Aerobic denitrification

Aerobic denitrification or co-respiration the simultaneous use of both oxygen (O2) and nitrate (NO3−) as oxidizing agents, performed by various genera of microorganisms.[1] This process differs from anaerobic denitrification not only in its insensitivity to the presence of oxygen, but also in that it has a higher potential to create the harmful byproduct nitrous oxide.[2]

Nitrogen, acting as an oxidant, is therefore reduced in a succession of four reactions performed by the enzymes nitrate, nitrite, nitric-oxide, and nitrous oxide reductases.[2] The pathway ultimately yields reduced molecular nitrogen (N2), as well as, when the reaction does not reach completion, the intermediate species nitrous oxide (N2O). A simple denitrification reaction proceeds as:

NO
3− → NO
2− → NO → N
2O → N
2 (g)

The respiration reaction which utilizes oxygen as the oxidant is:

C
6H
12O
6 (aq) + 6O
2 (g) → 6CO
2 (g) + 6H
2O

Classically, it was thought that denitrification would not occur in the presence of oxygen since there seems to be no energetic advantage to using nitrate as an oxidant when oxygen is available.[2] Experiments have since proven that denitrifiers are often facultative anaerobes[3] and that aerobic denitrification does indeed occur in a broad range of microbial organisms with varying levels of productivity, usually lower productivity than results from purely aerobic respiration. The advantages of being able to perform denitrification in the presence of oxygen are uncertain, though it is possible that the ability to adapt to changes in oxygen levels plays a role.[1] Aerobic denitrification may be found in environments where fluctuating oxygen concentrations and reduced carbon are available[4]. The relative harsh environment inspires the potential of denitrifiers to degrade toxic nitrate or nitrate under an aerobic atmosphere. Aerobic denitrifiers tend to work efficiently at 25 ~ 37°C and pH 7 ~ 8, when dissolved oxygen concentration is 3 ~ 5 mg/L and C/N load ratio is 5 ~ 10[4].

Environmental impact

Wastewater treatment

Water treatment often relies on the activity of anaerobically denitrifying bacteria in order to remove nitrate from water.[5] However, due to the absence of oxygen, nitrate cannot be fully reduced to dinitrogen, thus nitrate remains in the water or it is converted to nitrous oxide.[5] Lingering nitrate in drinking water poses a plethora of health risks, and both nitrate and nitrous oxide have major environmental impacts.[6][7] Some hazards include, carcinogenic nitrate ions in drinking water, or eutrophication caused by oxidized nitrogen seeding Algal blooms.[5][8] Conversely aerobic denitrification can further reduce oxidized nitrogen in a less specialized environment.[5] For instance, many aerobically denitrifying bacteria from the pseudomonas genus (Pm. stutzeri, Pm. mendocina and Pm. Putida) were shown to be isolated from the Liangshui river in China, and could be further used in bioaugmentation to clear contaminated water.[5] In addition to Pseudomas, Acinetobacter sp. SYF26 was isolated from the Hei He reservoir in China.[9] Genomic analysis revealed a napA gene encoding a periplasmic nitrate reductase, and a nirK and nirS for gene for the nitrite reductase (both enzymes needed in aerobic nitrate denitrification).[9]

Global warming

Nitrous oxide is a 200-300 times more potent greenhouse gas than carbon dioxide, accounting for 5% of the global green house gas effect.[10][7] During the reduction of nitrate in wastewater treatment, nitrous oxide is only released in the absence of appropriate oxygen regulation.[10] Some solutions to combat the release of nitrous oxide from wastewater treatment could be to use aerobically denitrifying organisms with the capacity to fully reduce nitrogen.[5] For instance, P. Denitrificans has shown to efficiently reduce nitrate to N2 in cultured media and wastewater.[10] Furthermore, TR2 strains of P. sturzeri and Pseudomonas sp. strain K50 were also shown to have substantially low levels of nitrous oxide production in water treatment.[10] Thus enriching activated sludge for aerobic denitrifying bacteria can be effective in combating the global warming effects of nitrous oxide in wastewater treatment.[10]

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See also

References
  1. Robertson, Lesley A.; J. Gijs Kuenen (22 April 1984). "Aerobic denitrification: a controversy revived". Archives of Microbiology. 139 (4): 351–354. doi:10.1007/BF00408378. ISSN 0302-8933.
  2. Lloyd, David (October 1993). "Aerobic denitrification in soils and sediments: from fallacies to facts". Trends in Ecology & Evolution. 8 (10): 352–356. doi:10.1016/0169-5347(93)90218-E. PMID 21236194.
  3. Schlesinger, William H. (1997). Biogeochemistry: An Analysis of Global Change. Academic Press. ISBN 978-0-12-625155-5.
  4. Ji, Bin; Yang, Kai; Zhu, Lei; Jiang, Yu; Wang, Hongyu; Zhou, Jun; Zhang, Huining (August 2015). "Aerobic denitrification: A review of important advances of the last 30 years". Biotechnology and Bioprocess Engineering. 20 (4): 643–651. doi:10.1007/s12257-015-0009-0. ISSN 1226-8372. S2CID 85744076.
  5. Lv, Pengyi; Luo, Jinxue; Zhuang, Xuliang; Zhang, Dongqing; Huang, Zhanbin; Bai, Zhihui (2017-08-30). "Diversity of culturable aerobic denitrifying bacteria in the sediment, water and biofilms in Liangshui River of Beijing, China". Scientific Reports. 7 (1): 10032. Bibcode:2017NatSR...710032L. doi:10.1038/s41598-017-09556-9. ISSN 2045-2322. PMC 5577267. PMID 28855587.
  6. Richard, Alyce M.; Diaz, James H.; Kaye, Alan David (2014). "Reexamining the Risks of Drinking-Water Nitrates on Public Health". The Ochsner Journal. 14 (3): 392–398. ISSN 1524-5012. PMC 4171798. PMID 25249806.
  7. Aronson, Emma L.; Allison, Steven D. (2012-07-31). "Meta-Analysis of Environmental Impacts on Nitrous Oxide Release in Response to N Amendment". Frontiers in Microbiology. 3: 272. doi:10.3389/fmicb.2012.00272. ISSN 1664-302X. PMC 3408851. PMID 22866053.
  8. Lloyd, David (1993-10-01). "Aerobic denitrification in soils and sediments: From fallacies to factx". Trends in Ecology & Evolution. 8 (10): 352–356. doi:10.1016/0169-5347(93)90218-E. PMID 21236194.
  9. Su, Jun-feng; Zhang, Kai; Huang, Ting-lin; Wen, Gang; Guo, Lin; Yang, Shao-fei (2015). "Heterotrophic nitrification and aerobic denitrification at low nutrient conditions by a newly isolated bacterium, Acinetobacter sp. SYF26". Microbiology. 161 (4): 829–837. doi:10.1099/mic.0.000047. PMID 25667009. S2CID 206197998.
  10. Takaya, Naoki; Catalan-Sakairi, Maria; Sakaguchi, Yasushi; Kato, Isao; Zhou, Zhemin; Shoun, Hirofumi (June 2003). "Aerobic Denitrification Bacteria That Produce Low Levels of Nitrous Oxide" (PDF). American Society for Microbiology: 3152–3157 – via http://aem.asm.org.
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