Colorado State University

In the De Long lab at Colorado State University, we have been inspired to address this challenge. We aim to develop a better understanding of the process of rewired anaerobic digestion. We have used microbes from anaerobic wastewater sludge and the stomachs from bison and cattle (known as the rumen). Our approach involves designing and assembling bioreactors to test the fatty acid production potential of these communities and then using DNA sequencing to figure out which microorganisms contribute to the production of desired fatty acids.

The microbial communities we use have piqued our interest because of their potential for fermenting organic residues. Microbes living in the rumen of cattle and bison play an essential role in the breakdown of difficult to digest feeds, like the ones we observed in food and animal residues. They can transform these feeds into short chain fatty acids for energy metabolism in their host animal. Although they have been characterized in animal nutrition investigations, a limited number of studies have used them to produce fatty acids from organic residues.

In a recent study published in Bioresource Technology, we used bison rumen in bioreactors fermenting cellulose for the first time and evaluated their potential for rewired anaerobic digestion (Rico, Reardon, & Susan, 2021). Interestingly, microbes from these communities were different from those present in cattle and sludge and produced the highest rates of butyric acid, the most economically valuable short-chain fatty acid. We found that microbes belonging to the genus Alistipes were potential contributors to production of this fatty acid. This type of microbes has been associated with butyric acid and carbohydrate decomposition in the human and termite gut but not in anaerobic digesters (Poulsen et al., 2014; Qing et al., 2019). These results are fascinating since isolating and growing these microbes in bioreactors could promote strategies to achieve more profitable butyric acid yields from organic residues.

Similarly, cattle rumen produced more propionic acid, which was linked to a potential symbiosis between species of Fibrobacter succinogenes and Prevotella ruminicola. These microbes are one of the most active bacteria in the rumen. Certain P. ruminicola species cannot use cellulose, but they can produce propionic acid (Strobel, 1992). On the other hand, F. succinogenes can breakdown cellulose to release sugars promoting the growth of other microbes (Burnet et al., 2015; Suen et al., 2011). These capabilities suggest that increased production of propionic acid in cattle rumen bioreactors could have been driven by the activity of F. succinogenes promoting propionic acid production by P. ruminicola. Propionic acid is a three-carbon short chain fatty acid with an increasing market in the food preservatives industry. Engineering microbial communities with these bacteria could enable the development of technologies that utilize food or animal residues instead of petroleum-based feeds to produce propionic acid.

Our study provided new insights into the ability of microbes to convert cellulose to different fatty acids. We found that we can get more or less of the fatty acids we want to produce depending on what set of microbes we use. However, rewired anaerobic digestion is still a complex and not-fully understood biochemical system. Our ongoing work uses advanced molecular biology tools to characterize genomes and fatty acid production pathways. This knowledge will be crucial in developing more sustainable chemical factories to integrate into current waste management systems. Microbial-based processes, like rewired anaerobic digestion, can utilize less energy than traditional petrochemical processes. They operate at lower temperatures and pressures and do not demand toxic solvents (like the ones used in petrochemical manufacturing). Using food and animal residues instead of petroleum feedstocks is a win-win opportunity to reduce the tremendous amount of landfilled waste and its associated carbon emissions. Furthermore, it is also an opportunity to rethink the establishment of alternative chemical production systems that are more just and sustainable to vulnerable communities currently affected by petrochemical systems worldwide.