Nitrous oxide production in coupled aerobic-anoxic nitrous decomposition operation (CANDO) bioreactors : microbial community structure and metabolic pathways
- Much as humans respire oxygen, so do microorganisms, but, unlike humans, microorganisms can also respire other electron acceptors for energy. In the nitrogen cycle, microbial respiration of nitrogen species involves reduction of nitrite (NO3−) to NO2-, NO2- to nitrous oxide (N2O), and N2O to N2. Of particular interest is the N2O-producing step due to its adverse impacts as a greenhouse gas, and its potential beneficial use as a renewable source of energy. Interest in N2O for energy production has increased with introduction of a new process for short-cut nitrogen removal and energy recovery - the coupled aerobic-anoxic nitrous decomposition operation (CANDO). CANDO comprises three steps: (1) ammonia (NH4+) oxidation to NO2−; (2) anoxic NO2− reduction to N2O; and (3) energy generation by N2O conversion to N2. For the anoxic step, reduction of nitrite to N2O is coupled to oxidation of intracellular polyhydroxybutyrate (PHB) - an intracellular biopolymer that is produced upon acetate addition. For energy generation, N2O is used as a co-oxidant with air of methane, yielding more energy than combustion with air alone. It is currently unclear why microorganisms produce N2O, and efforts to address that question have largely focused on characterizing and minimizing N2O emissions in conventional wastewater treatment plants. Maximizing N2O production and monitoring and characterization of CANDO communities provides a new window on the pathways and required functional genes. Enrichment communities and isolated pure cultures that efficiently removed NO2− (95-98%) and converted the majority to N2O (60-78%) providing insights into factors affecting physiology and ecology of N2O production. Under selection conditions favorable for N2O production (alternating exposure to acetate and nitrite), CANDO communities were dominated by strains of Comamonadaceae. Pure culture isolates were sequenced, and the likely pathway of N2O production was identified in which quinol-dependent nitric oxide reductase (qnor) produces N2O using reducing equivalents derived from the oxidation of intracellular PHB. One isolate, Alicycliphilus sp. CD02, contains polyphosphate kinase (ppk), the enzyme required for polyphosphate synthesis and production of PHB in Polyphosphate-Accumulating Organisms (PAOs) that are commonly used for soluble P removal in enhanced biological phosphorus removal (EBPR) systems. An unexpected observation was the lack of glycogen synthesis genes in CANDO isolates. This was surprising because conventional PAOs and GAOs use glycogen as a source of reducing equivalents for PHB synthesis. The ppk gene sequence in the CANDO isolate was also significantly different from those of conventional PAOs, suggesting a different role for polyphosphate (Poly-P) synthesis in Comamonadaceae.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Stanford University, Civil & Environmental Engineering Department.
|Spormann, Alfred M
|Spormann, Alfred M
|Statement of responsibility
|Submitted to the Department of Civil and Environmental Engineering.
|Thesis (Ph.D.)--Stanford University, 2017.
- © 2017 by Sunggeun Woo
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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