Metabolite labelling as a tool to define hierarchies in Clostridium acetobutylicum sugar usage and its relevance for biofuel production

The finite nature of fossil-based resources, as well as the environmental awareness of the effects of their exploitation, has led to an increasing interest in finding alternative ways to produce chemicals, materials and energy. In terms of energy, biofuels are a good option for decreasing the environmental effects caused by fossil fuels, and in the last years, a great effort has been made to find industrial feedstocks for their production that do not compete with human feeding. Agricultural lignocellulosic waste seems as a good alternative for the currently used feedstock (mainly corn), and several companies are investing great efforts in developing efficient pre-treatment of the material, as well as appropriate enzymatic cocktails, to ensure the release of monomeric sugars from this material. Although bioethanol is the most known and industrially produced biofuel, biobutanol is also a good alternative, showing, compared to ethanol, a higher energy content and lower volatility, being also less corrosive (Jin et al., 2011). A well-known route for the biological production of butanol is the clostridial ABE route. ABE stands for acetone, butanol and ethanol and defines a pathway shared for several Clostridium species, known as solventogenic Clostridia, that convert sugars into these three solvents during their growth (D€ urre, 1998). The ABE fermentation was industrially exploited in Europe and USA during World War I, to obtain acetone and to produce cordite, a propellant compound utilized in munition. At that time, butanol was a worthless co-product, although it raised in value just after the war, being used by the nitrocellulose lacquer industry (Gabriel, 1928). These industrial processes ended in 2004 due to economic reasons (Schiel-Bengelsdorf et al., 2013). Interestingly, in the last years, there is a rising interest in butanol fermentation, due to, in the one hand, the increasing interest in biofuels versus fossil fuels, and, in the other hand, the instability of the ethanol market, and the possibility of using butanol, not only as a fuel, but also as a bulk chemical. Several solventogenic Clostridia have been studied for their butanol production. The most widely studied strain is Clostridium acetobutylicum, followed by Clostridium beijerinckii. Clostridial strains are good candidates for fermenting lignocellulosic material, because they belong to the number of anaerobic bacteria that naturally contribute to the decomposition of lignocellulosic wastes in soils. In terms of butanol production, several mutants have been developed, both by random mutagenesis or engineered, that increase the butanol production of their parental strains. Usually, the butanol hyperproducing phenotype is obtained because the strain shows a higher tolerance to the solvent (butanol, Lin and Blaschek, 1983; Liu et al., 2013). Another strategy to increase butanol production relies on directing the metabolism of the bacteria to decrease the production of the other solvents, changing the normal 3:6:1 acetone:butanol:ethanol ratio, to a higher butanol ratio, either avoiding acid formation (Jang et al., 2012) or increasing their re-assimilation (Xu et al., 2014). Also, when fermenting a complex substrate, such as lignocellulosic wastes, the efficient utilization of all fermentable sugars is important, and some of the hyperproducing strains described show a better assimilation of C5 sugars (Ren et al., 2010). The recent work by Ludmilla Aristilde (Aristilde, 2017) uses metabolic labelling to elucidate the fate of the carbons from the different sugars present in a lignocellulosic sample, after fermentation by Clostridium acetobutylicum. She applies an already established method (high-performance LC-MS) to identify the metabolites that can clarify whether the carbons are being used for biofuels synthesis. The main sugars present in lignocellulosic Received 24 January, 2017; accepted 25 January, 2017. *For correspondence. E-mail [email protected]; Tel. +34 954553822; Fax +34 954557104. Microbial Biotechnology (2017) 10(3), 525–527 doi:10.1111/1751-7915.12696 Funding information No funding information provided.