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Changes in Glucose Fermentation Pathways as a Response to the Free Ammonia Concentration in Microbial Electrolysis Cells

Mahmoud, Mohamed, Torres, César I., Rittmann, Bruce E.
Environmental Science & Technology 2017 v.51 no.22 pp. 13461-13470
Clostridiaceae, Geobacteraceae, Lactobacillales, Methanobacterium, Methanobrevibacter, acetates, ammonia, ammonium nitrogen, anodes, bacteria, chemical inhibitors, electrons, environmental science, fermentation, fermenters, fuels, glucose, hydrogen, lactic acid, methane, methane production, methanogens, microbial communities, microbial electrolysis cells, mixed culture, oxidation, phylotype, propionic acid, toxic substances
When a mixed-culture microbial electrolysis cell (MEC) is fed with a fermentable substrate, such as glucose, a significant fraction of the substrate’s electrons ends up as methane (CH₄) through hydrogenotrophic methanogenesis, an outcome that is undesired. Here, we show that free ammonia-nitrogen (FAN, which is NH₃) altered the glucose fermentation pathways in batch MECs, minimizing the production of H₂, the “fuel” for hydrogenotrophic methanogens. Consequently, the Coulombic efficiency (CE) increased: 57% for 0.02 g of FAN/L of fed-MEC, compared to 76% for 0.18 g of FAN/L of fed-MECs and 62% for 0.37 g of FAN/L of fed-MECs. Increasing the FAN concentration was associated with the accumulation of higher organic acids (e.g., lactate, iso-butyrate, and propionate), which was accompanied by increasing relative abundances of phylotypes that are most closely related to anode respiration (Geobacteraceae), lactic-acid production (Lactobacillales), and syntrophic acetate oxidation (Clostridiaceae). Thus, the microbial community established syntrophic relationships among glucose fermenters, acetogens, and anode-respiring bacteria (ARB). The archaeal population of the MEC fed 0.02 g FAN/L was dominated by Methanobacterium, but 0.18 and 0.37 g FAN/L led to Methanobrevibacter becoming the most abundant species. Our results provide insight into a way to decrease CH₄ production and increase CE using FAN to control the fermentation step, instead of inhibiting methanogens using expensive or toxic chemical inhibitors, such as 2-bromoethanesulfonic acid.