Abstract
BackgroundPretreatment of lignocellulosic biomass can produce inhibitory compounds that are harmful for microorganisms used in the production of biofuels and other chemicals from lignocellulosic sugars. Selective inhibitor removal can be achieved with biodetoxification where microorganisms catabolize the inhibitors without consuming the sugars. We engineered the strictly aerobic Acinetobacter baylyi ADP1 for detoxification of lignocellulosic hydrolysates by removing the gene for glucose dehydrogenase, gcd, which catalyzes the first step in its glucose catabolism.ResultsThe engineered A. baylyi ADP1 strain was shown to be incapable of consuming the main sugar components of lignocellulosic hydrolysates, i.e., glucose, xylose, and arabinose, but rapidly utilized acetate and formate. Formate was consumed during growth on acetate and by stationary phase cells, and this was enhanced in the presence of a common aromatic inhibitor of lignocellulosic hydrolysates, 4-hydroxybenzoate. The engineered strain tolerated glucose well up to 70 g/l, and the consumption of glucose, xylose, or arabinose was not observed in prolonged cultivations. The engineered strain was applied in removal of oxygen, a gaseous inhibitor of anaerobic fermentations. Co-cultivation with the A. baylyi ADP1 gcd knockout strain under initially aerobic conditions allowed the strictly anaerobic Clostridium butyricum to grow and produce hydrogen (H2) from sugars of the enzymatic rice straw hydrolysate.ConclusionsWe demonstrated that the model organism of bacterial genetics and metabolism, A. baylyi ADP1, could be engineered to be an efficient biodetoxification strain of lignocellulosic hydrolysates. Only one gene knockout was required to completely eliminate sugar consumption and the strain could be used in production of anaerobic conditions for the strictly anaerobic hydrogen producer, C. butyricum. Because of these encouraging results, we believe that A. baylyi ADP1 is a promising candidate for the detoxification of lignocellulosic hydrolysates for bioprocesses.
Highlights
Pretreatment of lignocellulosic biomass can produce inhibitory compounds that are harmful for microorganisms used in the production of biofuels and other chemicals from lignocellulosic sugars
As A. baylyi strains are known for their inability to grow on most sugars as a sole carbon source, we considered that A. baylyi ADP1 would be an ideal organism for the detoxification of lignocellulosic hydrolysates
Characterization of the strains In order to characterize the growth of A. baylyi ADP1 and gcd knockout strain on a mixture of sugars and organic acids commonly found in most lignocellulosic hydrolysates, we cultivated the cells on approximately 10 milli molar (mM) of glucose, xylose, arabinose, formate, acetate, and levulinate (Fig. 1)
Summary
Pretreatment of lignocellulosic biomass can produce inhibitory compounds that are harmful for microorganisms used in the production of biofuels and other chemicals from lignocellulosic sugars. Selective inhibitor removal can be achieved with biodetoxification where microorganisms catabolize the inhibitors without consuming the sugars. We engineered the strictly aerobic Acinetobacter baylyi ADP1 for detoxification of lignocellulosic hydro‐ lysates by removing the gene for glucose dehydrogenase, gcd, which catalyzes the first step in its glucose catabolism. Kannisto et al Biotechnol Biofuels (2015) 8:198 form of acetate has a positive effect on ethanol yield of S. cerevisiae [4]. Even though acetate itself is not a strong inhibitor, its removal from lignocellulosic hydrolysates could decrease the overall toxicity of furfural. Acinetobacter baylyi ATCC 17976 accumulates more than 10 % of the dry weight of wax esters when grown on acetate under nitrogen limitation [8]
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