Search

As abundant carbohydrates in renewable feedstocks, such as pectin-rich and lignocellulosic hydrolysates, the pentoses arabinose and xylose are regarded as important substrates for production of biofuels and chemicals by engineered microbial hosts. Their efficient transport across the cellular membrane is a prerequisite for economically viable fermentation processes. Thus, there is a need for transporter variants exhibiting a high transport rate of pentoses, especially in the presence of glucose, another major constituent of biomass-based feedstocks. Here, we describe a variant of the galactose permease Gal2 from Saccharomyces cerevisiae (Gal2N376Y/M435I), which is fully insensitive to competitive inhibition by glucose, but, at the same time, exhibits an improved transport capacity for xylose compared to the wildtype protein. Due to this unique property, it significantly reduces the fermentation time of a diploid industrial yeast strain engineered for efficient xylose consumption in mixed glucose/xylose media. When the N376Y/M435I mutations are introduced into a Gal2 variant resistant to glucose-induced degradation, the time necessary for the complete consumption of xylose is reduced by approximately 40%. Moreover, Gal2N376Y/M435I confers improved growth of engineered yeast on arabinose. Therefore, it is a valuable addition to the toolbox necessary for valorization of complex carbohydrate mixtures.

In deep geological repository systems, containers for highly radioactive waste may encounter groundwater, which ultimately leads to the release of radionuclides. For the exploration of new atomic waste repositories, the retention of these radionuclides in hydrothermally grown barite and calcite has drawn much attention recently. Here, partition coefficients are important parameters as they are helpful to evaluate the immobilization potential. To derive meaningful partition coefficients, only parts of the crystals in equilibrium with their host fluid should be considered. Differences in partition coefficients between synthetic and natural calcite have been observed and might at least partially stem from such analytical complications. This study presents a method which combines both, the advantages of visualizing growth structures at the µm scale and element sensitivity down to the ppm level via LA-ICP-MS. Objects of investigation are natural barites from a borehole in the Äspö Hard Rock Laboratory, Sweden, that have been sampled together with their host fluid. High-resolution mappings with a spot size of 2 µm allow us to define discrete growth zones and calculate equilibrium partition coefficients based on pixel clusters that represent the latest crystallization events. Additional LA-ICP-MS spot analyses substantiate the lower error values of our method and provide partition coefficients for twelve elements. La and Sr are assumed to behave like radionuclides, due to similar chemical properties. Preliminary barite partition coefficients for La and Sr are 3-4 orders of magnitude below calcite values of the same sampling location, indicating a much lower immobilization potential of barite compared to calcite.