The nitrogen cycle

Abstract

Nitrogen is the fourth most abundant element in cellular biomass, and it comprises the majority of Earth's atmosphere. The interchange between inert dinitrogen gas (N2) in the extant atmosphere and 'reactive nitrogen' (those nitrogen compounds that support, or are products of, cellular metabolism and growth) is entirely controlled by microbial activities. This was not the case, however, in the primordial atmosphere, when abiotic reactions likely played a significant role in the inter-transformation of nitrogen oxides. Although such abiotic reactions are still important, the extant nitrogen cycle is driven by reductive fixation of dinitrogen and an enzyme inventory that facilitates dinitrogen-producing reactions. Prior to the advent of the Haber-Bosch process (the industrial fixation of N2 into ammonia, NH3) in 1909, nearly all of the reactive nitrogen in the biosphere was generated and recycled by microorganisms. Although the Haber-Bosch process more than quadrupled the productivity of agricultural crops, chemical fertilizers and other anthropogenic sources of fixed nitrogen now far exceed natural contributions, leading to unprecedented environmental degradation.

Design and evaluation of primers targeting genes encoding NO-forming nitrite reductases: implications for ecological inference of denitrifying communities

“…This can severely affect the ecological interpretation of results obtained, leading to an underestimation of the genetic potential for denitrification when certain clades are not targeted by the primer set, or possibly an overestimation if primers target homologous sequences that are not involved in denitrification. Moreover, nitrite reduction catalyzed by NirS and NirK occurs for different reasons in microorganisms and in different pathways in the nitrogen cycle1. Together, this advocates for clade-specific primers for ecological studies rather than using broad range primers.…”

“…Nitrite reduction is a key step in the nitrogen cycle and is considered a branching point due to its role in several N-cycle pathways1. The reduction of nitrite can be a detoxification process in some organisms2, while in others it is part of energy conservation as in the processes denitrification (including nitrifier denitrification3) and anaerobic ammonia oxidation4.…”

“…The reduction of nitrite can be a detoxification process in some organisms2, while in others it is part of energy conservation as in the processes denitrification (including nitrifier denitrification3) and anaerobic ammonia oxidation4. Moreover, nitrite reductases are involved in assimilatory and respiratory ammonification1. Of particular importance are microbial communities that perform nitrogen reduction pathways that result in the formation of gaseous products, since these processes account for net removal of available nitrogen from the ecosystem.…”

Effect of Temperature on Oxygen Profiles and Denitrification Rates in Freshwater Sediments

“…At the same time, nitrogen pollution is expected to increase, due to intensification of agricultural and industrial activities. This can amplify eutrophication effects in freshwater and coastal habitats, and increase the emission of the potent greenhouse gas N 2 O (Erisman et al 2013;Stein and Klotz 2016). Currently, the altered flow of reactive nitrogen into ecosystems is already one of the most pressing global environmental issues (Steffen et al 2015).…”

“…Denitrifiers use organic carbon (canonical heterotrophic denitrification), sulphide, or hydrogen (chemolithoautotrophic denitrification) as an electron donor (Burgin et al 2012). Or, in the special case of methane-oxidation dependent denitrification, methane reduction and oxidation are coupled to nitrite dismutation (Ettwig et al 2010;Stein and Klotz 2016). Because of the prevailing nitrate and oxygen concentrations, freshwater denitrification activities are often highest at the oxic-anoxic boundary layer in the sediment, in particular at sites where the diffusion path of nitrate from the overlying water to the anoxic denitrification zone is small, i.e.…”