ICMCS seminar - N2O sink or source? The impact of the anoxic switch on denitrifying bacteria.

Title: N₂O sink or source? The impact of the anoxic switch on denitrifying bacteria.

Penot, Anne-Maelle¹*; Chatzisymeon, Efthalia²; MacPhee, Cait¹; Domingo-Felez, Carlos³; Free, Andrew⁴; Melaugh, Gavin¹

¹School of Physics and Astronomy, University of Edinburgh, United Kingdom; ²School of Engineering, University of Edinburgh, United Kingdom; ³James Watt School of Engineering, University of Glasgow, United Kingdom; ⁴School of Biological Sciences, University of Edinburgh, United Kingdom

*Presenting author email: a.m.m.penot [at] ed.ac.uk

Abstract: Biological removal of nitrogen in wastewater treatment (WWT) is responsible for significant emissions of nitrous oxide (N₂O) – a potent greenhouse gas – which account for up to 75% of the carbon footprint of WWT plants [1]. Aeration patterns strongly influence N₂O dynamics, but findings are often contradictory: intermittent aeration, for instance, is linked to high N₂O emissions in some studies [2, 3] but is used as a mitigation strategy in others [4, 5]. To resolve these inconsistencies, a deeper understanding of microbial behaviour around the aerobic-anoxic transition is needed—particularly regarding the denitrification pathway, which can act as a net source or sink for N₂O.

In this talk, I will present results from my PhD project, which aims to understand the N₂O production and consumption by the rapid denitrifier Comamonas denitrificans around the aerobic-anoxic switch [6].

We first observe a large accumulation of N₂O immediately following the onset of anoxia in batch cultures of C. denitrificans. Further investigation reveals negligible N₂O consumption during the aerobic and early-anoxic phases. As anoxia persists, N₂O consumption increases substantially, peaking at about 6.5 pmol.h^-1.cell^-1 – several orders of magnitude higher than previously reported for Paracoccus denitrificans [7].

We hypothesize that delayed expression of the nosZ gene, encoding N₂O reductase, underlies this transient N₂O accumulation in C. denitrificans – marking a key physiological difference from well-characterised denitrifiers like P. denitrificans [8]. These findings suggest that N₂O generation around the aerobic-anoxic switch is controlled by organism-specific regulation of denitrification genes.

References:

[1] Daelman, M.R.J., van Voorthuizen, E.M., van Dongen, L.G.J.M., Volcke, E.I.P. and van Loosdrecht, M.C.M. (2013). Methane and Nitrous Oxide Emissions from Municipal Wastewater Treatment – Results from a Long-term Study. Water Science and Technology, 67(10), pp.2350–2355.

[2] Dotro, Gabriela, et al. "A review of the impact and potential of intermittent aeration on continuous flow nitrifying activated sludge." Environmental technology 32.15 (2011): 1685-1697.

[3] Castro-Barros, Celia Maria, et al. "Effect of aeration regime on N2O emission from partial nitritation-anammox in a full-scale granular sludge reactor." Water Research 68 (2015): 793-803.

[4] Duan, Haoran, et al. "Mitigating nitrous oxide emissions at a full-scale wastewater treatment plant." Water research 185 (2020): 116196.

[5] Blum, Jan-Michael, Marlene Mark Jensen, and Barth F. Smets. "Nitrous oxide production in intermittently aerated Partial Nitritation-Anammox reactor: oxic N2O production dominates and relates with ammonia removal rate." Chemical Engineering Journal 335 (2018): 458-466.

[6] Gumaelius, Lena, et al. "Comamonas denitrificans sp. nov., an efficient denitrifying bacterium isolated from activated sludge." International journal of systematic and evolutionary microbiology 51.3 (2001): 999-1006.

[7] Suenaga, Toshikazu, et al. "Biokinetic characterization and activities of N2O-reducing bacteria in response to various oxygen levels." Frontiers in microbiology 9 (2018): 697.

[8] Qu, Zhi, et al. "Transcriptional and metabolic regulation of denitrification in Paracoccus denitrificans allows low but significant activity of nitrous oxide reductase under oxic conditions." Environmental microbiology 18.9 (2016): 2951-2963.

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