Mineral medium enriched for an acetate-butyrate type fermentation, potential glucose storage and the class ofClostridia
The product formation spectrum from the mineral medium enrichment culture was evaluated to identify the most dominant catabolic route. It was found that our flux-based model fitted best when 42% of the glucose was converted through the acetate-butyrate pathway involving electron bifurcation (Table S5). The microbial community was populated by two genera from the class of Clostridia : an Ethanoligenenspopulation and Clostridium population. The Ethanoligenenspopulation showed dominance, as Ruminococcaceae were dominant while Clostridium was a minority which was shown with fluorescentin situ hybridisation (Figure 3F and 3G). Ethanoligenens harbinense is a strictly anaerobic species known to produce ethanol, acetate and butyrate from carbohydrates (Tang et al., 2012; Xing et al., 2006). Cluster 12 (sensu strcito XII) of Clostridium was identified (Figure 3A and 3F). Clostridium pasteurianum is a well-studied species in this cluster (SILVA release 138), and is known for acetate-butyrate production involving electron bifurcation (Buckel and Thauer, 2013). This organism has also been found in a fermentative granular enrichment culture which stored poly-glucose (Tamis et al., 2015). In the mineral medium enrichment, effectively 20% of the glucose was potentially metabolised via a carbon storage pool (Figure S6A). The storage response in the mineral medium enrichment causes uncoupling of substrate uptake and growth. The community thereby maximises its substrate uptake rate (competitive advantage) while growing at a more balanced growth rate over the SBR cycle.