The Contribution of Autotrophic Nitrogen Oxidizers to Global Nitrogen Conversion

*Hui-Ping Chuang, Akiyoshi Ohashi and Hideki Harada*

## **Abstract**

The accumulation of ammonium (NH4 + -N) and nitrous oxide (N2O-N) in the environment is causing concern due to their ecological impacts and contribution to global warming. Autotrophic nitrogen oxidizers, including aerobic ammonium-oxidizing archaea and bacteria, anaerobic ammonium oxidizer and nitrite oxidizers, play a crucial role in the nitrogen cycle by facilitating the removal of nitrogenous residues from the environment. Nitrogen oxides (NOx) like nitrite (NO2 − -N) and nitrate (NO3 − -N) are produced as key immediate products during the conversion of NH4 + -N or N2O-N. Additionally, these autotrophic microbes utilize carbon dioxide (CO2) for cell synthesis, thereby mitigating the greenhouse effect. Preliminary results pointed out that nitrogen oxidizers could effectively remove NH4 + -N and NOx from sewage and wastewater systems at the loading rate lower than 0.5 kg N/m3 -day. Moreover, this family could also reduce the greenhouse N2O-N through oxidizing pathway, attaining the maximum reduction of 25.2-fold the annual N2O production.

**Keywords:** autotrophic nitrogen oxidizers, biological technologies, greenhouse effect, nitrogen cycle, sponge media

## **1. Introduction**

The ever-increasing nitrogen pollution in the environment is getting attention in recent years, particularly regarding the high warming-potential nitrous oxide (N2O) and the discharge of the concerned ammonium (NH4 + -N). First, the total emissions of N2O reached 336.33 ppb (parts per billion) [1], accounting for 6.2% of GHGs (greenhouse gases), and its heat-capturing capacity is approximately 298 times higher than that of carbon dioxide (CO2), calculated over a 100-year period. The escalating rate of GHG emissions will accelerate global warming, leading to a projected 1.5°C temperature increase before 2030. N2O is also a primary contributor to ozone depletion, along with chlorofluorocarbons (CFCs), which amplifies the impact on the extreme climate [2]. Furthermore, nitrogen oxides (NOx, including NO and NO2) readily dissolve in water vapor, leading to the formation of acid rain, which releases heavy metals from soil, indirectly poisoning various organisms and causing ocean acidification. Recent findings indicate that N2O accumulation results from the quantitative release of marine and terrestrial environments, influenced by

the three major human activities of agriculture, chemical factories and wastewater treatment plants (WWTP) [3].

Ammonium (NH4 + -N) is another concerned compound, primarily originating from sources, such as animal husbandry, industrial and domestic sewage. Its impact is multifaceted, affecting gas, liquid and solid phases. The Environmental Protection Agency of Taiwan (Taiwan EPA) has set the regulatory limits of NH4 + -N and total nitrogen (TN) in the discharge, with the limits being lower than 30 and 35 milligrams nitrogen per liter (mgN/L), respectively. These regulations are set to be enforced in 2024. However, many industries and sewage treatment plants face challenges in treating the wastewater containing nitrogenous compounds to meet the EPA regulation. Moreover, NH4 + -N in the systematic environment can convert to pungent ammonia (NH3) (pKa of NH4 + /NH3 = 9.25) under high pH conditions. NH3 is a controlled component under the Convention on Long-Range Transboundary Air Pollution (CLRTAP), as outlined in the Gothenburg Protocol [3].

Various methods have been employed to eliminate nitrogen pollutants, such as NH4 + -N and N2O, including ion exchange resins, physical/biological adsorption and biological filtration [4], as well as thermal catalytic cracking and photocatalytic decomposition [5]. However, these treatment technologies often require significant initial investments and ongoing costs for consumable replacements. In recent years, biological treatment technologies with lower costs have been widely applied for the transformation of NH4 + -N in diverse environmental settings, encompassing processes such as nitrification, denitrification and other nitrogen-removal procedures [6]. Among them, the reduction of N2O in the last stage of the denitrification process has become the prevailing method for N2O elimination in the aquatic system. Achieving a N2O conversion rate of over 70% is possible when there is an ample carbon source available in the system. However, in low-carbon environments, the release of N2O becomes unfavorable for denitrifiers relying on high-carbon-to-nitrogen (C/N)-ratio food sources. Hence, autotrophic nitrogen oxidizers, including ammonia-oxidizing microorganism (AOM) and complete nitrifying bacteria (complete ammonia oxidizer, comammox), are potentially valuable contributors to reducing the residual N2O level in the atmosphere. These autotrophic nitrogen oxidizers offer the advantages of low cost and energy consumption.

Numerous chemical reactions based on the nitrogen cycle [3] have been identified for NH4 + -N removal, encompassing a total of 14 reactions. However, most of these reactions have primarily been investigated in laboratory-scale systems. In Taiwan, the aerobic activated sludge tank has emerged as a popular NH4 + -N removal system in the water treatment plants. Nevertheless, meeting the regulations set by the Environmental Protection Agency to reduce the total nitrogen requirement to 35 mgN/L by the year 2024 poses a significant challenge. Furthermore, the complete removal of NH4 + -N and nitrate (NO3 − -N) in urban sewage with low NH4 + -N concentration (<50 mgN/L) and wastewater with a low C/N ratio is problematic due to slow-growth rate of microorganisms and insufficient carbon sources. Consequently, treatment processes based on the mechanisms of autotrophic microbes have been proposed as valuable tools for the elimination of nitrogenous compounds in the wastewater.

In this chapter, we will explore the chemical substances and functional microorganisms that play pivotal roles in the nitrogen cycle. Additionally, we will investigate the utilization of two sponge-based biological systems to enhance the growth rate of the slow-growth functional microbes. Specifically, we will focus on the application of autotrophic nitrogen oxidizers for mitigating residual nitrogen pollutants in the environment.

*The Contribution of Autotrophic Nitrogen Oxidizers to Global Nitrogen Conversion DOI: http://dx.doi.org/10.5772/intechopen.112709*

## **2. Impact of nitrogen pollution on the environment**

Water pollution in Taiwan is primarily attributed to the discharge of pollutants from factories, livestock excretion and domestic sewage, accounting for 34.08, 2.75 and 63.17% of the total annual discharge of 3.2 billion cubic meters, respectively. Nitrogenous compounds originate from petrochemical industry (45%), high-tech industry (23%), pig manure, urine wastewater (16%) and domestic sewage (14%). A continuous release of nitrogen pollutants into the atmosphere and water bodies can lead to eutrophication and hypoxia in aquatic ecosystems (resulting from liquidous NH4 + -N), detrimentally affecting native plant species (associated with liquidous NO3 − -N), causing acid rain (resulting from gaseous NO2), contributing to global warming (related to gaseous N2O) and posing various other environmental challenges. The concentration of NH4 + -N in the petrochemical industrial wastewater ranges from 10 to 300 mgN/L (with a C/N ratio of approximately 3) [6], while domestic sewage typically contains 18.8 ± 5.71 mgN/L of NH4 + -N and 20.4 ± 6.38 mgN/L of total nitrogen (TN). The discharge of such sewage into natural water bodies accounts for 41.6% of national river pollution, with 7.2% classified as severe pollution (NH4 + -N > 3.0 mgN/L) (National Environmental Water Quality Monitoring Annual Report in 2022). These findings have further implications for groundwater systems, where 42.0% of regions exceed regulatory limits. Particularly, the Taipei Basin (<0.01 ~ 8.89 mgN/L) and the Jianan Plain (<0.01 ~ 8.76 mgN/L) exhibit the most severe contamination levels (statistical data obtained from the National Environmental Water Quality Monitoring System in 2022).

On the other hand, N2O, a well-known greenhouse gas, has garnered significant attention due to in contribution to global warming, reaching 336.33 ppb in December 2022 [1]. Approximately 4 teragrams (Tg) per year of nitrogen is released into the atmosphere from oceanic sources, while terrestrial source contributes around 12 Tg per year of nitrogen [7]. Human activities account for 40% of total greenhouse gas emission, with specific sectors making varying contributions [8]. Agricultural soil management is responsible for 74% of emissions, wastewater treatment for 6%, stationary combustion for 5%, chemical production and other product uses for 5%, manure management for 5%, transportation for 4% and other activities for 1% [9]. In terms of industrial processes, the largest amount of N2O is produced from nitric acid (HNO3), with an annual emission of about 400 metric tons [10]. Biological nitrogen-removal systems, including the activated sludge system (0.06%), nitrification (2.7–9%) [11], partial nitrification (nitritation), anaerobic ammonia oxidation (anammox), nitritation-anammox procedure (1.3–2.2%) [12], nitrifier denitrification, denitrification (0.6–1.9%) [13] and nitrification-denitrification process (1.9–8.5%) [14], have been identified as potential sources of N2O emission [15]. Nearly 70% of these emissions are attributed to the NH4 + -N oxidation process, resulting in a nitrogen conversion of 27% with equivalent to 600 parts per million by volume (ppmv) [16]. In the family of microbes involved in the nitrogen cycle, aerobic ammonia-oxidizing bacteria (aerAOB) have been found to release higher amounts of N2O compared to aerobic ammonia-oxidizing archaea (aerAOA) and comammox bacteria [17].

## **3. Elimination of nitrogen pollutants from the surroundings**

Nitrogen compounds, characterized by low molecular weight and high reactivity, have the ability to rapidly disperse into gas phase (atmosphere), liquid phase (various water bodies) and solid phase (soil or sediment), posing biological hazards and contributing to global warming. Many countries or organization, including the United States, Japan and the European Union, have implemented regulations to control the nitrogen concentrations in wastewater discharge, with limits set at less than 60 mgN/L. In Taiwan, the regulations will further restrict the total nitrogen content in public sewer systems to below 35 mgN/L by 2024. Of particular concern is N2O, which possesses a high greenhouse potential (GHP) and is approximately 298-fold more potent than CO2 in terms of its heat-trapping capacity. The significant impact of N2O on global temperature rise cannot be ignored. The 2015 Paris Agreement, signed by 200 countries, aims to mitigate the rate of global warming and limit the temperature increase to within 2°C by the end of the twenty-first century. This collective effort reflects the global commitment to combat the effects of N2O and other greenhouse gases on climate change.

The commonly physical and chemical methods are employed for the removal of NH4 + -N from wastewater. These methods include air stripping, ion exchange, reverse osmosis, electrodialysis and breakpoint chlorination, among others. They enable the efficient conversion or recovery of different forms of ammonium. However, these techniques are often associated with high operational costs and the challenge of disposing of secondary compounds, which limits their economic viability. In the case of N2O reduction, thermocatalytic methods [18] and photocatalytic methods [5] have been utilized for N2O decomposition. The use of thermocatalysis dates back to the 1950s [19], and it involves the utilization of various media such as metals, reducing oxides and zeolites [18]. Photocatalytic methods commonly employ zerovalent zinc [20]. Despite their effectiveness, physicochemical techniques are rarely used for N2O reduction in wastewater treatment plants. This is primarily attributed to the high levels of dissolved oxygen and the low concentration of N2O typically found in the water field [3].

Considering the aforementioned challenges, the environment-friendly and cost-effective biological treatment technologies present a promising approach for addressing the residual amounts of NH4 + -N or N2O in the environment. In the case of wastewater treatment, the selection of the appropriate biological treatment system depends on the prevailing C/N ratio. A nitrification-denitrification system is suitable for high C/N ratios, whereas a nitritation-anammox process is more effective for low C/N ratios [21]. In terms of N2O reduction, two main reaction pathways are commonly considered. The first pathway describes denitrification, where N2O is reduced to N2 as the final stage of the process [22]. The second pathway involves NO3 − -N ammonification [23]. Our research team has also been exploring an alternative elimination pathway involving oxidation [24]; however, the precise mechanism underlying NH2OH generation in this process is still not fully understood. Further investigations are needed to unravel this key aspect.

## **4. Pathway of nitrogen transformation**

Nitrogen is a vital element in the biosphere, playing a crucial role in atmospheric composition and the metabolic processes of living organisms. The nitrogen cycle encompasses various catabolic and anabolic reactions that drive the transformation of nitrogen compounds [25]. Five catabolic reactions include nitritification, nitratification, denitrification, dissimilatory nitrate reduction and anaerobic ammonium oxidation. These processes involve the conversion of nitrogenous compounds to

### *The Contribution of Autotrophic Nitrogen Oxidizers to Global Nitrogen Conversion DOI: http://dx.doi.org/10.5772/intechopen.112709*

different forms, facilitating their cycling within ecosystems. On the other hand, three anabolic reactions encompass ammonium uptake, assimilatory nitrate reduction and nitrogen fixation, which are responsible for the incorporation of nitrogen into organic molecules and the production of nitrogenous compounds essential for life processes. Additionally, ammonification is a crucial step in the nitrogen cycle, where organic nitrogen is converted back to ammonium. Understanding the intricate pathways of nitrogen transformation is essential for comprehending the dynamics of nitrogen cycling in various environmental systems.

The fundamental mechanisms of nitrogenous oxidation and reduction are nitrification and denitrification. Nitrification can be categorized into autotrophic nitrification and heterotrophic nitrification by different microbial communities. The former is carried out by aerobic autotrophic ammonia and nitrite oxidizers, and the latter is catalyzed by fungi as well as heterotrophic bacteria [26]. Autotrophic nitrification is a chemolithotrophic oxidation of ammonia to nitrate under strict aerobic conditions and conducted in two sequential oxidative stages: ammonia oxidation and nitrite oxidation. The yield of cells per unit of ammonia oxidizer (AOB) as the genus *Nitrosomonas* is approximately 0.15 mg cells/mg NH4 + -Noxidized, while for nitriteoxidizing bacteria (NOB) such as the genus *Nitrobacter*, it is around 0.02 mg cells/mg NO2 − -Noxidized. Oxygen consumption during these reactions is estimated to be 3.16 mg O2/mg NH4 + -Noxidized and 1.11 mg O2/mg NO2 − -Noxidized, respectively. Additionally, alkalinity in the form of 7.07 mg CaCO3/mg NH4 + -N is required for ammonium oxidation to maintain pH stability in the system.

Denitrification is the reduction of the oxidized nitrogen compounds (NO2 − -N and NO3 − -N) to dinitrogen (N2) through the intermediate production of nitrogen oxide (NO) and N2O. This transformation occurs via three distinct pathways, including respiratory denitrification, aerobic denitrification and lithoautotrophic denitrification. In the respiratory denitrification, heterotrophic microorganisms use nitrite and/ or nitrate as electron acceptors, while organic matter serves as the carbon and energy source in the absence of oxygen [27]. In environmental biotechnology applications, a variety of electron donors and carbon sources, such as methanol, acetate, glucose, ethanol and others, can be used to facilitate respiratory denitrification. Next, aerobic denitrification is complete denitrification occurring at high dissolved oxygen (DO) concentration, and heterotrophic organisms are responsible for corespiration of nitrate and oxygen and they are widespread in the environment [28]. Third, autotrophic denitrifiers catalyzed the lithoautotrophic denitrification using inorganic sulfur compounds, hydrogen or ammonia as electron donors [28]. These specialized microorganisms play a crucial role in the removal of nitrogen compounds in specific ecological niches.

Nitrous oxide (N2O) serves as a common intermediate in various nitrogen treatment systems. Within the nitrogen cycle, N2O is primarily produced through three metabolic mechanisms, namely (1) oxidation of hydroxylamine (NH2OH) [30], (2) nitrifier denitrification [31] and (3) anoxic nitrite reduction. First of all, NH2OH oxidation plays a crucial role in ammonium oxidation and is a significant reaction leading to N2O production. This process is catalyzed by hydroxylamine dehydrogenase (HAO) enzymes [32]. Two pathways have been identified for NH2OH oxidation: (a) NH2OH is first oxidized to NOH, which is subsequently chemically converted to N2O [33]; (b) NH2OH is first oxidized to NO, followed by enzymatic reduction to N2O mediated by cytochrome c554 (cyt c554) [34].

The second is nitrifier denitrification, and the main player as *Nitrosomonas europaea* and other AOBs can reduce NO2 − to NO, N2O or N2 in the absence of oxygen [35]. Two enzymes involved in this reaction are nitrite reductase (NIR) [36] and nitric oxide reductase (NOR) [37]. NIR enzymes catalyze the reduction of NO2 − -N to NO and subsequently, NOR enzymes facilitate the reduction of NO to N2O [38]. Studies have shown that *N. europaea* lacking NIR enzyme produced four fold higher amounts of N2O compared to the wild type with NIR enzyme, indicating the role of NIR in supporting HAO enzymes to enhance nitrification activity [39]. Notably, strains lacking NOR enzymes did not exhibit a significant effect on N2O production [37]. These research findings suggest that NIR enzymes can support the function of HAO enzymes to raise the nitrification activity under the sufficient electron source. Therefore, nitrifier denitrification is not the main source of N2O emission under normal situation of microbial growth. Overall, the metabolic pathways involving NH2OH oxidation and nitrifier denitrification contribute to the production of N2O within the nitrogen cycle. Further investigation into these mechanisms is necessary to better understand N2O emissions and develop effective strategies for its mitigation.

On the other hand, the release of N2O can occur under four different environmental conditions during the anoxic nitrite reduction. The first condition is N2O accumulation due to the inhibition of nitrous oxide reductase (N2OR) under DO concentration attaining 0.2–0.5 mg/L [40]. The second condition arises when N2OR becomes inactive, interrupting the reduction of N2O to N2 at low pH levels [23]. The third condition occurs when there is an insufficient electron source that resulted from a low biodegradable organic load [41]. Lastly, nitrite (NO2 − -N) as the electron acceptor is more prone to induce the N2O accumulation catalyzed by NIR/NOR enzymes, with a conversion rate of 55% per N transformation, compared to nitrate (NO3 − -N) at 0.8%/N transformation [42]. It is important to note that anammox bacteria and nitrite oxidizers are unlikely contributors to N2O production, as the pathways for their potential generation of N2O have been elucidated. Instead, four factors, including microaerobic environment, insufficient electron source, NO2 − -N accumulation and acidification, likely stimulate ammonia oxidizers and denitrifiers to produce N2O in wastewater treatment systems. Furthermore, NO2 − -N accumulation and acidification also promote abiotic decomposition processes that contribute to N2O emission.

To further eliminate the presence of high-GHP-N2O, two biological reduction mechanisms have been identified: (1) N2O reduction during denitrification and (2) NO3 − -N ammoniation. During denitrification, nitrous oxide reductase (N2OR) catalyzes the reduction of N2O to N2 [43]. In addition, studies have revealed the growth and activity of *Rhodobacter capsulatus* and *Wolinella succinogens* in the presence of high N2O concentration [44, 45]. However, N2O becomes the final product of denitrification at low C/N ratio in an influent, making it challenging to further initiate the reduction of N2O to N2. In terms of NO3 − -N ammoniation, *Bacillus vireti* utilize N2O oxidized to NOx by activating the NOS operon under anaerobic condition, while simultaneously synthesizing microbial cells.

## **5. Biological technologies based on the nitrogen cycle**

The biological nitrogen cycle encompasses 14 currently known biochemical conversion mechanisms, which can be broadly categorized into nitrification, comammox, denitrification, anammox, nitrate assimilation, respiratory ammonification (dissimilatory nitrate reduction to ammonia (DNRA)) and nitrogen fixation. Nitrification is a well-established process employed in wastewater treatment plants, where NH4 + -N is

### *The Contribution of Autotrophic Nitrogen Oxidizers to Global Nitrogen Conversion DOI: http://dx.doi.org/10.5772/intechopen.112709*

sequentially oxidized to NO3 − -N via three steps with the intermediates of NH2OH and NO2 − -N. In contrast, comammox performs the direct one-step oxidation of NH4 + -N to NO3 − -N [46]. Furthermore, NO3 − -N serves as the inducer for initiating denitrification, which involves a four-stage reduction of NO3 − -N to N2 with the intermediates of NO2 − -N, NO and N2O.

In contrast to high-carbon-demanding denitrification, autotrophically anaerobic ammonia oxidation (anammox) has attracted significant attention due to its low energy consumption and minimal sludge production. Anammox is an energetically favorable reaction that utilizes NH4 + -N and NO2 − -N/nitrogen oxide (NO) as electron donors and acceptors to yield gaseous nitrogen. Furthermore, anammox organisms utilize CO2 as the sole carbon source for cellular material synthesis [47]. Notably, hydrazine (N2H4) plays a crucial role as an electron donor in the conversion of NO2 − -N to NH2OH in the anammox process, distinguishing it from other nitrogen removal processes.

To achieve the comprehensive removal of nitrogen compounds, various biochemical reactions and their combinations have been applied. For instance, nitrificationdenitrification process has been recommended for the wastewater containing a high C/N ratio, whereas nitritation-anammox system is suitable for the influent with a low C/N ratio. In the two-stage nitrification-denitrification process, organic matter in wastewater is initially degraded to lighten the inhibition of autotrophic nitrifiers. The resulting NH4 + -N is then further oxidized to NO3 − -N by nitrification. NO3 − -N can be circularly used as electron acceptor for denitrification, leading to 70–90% nitrogen removal after the long-term operation. In comparison to two-stage nitrificationdenitrification, a single reactor that combines the advantages of both reactions has been developed, known as the SHARON process (the acronym for Single reactor High activity Ammonium Removal Over Nitrite) [48].

In the case of nitritation-anammox, NH4 + -N undergoes partial oxidation to NO2 − -N by supplying 75% of the required oxygen, as opposed to the complete oxidization of NO3−-N. Subsequently, NH4 + -N and NO2 − -N are reduced to N2. The partial oxidation of NH4 + -N is also known as partial nitrification [49], as it directly provides the necessary substrates for the anammox family without extra energy consumption. Throughout this process, two groups of autotrophic microbes work together to convert NH4 + -N into N2, making it well suited for the wastewater with low organic content. In comparison to the two-stage system, a single reactor is employed to facilitate the growth of both autotrophic aerobically and anaerobically ammonia oxidizers, which are responsible for the transformation of NH4 + -N to N2. This process is commonly referred to as CANON (the acronym for Completely Autotrophic Nitrogen removal Over Nitrite) [50].

To simplify the understanding of the system's functionality, we focus on the characteristics of an ammonia oxidizer such as the genus *Nitrosomonas*. One system that controls the activity of nitrification and denitrification through the regulation of oxygen is known as the OLAND process (Oxygen-Limited Autotrophic Nitrification and Denitrification) [51]. Another system, referred to as the NOx process [52], operates by regulating the levels of NOx (NO/NO2) to facilitate nitrification and denitrification. Additionally, the archaeal family can anaerobically oxidize methane (CH4) coupled with NO3 − -N reduction, known as N-damo [51]. In comparison to anammox, N-damo achieved a further reduction of 0.19 mM CH4 while utilizing 1 mM NH4 + -N [53]. On the other hand, aerobic deammonification directly converts NH4 + -N to N2 and NO2 − -N via NH2OH, although the detailed mechanism of this process is not yet well understood [54].

Recently, significant attention has been given to the production of GHP-N2O through four reactions involved in the nitrogen cycle, including NH2OH oxidization [30], nitrifier denitrification [31], comammox [46] and NO2 − -N reduction [37]. During NH2OH oxidation, it is believed that hydroxylamine oxidase (NH2OH oxidase, HAO) or nitric oxide reductase (NO reductase, NOR) present in microbial cells catalyzes the oxidation or reduction pathways for N2O formation [30]. Genus *Nitrosomonas*, in the process of nitrifier denitrification, utilizes NH4 + -N or nitrogen oxides to produce N2O under anoxic condition [55]. The comammox reaction, facilitated by the genus *Nitrospira*, also leads to the release of N2O [56]. The final pathway is NO2 − -N reduction, which occurs under conditions of high dissolved oxygen [56], low pH [23], insufficient organic loading [43] and NO2 − -N in replacement of NO3 − -N as electron acceptor [42]. The main mechanism of N2O reduction is the reduction of N2O to N2 catalyzed by N2OR enzyme (clade II nosZ) in denitrifiers [57]. In addition, there are still unclear mechanisms of N2O elimination, including the co-metabolism of NO3 − -N ammonification [23] and N2O nitrification [24].

## **6. Key microbes involved in the nitrogen cycle**

The nitrogen cycle involves the participation of six prominent groups of microorganisms responsible for 14 biological reactions. These groups are aerobic ammonia-oxidizing bacteria (aerAOB), aerobic ammonia-oxidizing archaea (aer-AOA), anaerobic ammonia-oxidizing bacteria (anAOB or anammox bacteria (AMX)), nitrite-oxidizing bacteria (NOB), denitrifying microbes (DENer) and nitrogen-fixing bacteria (NFB).

The first group is aerobic ammonia-oxidizing bacteria (aerAOB), and it uses ammonia monooxygenase (AMO) and hydroxylamine dehydrogenase (HAO) to catalyze the oxidation of NH4 + -N to NO2 − -N via NH2OH. This family comprises six genera of *Nitrosomonas*, *Nitrosolobus*, *Nitrosovibrio*, *Nitrosospira*, *Nitrosococcus* and Candidatus *Nitrosoglobus* [58] within two bacterial phyla of β- and *γ*-*proteobacteria* [25]. Among them, the genus *Nitrosomonas* is not only an obligate autotrophic nitrifier but can also act as a denitrifier, reducing NO2 − -N using hydrogen (H2) as an electron donor [31]. Furthermore, the oxidation of NH2OH to NO is initially catalyzed by HAO enzyme, and then NO is converted to NO2 − -N by nitric oxide oxidoreductase (NOO) [56]. However, the oxidation of NH2OH to N2O occurs with NO2 − -N as the electron acceptor in the absence of oxygen [59].

The second group is aerobic ammonia-oxidizing archaea (aerAOA), and it catalyzes the similar mechanism of NH4 + -N oxidation as aerobic ammonia-oxidizing bacteria (aerAOB). However, there is a distinction in the process: the intermediate of NO in the NH2OH oxidation is rapidly consumed, and no free NO is released to the atmosphere. This family encompasses 10 genera of *Nitrosoarchaeum*, *Nitrosopumilus*, *Cenarchaeum*, *Nitrososphaera*, Candidatus *Nitrosocaldus*, Candidatus *Nitrosotalea* [60], Candidatus *Nitrosotenuis* [61], Candidatus *Nitrosopelagicus*, Candidatus *Nitrosocosmicus* [62] and Candidatus *Nitrosomarinus* [63] within the phylum *Thaumarchaeota*. These archaea are prevalent in the ocean and interact with other team players in the system, contributing to one third of total N2O emission.

The third group is anaerobic ammonia-oxidizing bacteria (anAOB or AMX), and it performs the reduction of NH4 + -N and NO/NO2 − -N to N2. Three families of anammox microbes with distinct biogeographical distributions have been identified: freshwater Candidatus *Brocadiaceae* (including the genera *Brocadia*, *Kuenenia*, *Anammoxoglobus* and *The Contribution of Autotrophic Nitrogen Oxidizers to Global Nitrogen Conversion DOI: http://dx.doi.org/10.5772/intechopen.112709*

*Jettenia*) [64], marine Candidatus *Scalinduaceae* (represented by the genus *Scalindua*) [65] and marine Candidatus *Bathyanammoxibiaceae* [66] in the order Candidatus *Brocadiales* within the phylum *Planctomycetes* [67]. Notably, *Anammoxoglobus propionicus* exhibits the ability to reduce NO2 − -N by simultaneously utilizing NH4 + -N and propionate as electron donors [68]. Furthermore, *Kuenenia stuttgartiensis* is capable of performing dissimilatory NO3 − -N reduction to NH4 + -N (DNRA) [69]. Additionally, this group demonstrates the capacity for carbon fixation under anaerobic condition [26], making it advantageous for GHP-CO2 elimination applications.

The fourth group is nitrite-oxidizing bacteria (NOB) and it conducts the oxidation of NO2 − -N to NO3 − -N. This family includes seven genera: *Nitrobacter* in the phylum α-*proteobacteria*, Candidatus *Nitrotoga* in the phylum β-*proteobacteria*, *Nitrococcus* in the phylum *γ*-*proteobacteria*, *Nitrospira* in the phylum *Nitrospirota*, both of *Nitrospina* and Candidatus *Nitromaritima* in the phylum *Nitrosponota* and *Nitrolancea* in the phylum *Thermomicrobiota* [70, 71]. Notably, the widely distributed *Nitrospira* are further divided into canonical nitrite-oxidizing *Nitrospira* (canonical-*Nitrospira*) and comammox-*Nitrospira* [72]. In comparison of canonical-*Nitrospira*, comammox-*Nitrospira* catalyzes the complete oxidization of NH4 + -N to NO3 − -N. For example, Candidatus *Nitrospira inopinata* exhibits higher affinility of NH4 + -N than ammonia oxidizer under the limited NH4 + -N condition [73]. Moreover, Candidatus *Nitrologa* has been discovered in marine environments and demonstrates tolerance to high salinity [72].

The fifth group is denitrifying microbes (DENer), and they possess the ability to reduce nitrogen oxides (such as NO3 − -N, NO2 − -N, NO and N2O) to N2 under anaerobic, micro-aerophilic and occasionally aerobic conditions. This diverse family can be categorized into heterotrophs, autotrophs and mixotrophs based on their energy source. While heterotrophic denitrifier commonly utilizes organics as electron donor, autotrophic denitrifier (AuDen) primarily uses H2, reduced inorganic sulfur compounds (RISCs, such as S<sup>0</sup> , S2− and S2O3 2−), sulfite (SO3 2−), thiocyanate (SCN− ), iron oxides (e.g., iron disulfide (FeS2), Fe2+ and Fe0 or zerovalent iron (ZVI)) and trivalent arsenic (As3+). The autotrophic families belong to various phyla including α -, β-, γ- and ε-*proteobacteria* [74]. It is noteworthy that certain nondenitrifying microbes possess the N2OR enzyme that can directly utilize the residual N2O in the environment as a source of energy and nutrients, including the genera *Anaeromyxobacter*, *Dyadobacter, Gemmatimonas*, *Ignavibacterium*, *Melioribacter* and *Pedobacter*. They potentially play a role in the elimination of GHP-N2O [75]. Lastly, nitrogen-fixing bacteria (NFB) catalyze the reduction of N2 to NH3 through the process of nitrogen fixation. They are widely distributed in various environemts, including the phyla *Proteobacteria*, *Chlorobi*, *Firmicutes* and *Cyanobacteria*, and three methanogenic archaea of the genera *Methanosarcina*, *Methanococcus* and *Methanothermobacter* [76].
