Wheat straw, or the dried wheat stalks left after grain production, is a promising source of biofuels and basic chemicals. Before straw can be turned into useful products by biorefineries, however, the polymers that make it up should be broken down into their building blocks.
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Scientists have now discovered that microbes from the intestines of some species of termites can help break down lignin, a typically hard polymer in straw. The study results were published in the journal ACS Sustainable Chemistry & Engineering.
The three essential polymers in straw and other dried plant material – lignin, hemicelluloses, and cellulose – are nested into a complex 3D structure. Cellulose and hemicelluloses are polysaccharides that can be broken down into sugars and converted into fuel in bioreactors.
In contrast, lignin is an aromatic polymer that can be modified into useful industrial chemicals. Enzymes extracted from mushrooms can break down lignin, which is considered to be the hardest of the three polymers to collapse. However, researchers are looking for bacterial enzymes that are easier to synthesize.
In a previous study, Guillermina Hernandez-Raquet and co-workers showed that gut microbes from four species of termites can break down lignin in anaerobic bioreactors. Presently, with Yuki Tobimatsu and Mirjam Kabel, they wanted to take a closer look at the process by which wood-eating insect microbes break down lignin in wheat straw and determine the changes they make to this material.
The team added 500 viscera from four higher termite species each to various anaerobic bioreactors, and added wheat straw as the sole source of carbon. After a period of 20 days, they adjusted the composition of the digested straw to that of the unrefined straw. All gut microbiomes decomposed lignin (up to 37%), although they were more effective at breaking down cellulose (41%) and hemicelluloses (51%).
Lignin residues in the straw had experienced structural and chemical fluctuations, such as: B. the oxidation of some of its subunits. The team suggested that the effective breakdown of hemicelluloses by the microbes may also have improved the breakdown of lignin, which is crosslinked with the polysaccharides.
In future studies, researchers aim to find the enzymes, microorganisms, and lignin degradation pathways that are behind such effects that could be used in lignocellulosic biorefineries.
This study was financially supported by the PHC Sakura program, the Japanese Society for the Advancement of Science, the Research Institute for Sustainable Humanosphere, the ZELCOR program Horizon 2020 of the European Union, the French National Research Institute for Agriculture, Food and Environment, Languedoc Region – Roussillon Midi-Pyrénées grant and the 3BCAR project of the Carnot Institute.
Dumond, L. et al. (2021) Contribution of termite intestinal microbiota to the delignification of wheat straw in anaerobic bioreactors. ACS Sustainable Chemistry & Engineering. doi.org/10.1021/acssuschemeng.0c07817.