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Microbial Biotechnology
Published in Firdos Alam Khan, Biotechnology Fundamentals, 2020
Denitrification is the utilization of nitrate (NO3−) as a terminal electron acceptor (Figure 5.7). It is a widespread process that is used by many members of Proteobacteria. Many facultative anaerobes use denitrification because nitrate, like oxygen, has a high reduction potential. Many denitrifying bacteria can also use ferric iron (Fe3+) and some organic electron acceptors. Denitrification involves the stepwise reduction of nitrate to nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2) by the enzymes nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase, respectively. Protons are transported across the membrane by the initial NADH reductase, quinones, and nitrous oxide reductase to produce the electrochemical gradient critical for respiration. Some organisms (such as E. coli) only produce nitrate reductase and therefore can accomplish only the first reduction, leading to the accumulation of nitrite. Others (such as Paracoccus denitrificans and Pseudomonas stutzeri) reduce nitrate completely. Complete denitrification is an environmentally significant process because some intermediates of denitrification (nitric oxide and nitrous oxide) are important greenhouse gases (GHGs) that react with sunlight and ozone to produce nitric acid, a component of acid rain. Denitrification is also important in biological wastewater treatment, where it is used to reduce the amount of nitrogen released into the environment.
Microbial biotechnology
Published in Firdos Alam Khan, Biotechnology Fundamentals, 2018
Denitrification is the utilization of nitrate (NO3–) as a terminal electron acceptor (Figure 5.7). It is a widespread process that is used by many members of Proteobacteria. Many facultative anaerobes use denitrification because nitrate, like oxygen, has a high reduction potential. Many denitrifying bacteria can also use ferric iron (Fe3+) and some organic electron acceptors. Denitrification involves the stepwise reduction of nitrate to nitrite (N02–), nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2) by the enzymes nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase, respectively. Protons are transported across the membrane by the initial NADH reductase, quinones, and nitrous oxide reductase to produce the electrochemical gradient critical for respiration. Some organisms (such as Escherichia coli) only produce nitrate reductase and therefore can accomplish only the first reduction, leading to the accumulation of nitrite. Others (such as Paracoccus denitrificans and Pseudomonas stutzeri) reduce nitrate completely. Complete denitrification is an environmentally significant process because some intermediates of denitrification (nitric oxide and nitrous oxide) are important greenhouse gases that react with sunlight and ozone to produce nitric acid, a component of acid rain. Denitrification is also important in biological wastewater treatment, where it is used to reduce the amount of nitrogen released into the environment.
The Importance of Microbes in Organic Matter Composting
Published in Gustavo Molina, Zeba Usmani, Minaxi Sharma, Abdelaziz Yasri, Vijai Kumar Gupta, Microbes in Agri-Forestry Biotechnology, 2023
Zimin Wei, Junqiu Wu, Xiaomeng Chen, Haishi Qi, Mingzi Shi, Yufeng Chen, Yue Zhao, Xu Zhang, Xinyu Xie
Furthermore, composting systems rely on heterotrophic denitrification for nitrate reduction. Denitrification leads to a great loss of nitrogen and serious environment pollution during composting. Denitrification is a four-step biochemical process: (1) NO3- is reduced to NO2−, which is catalyzed by nitrate reductase encoded by napG and napA; (2) NO2− is reduced to NO, which is catalyzed by nitrite reductase encoded by nirS; (3) NO is reduced to N2O, which is catalyzed by nitric oxide reductase encoded by nor; and (4) N2O is reduced to N2, which is catalyzed by nitrous oxide reductase encoded by nosZ (Smith et al., 2007; Sánchez & Minamisawa, 2008). Among these key enzymes of denitrification, the nosZ-encoding nitrous oxide reductase is the most important one for its ability to reduce air pollution during denitrification. Denitrifying microbes may carry one or more types of denitrification functional enzymes. Complete denitrifying microbes can successively reduce nitrate to N2 with transient accumulation of intermediates (nitrite, NO and N2O), which have the ability to synthesize all fully functional enzymes. Relatively, denitrifying microbes lack one or more functional enzymes that are considered truncated denitrifying microbe (Moreno-Vivian et al., 1999). Truncated denitrifying microbes have been frequently observed in the composting, which are physiological and regulatory characteristics diverse. Therefore, partial denitrification may occur during composting containing truncated denitrifying microbes. However, significant accumulation of nitrite or N2O may also be conducted by some complete denitrifying microbes during composting (Lycus et al., 2018). It may be due to different denitrification functional enzymes with different reducing abilities under specific conditions. Besides denitrification, the denitrifying microbes can participate in various metabolic pathways (Lycus et al., 2017). For instance, denitrifying bacteria Azoarcus, Georgfuchaia, Rhodoferax and Sulfuritalea play pivotal roles in degrading polycyclic aromatic hydrocarbons (PAHs). Paracoccus and Pseudomonas, the typical aerobic denitrifying microbes, can aerobically reduce nitrate, nitrite to nitrogen or N2O. Recent studies show that some autotrophic microbes can perform denitrification with inorganic C sources (CO2, CO32−), such as Thiobacillus. Denitrifying polyphosphate bacteria is metabolically diverse, which perform nitrate reduction while accumulating phosphorus. In general, the real-time quantification of nitrite reductase functional genes nirSor nirK is the gold standard for the presence of denitrifying microorganisms (Kraft et al., 2014; Lycus et al., 2018).
Denitrification-induced carbonate precipitation by bio-composite material with Pseudomonas aeruginosa for simultaneous nitrate and cadmium remediation
Published in Human and Ecological Risk Assessment: An International Journal, 2023
Chaolin Fang, Ruitao Lin, Kaixuan Xu, Varenyam Achal
Common methods for such water pollution treatment mainly include bioremediation, permeable reactive barriers, air sparging, and adsorption (He et al. 2020a; Zhao et al. 2021). Among them, biological denitrification, a type of bioremediation, has gained increasing interest for nitrate removal; however, it’s efficiency in simultaneous remediation of heavy metals is often in question. Denitrification is sensitive to heavy metals, and the inhibition of denitrification is dependent on the amount of heavy metals in surrounding medium (He et al. 2020b). It has been found that aerobic denitrifying activity decreases with increasing heavy metal concentrations due to their corresponding inhibition on the related enzymes (Gui et al. 2017). The process of denitrification is catalyzed mainly by denitrifying enzymes including nitrite reductase, nitric oxide reductase, and nitrous oxide reductase, from nitrate to nitrogen gas (Heylen et al. 2006). However, these enzyme activities have not been documented at large in heavy metal remediation with denitrification.