Nitrate reducing under anaerobic conditions where some bacteria utlilize Nitrate instead of Oxygen as terminal electrone acceptor is termed dissimilatory. Nitrate-reducing bacteria are mostly heterotrophic and often facultative anaerobic, with the ability to switch between oxygen and nitrate respiration depending on the environmental conditions.According to the electron acceptor, both oxygen and nitrate could be electron acceptor for the respiration for microorganisms in activated sludge. The oxygen is preferred over the nitrate because of its higher redox potential and more energy produced in respiration. an excessive increase in nitrate concentration (>10 mg l−1 NO3−-N) in the environment leads to nitrate pollution which has become a global environmental problem.Generally, the number of nitrate-reducing bacteria in deep crystalline bedrock groundwater have been predicted to be low, based on studies focusing on detection of essential genes (e.g., nitrate reductase, narG) for denitrification in deep crystalline bedrock environments . Nevertheless, these bacteria, otherwise below the detection limit, have been shown to significantly increase the transcription of narG genes in response to increased concentration of methane together with sulfate under N2 atmosphere in deep groundwater . In addition, it has been shown that certain ε-proteobacterial lineages couple reduction of nitrate to simultaneous oxidation of sulfide. Sulfur: Sulfur cycling, which is primarily driven by hydrogen sulfide and other reduced sulfur compounds serving as electron donors for sulfur-oxidizing microorganisms, is tightly interwoven with other important element cycles (carbon, nitrogen, iron, manganese) (Wasmund et al., 2017). Alternatively, sulfur (e.g., sulfate) can also act as a terminal electron acceptor, once more energetically favorable electron acceptors such as oxygen, nitrate/nitrite, and iron and manganese oxides are depleted.

Nitrates and nitrites are ubiquitous in the environment and commonly found in human diets. Nitrates are widely used as fertilizers in agriculture, resulting in high levels of nitrate accumulation in a variety of vegetables. Vegetables are the primary source of exposure to ingested nitrates, comprising nearly 80% of the total nitrate intake in a typical human diet.1 In contrast to nitrates.

Some microbes are capable of using nitrate as their terminal electron accepter. The ETS used is somewhat similar to aerobic respiration, but the terminal electron transport protein donates its electrons to nitrate instead of oxygen. Nitrate reduction in some species (the best studied being E. coli) is a two electron transfer where nitrate is reduced to nitrite. Electrons flow through the quinone pool and the cytochrome b/c1 complex and then nitrate reductase resulting in the transport of protons across the membrane as discussed earlier for aerobic respiration.

N03- + 2e- + 2H+N02-+ H20

This reaction is not particularly efficient. Nitrate does not as willingly accept electrons when compared to oxygen and the potential energy gain from reducing nitrate is less. If microbes have a choice, they will use oxygen instead of nitrate, but in environments where oxygen is limiting and nitrate is plentiful, nitrate reduction takes place.

@ Mohankrishnan, representatives of Neisseria, Rothia, Veillonella, Actinomyces, Corynebacterium, Haemophilus, and Kingella reduce nitrate to nitrite (Grant and Payne, 1981; Doel et al., 2005; Hyde et al., 2014). However, the bacteria active under nitrate rich waste-water conditions are Methanosarcina, Methanosaeta (Van Lier et al. 2008; Zinder and Mah 1979) and Clostridium (Lisle et al.).,

Nitrate reducing bacteria, such as Neisseria, Rothia, Actinomyces, and Kingellaو

Staphylococcus sciuri, followed by Staphylococcus intermedius,Pasteurella Spp.,and finally Streptococcus spp.