Microorganisms Enhance Denitrification of Low Carbon-to-Nitrogen Ratio Municipal Wastewater and Reduce N<sub>2</sub>O Emission: Metabolites Promote Carbon Flux to Nitrate Bioreduction
Zhiyi Qin, Qimeng Liu, Lei Dong, Xin Zhang, Chun-Jian Ding, Yinguang Chen
Abstract
Denitrifying carbon source is mainly used for microbial growth and proliferation, substrate transport, and nitrate bioreduction. This paper reported a new approach to dramatically promote denitrification and reduce N 2 O emission of low carbon-to-nitrogen ratio (C/N) municipal wastewater by introducing two microorganisms to increase carbon flux to nitrate bioreduction without increasing the external carbon source. At a C/N of 3, the total nitrogen removal efficiency of the model denitrifier ( P. denitrificans ) was increased from 48.1% to 87.4%, with a 63.2% reduction in N 2 O emission after adding S. oneidensis MR-1 and B. subtilis . Mechanism investigation revealed that the carbon flux for P. denitrificans growth and proliferation remained unchanged but decreased for substrate transport, resulting in more carbon flux to produce more NADH for nitrate bioreduction. Proteomics analysis showed that the decreased carbon flux for substrate transport was due to biosurfactants (surfactin, lichenysin, and fengycin) secreted by B. subtilis, increasing P. denitrificans cell membrane permeability and decreasing substrate uptake via active transport. Moreover, metabolites (riboflavin, heme, and cytochromes) from S. oneidensis MR-1 improved electron transfer, while lichenysin improved P. denitrificans surface hydrophilicity, which promoted the entry of metal ions associated with denitrification into the cell, increasing denitrifying enzyme activity, particularly N 2 O reductase, ultimately enhancing denitrification and reducing N 2 O emission.