Development of Enterobacter aerogenes fuel cells: From in situ biohydrogen oxidization to direct electroactive biofilm

This study described an Enterobacter aerogenes-catalyzed microbial fuel cell (MFC) with a carbon-based anode that exhibited a maximum power density of 2.51 W/m³ in the absence of artificial electron mediators. The MFC was started up rapidly, within hours, and the current generation in the early stage was demonstrated to result from in situ oxidation of biohydrogen produced by E. aerogenes during glucose fermentation. Over periodic replacement of substrate, both planktonic biomass in the culture liquid and hydrogen productivity decreased, while increased power density and coulombic efficiency and decreased internal resistance were unexpectedly observed. Using scanning electron microscopy and cyclic voltammetry, it was found that the enhanced MFC performance was associated with the development of electroactive biofilm on the anodic surface, proposed to involve an acclimation and selection process of E. aerogenes cells under electrochemical tension. The significant advantage of rapid start-up and the ability to develop an electroactive biofilm identifies E. aerogenes as a suitable biocatalyst for MFC applications.     

Fluorescence in situ hybridization of uncultured zoosporic fungi: Testing with clone-FISH and application to freshwater samples using CARD-FISH

Recently, molecular environmental surveys of the eukaryotic microbial community in lakes have revealed a high diversity of sequences belonging to uncultured zoosporic fungi commonly known as chytrids. These microorganisms have two different stages in their life cycle and are known as algal parasites (i.e. host-attached infective sporangia) and as food sources for zooplankton (i.e. free-living zooflagellate propagules) in aquatic systems. However, because of their small size and their lack of distinctive morphological features, traditional microscopy does not allow the detection of chytrids, particularly of zoospores which have probably been misidentified as phagotrophic flagellates in previous studies. Hence, quantitative data on chytrids in natural environments is missing. We have adapted a clone-FISH approach known from prokaryotes to optimize the hybridization conditions of a designed oligonucleotidic probe specific to Chytridiales (i.e. the largest group of the true-fungal division of Chytridiomycota), before application to natural samples using the CARD-FISH approach. When these conditions were applied, the CARD-FISH assay demonstrated high specificity and sensitivity, and offers a promising tool for quantitative assessment of natural zoosporic fungi, primarily of zoospores which contributed up to 60% of the total abundance of heterotrophic flagellates. Although the field results from the CARD-FISH approach were considered preliminary and mainly as ‘proof of concept’, findings were consistent with ecological considerations known from pelagic habitats and host versus parasite populations, with recurrent ecological patterns in two contrasting lake ecosystems. We conclude that this approach will contribute to a better understanding of the ecological significance of zoosporic organisms in microbial food webs of pelagic ecosystems.     

Biotransformation of glycerol to 3-hydroxypropionaldehyde: Improved production by in situ complexation with bisulfite in a fed-batch mode and separation on anion exchanger

3-Hydroxypropionaldehyde (3HPA) is an important C3 chemical that can be produced from renewable glycerol by resting whole cells of Lactobacillus reuteri. However the process efficiency is limited due to substrate inhibition, product-mediated loss of enzyme activity and cell viability, and also formation of by-products. Complex formation of 3HPA with sodium bisulfite and subsequent binding to Amberlite IRA-400 was investigated as a means of in situ product recovery and for overcoming inhibition. The adsorption capacity and -isotherm of the resin were evaluated using the Langmuir model. The resin exhibited maximum capacity of 2.92 mmol complex/g when equilibrated with 45 mL solution containing an equilibrium mixture of 2.74 mmol 3HPA-bisulfite complex and 2.01 mmol free 3HPA. The dynamic binding capacity based on the breakthrough curve of 3HPA and its complex on passing a solution with 2.49 mmol complex and 1.65 mmol free 3HPA was 2.01 mmol/g resin. The bound 3HPA was desorbed from the resin using 0.20 M NaCl with a high purity as a mixture of complexed- and free 3HPA at a ratio of 0.77 mol/mol. Fed-batch biotransformation of glycerol (818.85 mmol) with in situ 3HPA complexation and separation on the bisulfite-functionalized resin resulted in an improved process with consumption of 481.36 mmol glycerol yielding 325.54 mmol 3HPA at a rate of 17.13 mmol/h and a yield of 68 mol%. Also, the cell activity was maintained for at least 28 h.