Co-fermentation of cellobiose and xylose by mixed culture of recombinant <i>Saccharomyces cerevisiae</i> and kinetic modeling

<div><p>Efficient conversion of cellulosic sugars in cellulosic hydrolysates is important for economically viable production of biofuels from lignocellulosic biomass, but the goal remains a critical challenge. The present study reports a new approach for simultaneous fermentation of cellobiose and xylose by using the co-culture consisting of recombinant <i>Saccharomyces cerevisi</i>ae specialist strains. The co-culture system can provide competitive advantage of modularity compared to the single culture system and can be tuned to deal with fluctuations in feedstock composition to achieve robust and cost-effective biofuel production. This study characterized fermentation kinetics of the recombinant cellobiose-consuming <i>S</i>. <i>cerevisiae</i> strain EJ2, xylose-consuming <i>S</i>. <i>cerevisiae</i> strain SR8, and their co-culture. The motivation for kinetic modeling was to provide guidance and prediction of using the co-culture system for simultaneous fermentation of mixed sugars with adjustable biomass of each specialist strain under different substrate concentrations. The kinetic model for the co-culture system was developed based on the pure culture models and incorporated the effects of product inhibition, initial substrate concentration and inoculum size. The model simulations were validated by results from independent fermentation experiments under different substrate conditions, and good agreement was found between model predictions and experimental data from batch fermentation of cellobiose, xylose and their mixtures. Additionally, with the guidance of model prediction, simultaneous co-fermentation of 60 g/L cellobiose and 20 g/L xylose was achieved with the initial cell densities of 0.45 g dry cell weight /L for EJ2 and 0.9 g dry cell weight /L SR8. The results demonstrated that the kinetic modeling could be used to guide the design and optimization of yeast co-culture conditions for achieving simultaneous fermentation of cellobiose and xylose with improved ethanol productivity, which is critically important for robust and efficient renewable biofuel production from lignocellulosic biomass.</p></div

Values of weighing factors of in the model for cellobiose and xylose co-fermentation by the co-culture of <i>S</i>. <i>cerevisiae</i> EJ2 and SR8.

<p>Values of weighing factors of in the model for cellobiose and xylose co-fermentation by the co-culture of <i>S</i>. <i>cerevisiae</i> EJ2 and SR8.</p

Xylose fermentation by <i>S</i>. <i>cerevisiae</i> SR8.

<p>Experimental and model simulated profiles of (A) cell growth, (B) xylose consumption and (C) ethanol production at different initial xylose concentrations using <i>S</i>. <i>cerevisiae</i> strain SR8. Lines represent model predictions and symbols represent the means of duplicate experimental results.</p

Modification of Fatty Acids in Membranes of Bacteria: Implication for an Adaptive Mechanism to the Toxicity of Carbon Nanotubes

We explored whether bacteria could respond adaptively to the presence of carbon nanotubes (CNTs) by investigating the influence of CNTs on the viability, composition of fatty acids, and cytoplasmic membrane fluidity of bacteria in aqueous medium for 24 h exposure. The CNTs included long single-walled carbon nanotubes (L-SWCNTs), short single-walled carbon nanotubes (S-SWCNTs), short carboxyl single-walled carbon nanotubes (S-SWCNT-COOH), and aligned multiwalled carbon nanotubes (A-MWCNTs). The bacteria included three common model bacteria, <i>Staphyloccocus aureus</i> (Gram-positive), <i>Bacillus subtilis</i> (Gram-positive), and <i>Escherichia coli</i> (Gram-negative), and one polybrominated diphenyl ether degrading strain, <i>Ochrobactrum</i> sp. (Gram-negative). Generally, L-SWCNTs were the most toxic to bacteria, whereas S-SWCNT-COOH showed the mildest bacterial toxicity. <i>Ochrobactrum</i> sp. was more susceptible to the toxic effect of CNTs than <i>E. coli</i>. Compared to the control in the absence of CNTs, the viability of <i>Ochrobactrum</i> sp. decreased from 71.6−81.4% to 41.8–70.2%, and <i>E. coli</i> from 93.7−104.0% to 67.7–91.0% when CNT concentration increased from 10 to 50 mg L^–1. The cytoplasmic membrane fluidity of bacteria increased with CNT concentration, and a significant negative correlation existed between the bacterial viabilities and membrane fluidity for <i>E. coli</i> and <i>Ochrobactrum</i> sp. (<i>p</i> < 0.05), indicating that the increase in membrane fluidity induced by CNTs was an important factor causing the inactivation of bacteria. In the presence of CNTs, <i>E. coli</i> and <i>Ochrobactrum</i> sp. showed elevation in the level of saturated fatty acids accompanied with reduction in unsaturated fatty acids, compensating for the fluidizing effect of CNTs. This demonstrated that bacteria could modify their composition of fatty acids to adapt to the toxicity of CNTs. In contrast, <i>S. aureus</i> and <i>B. subtilis</i> exposed to CNTs increased the proportion of branched-chain fatty acids and decreased the level of straight-chain fatty acids, which was also favorable to counteract the toxic effect of CNTs. This study suggests that the bacterial tolerances to CNTs are associated with both the adaptive modification of fatty acids in the membrane and the physicochemical properties of CNTs. This is the first report about the physiologically adaptive response of bacteria to CNTs, and may help to further understand the ecotoxicological effects of CNTs

Kinetic model parameters of cellobiose and xylose fermentation by <i>S</i>. <i>cerevisiae</i> EJ2 and SR8, respectively.

<p>Kinetic model parameters of cellobiose and xylose fermentation by <i>S</i>. <i>cerevisiae</i> EJ2 and SR8, respectively.</p

Cellobiose fermentation by <i>S</i>. <i>cerevisiae</i> EJ2.

<p>Experimental and model simulated profiles of (A) cell growth, (B) cellobiose consumption and (C) ethanol production at different initial cellobiose concentrations for <i>S</i>. <i>cerevisiae</i> strain EJ2. Lines represent model predictions and symbols represent the means of duplicate experimental results.</p

Bioavailability of Pyrene Associated with Suspended Sediment of Different Grain Sizes to <i>Daphnia magna</i> as Investigated by Passive Dosing Devices

Suspended sediment (SPS) is widely present in rivers around the world. However, the bioavailability of hydrophobic organic compounds (HOCs) associated with SPS is not well understood. In this work, the influence of SPS grain size on the bioavailability of SPS-associated pyrene to Daphnia magna was studied using a passive dosing device, which maintained a constant freely dissolved pyrene concentration (<i>C</i>_free) in the exposure systems. The immobilization and protein as well as enzymatic activities of Daphnia magna were investigated to study the bioavailability of SPS-associated pyrene. With <i>C</i>_free of pyrene ranging from 20.0 to 60.0 μg L^–1, the immobilization of Daphnia magna in the presence of 1 g L^–1 SPS was 1.11–2.89 times that in the absence of SPS. The immobilization caused by pyrene associated with different grain size SPS was on the order of 50–100 μm > 0–50 μm > 100–150 μm. When pyrene <i>C</i>_free was 20.0 μg L^–1, the immobilization caused by pyrene associated with 50–100 μm SPS was 1.42 and 2.43 times that with 0–50 and 100–150 μm SPS, respectively. The protein and enzymatic activities of Daphnia magna also varied with the SPS grain size. The effect of SPS grain size on the bioavailability of SPS-associated pyrene was mainly due to the difference in SPS ingestion by Daphnia magna and SPS composition, especially the organic carbon type, among the three size fractions. This study suggests that not only the concentration but also the size distribution of SPS should be considered for the development of a biological effect database and establishment of water quality criteria for HOCs in natural waters

Co-fermentation of cellobiose and xylose by the co-culture system at different initial sugar concentrations and inoculum sizes using <i>S</i>. <i>cerevisiae</i> strain EJ2 and SR8.

<p>The initial sugar concentrations are 40 g/L cellobiose+40 g/L xylose (A) and 80 g/L cellobiose+40 g/L xylose (B and C). The initial cell densities of EJ2+SR8 were 0.45 g dry cell weight/L+0.45 g dry cell weight/L (A and B) and 0.45 g dry cell weight /L+0.9 g dry cell weight /L (C). Lines represent model predictions and symbols represent experimental data (blue solid line, model curve of cellobiose consumption; blue dash line, model curve of xylose consumption; red solid line, model curve of cell growth; red dash line, model curve of ethanol production; blue solid circle, cellobiose concentration (g/L); blue hallow circle, xylose concentration (g/L); red solid circle, cell density (g dry cell weight /L); red hallow circle, ethanol concentration (g/L)).Experimental results are the means of duplicate experiments; error bars indicating standard deviations are not visible when smaller than the symbol size.</p

Co-culture of two specialist strains.

<p>Co-culture is adjustable to achieve simultaneous fermentation of cellobiose and xylose at different substrate concentration ratios, while single recombinant strain is unable to deal with varying substrate compositions.</p