose-rich material. A recombinant xylose-fermenting strain of <it>Saccharomyces cerevisiae </it>(TMB3400) was used for this purpose in fed-batch SSCF experiments of steam-pretreated wheat straw.</p> <p>Results</p> <p>By using both enzyme and substrate feeding, the xylose conversion in SSCF could be increased from 40% to 50% in comparison to substrate feeding only. In addition, by this design of the feeding strategy, it was possible to process a WIS content corresponding to 11% in SSCF and obtain an ethanol yield on fermentable sugars of 0.35 g g^-1.</p> <p>Conclusion</p> <p>A combination of enzyme and substrate feeding was shown to enhance xylose uptake by yeast and increase overall ethanol yield in SSCF. This is conceptually important for the design of novel SSCF processes aiming at high-ethanol titers. Substrate feeding prevents viscosity from becoming too high and thereby allows a higher total amount of WIS to be added in the process. The enzyme feeding, furthermore, enables keeping the glucose concentration low, which kinetically favors xylose uptake and results in a higher xylose conversion.</p