María Dolores Pérez Hidalgo

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

This is a highlight on article ‘Metabolite labelling reveals hierarchies in Clostridium acetobutylicum that selectively channel carbons from sugar mixtures towards biofuel precursors’ by Ludmilla Aristilde.

Fermentation of glycerol by a newly discovered anaerobic bacterium: adding value to biodiesel production

In the last years, there is a rising interest in substituting fossil-derived fuels by biofuels, due mostly to environmental reasons and the finite nature of the former ones. Biodiesel, together with bioethanol, are the two more volumetrically produced biofuels worldwide. Moreover, in the last decade, biodiesel production in Europe contributed to more than 80% of global biodiesel production (Demirbas and Balat, 2006), with an estimated production of over 10 million tons in 2015 and a production capacity of 23 million tons (Patil et al., 2017). By definition, biodiesel is any liquid fuel derived from organic acids, such as vegetable oil or animal fat, that can be used in standard diesel engines. It can be used either alone or blended with petro-diesel in different proportions. Biodiesel consists of long-chain alkyl esters and is typically made by chemically reacting lipids with an alcohol. During its synthesis, a considerable amount of glycerol is produced. Glycerol (1, 2, 3-propanetriol) is a simple trivalent alcohol that is naturally found as the backbone of animal and plants triglycerides. Although it has wide applications in different industries (food, pharmaceutical, cosmetics, tobacco. . .), its increased co-production in biodiesel industries has made it a waste product instead of a valuable co-product; moreover, the glycerol obtained during biofuel synthesis cannot be directly use in any industrial application due to the impurities that contains. On the other hand, its chemical composition makes it a better feedstock in yield terms than sugars for fermentation into reduced products, such as ethanol or H2 (Murarka et al., 2008). In this issue of Microbial Biotechnology, Patil et al. (2017) describe the use of a newly discovered anaerobic bacterium that ferments glycerol. This anaerobic bacterium, Anaerobium acetethylicum, converts glycerol into two interesting biofuels: ethanol and hydrogen, with very little amounts of undesired co-products. Bio-ethanol is considered an alternative to fossil fuels, being renewable and with potential to reduce particulate emissions (Hansen et al., 2005). As mentioned before, it is the most common biofuel produced worldwide and can be used in gasoline engines, either in its pure form, or blended with gasoline. Hydrogen is a very interesting biofuel. In terms of mass, its energy content is higher than any other fuel, and its use in fuel cells ensures production of pollutionfree electricity. However, hydrogen is still mostly produced by steam reforming from hydrocarbons, although there is a huge research interest in its bio-based production, using either photosynthetic organisms, such as Rhodobacter capsulatus or Chlamydomonas reinhardtii (Scoma et al., 2012; Abo-Hashesh et al., 2013), or by anaerobic fermentation of sugars with different microbes, such as strains from the Clostridium or Enterobacter genera (Hung et al., 2011). Conversion of glycerol into these two valuable biofuels seems therefore an excellent way to add value to the well-established biodiesel industry. Anaerobium acetethylicum was recently isolated from sludge samples obtained from a biogas reactor at Germany. It was described as able to ferment gluconate, although the authors also reported growth on glycerol under strict anoxic conditions (Patil et al., 2015). It has been taxonomically classified into the order Clostridiales, and its genome has been sequenced (Patil et al., 2017). In their current article, the authors described optimal conditions for glycerol fermentation to ethanol and hydrogen, with very low production of other fermentation products (Patil et al., 2017). A. acetethylicum can grow in up to 1500 mM of glycerol, in the total absence of complex organic supplements, and the maximum ethanol production observed was 60 mM. As mentioned above, little production of undesirable co-products (acetate, formate and propylene glycol) was observed, although the authors reported the presence of a fermentation product Received 2 March, 2017; accepted 6 March, 2017. *For correspondence. E-mail epuerta@us.es; Tel. +34 954553822; Fax +34 954557104. Microbial Biotechnology (2017) 10(3), 528–530 doi:10.1111/1751-7915.12709 Funding Information No funding information provided.