Search

Ammonia oxidizers are key players in the global nitrogen cycle and are responsible for the oxidation of ammonia to nitrite, which is further oxidized to nitrate by other microorganisms. Their activity can lead to adverse effects on some human-impacted environments, including water pollution through leaching of nitrate and emissions of the greenhouse gas nitrous oxide (N2O). Ammonia monooxygenase (AMO) is the key enzyme in microbial ammonia oxidation and shared by all groups of aerobic ammonia oxidizers. The AMO has not been purified in an active form, and much of what is known about its potential structure and function comes from studies on its interactions with inhibitors. The archaeal AMO is less well studied as ammonia oxidizing archaea were discovered much more recently than their bacterial counterparts. The inhibition of ammonia oxidation by aliphatic alcohols (C1-C8) using the model terrestrial ammonia oxidizing archaeon 'Candidatus Nitrosocosmicus franklandus' C13 and the ammonia oxidizing bacterium Nitrosomonas europaea was examined in order to expand knowledge about the range of inhibitors of ammonia oxidizers. Methanol was the most potent specific inhibitor of the AMO in both ammonia oxidizers, with half-maximal inhibitory concentrations (IC50) of 0.19 and 0.31 mM, respectively. The inhibition was AMO-specific in 'Ca. N. franklandus' C13 in the presence of C1-C2 alcohols, and in N. europaea in the presence of C1-C3 alcohols. Higher chain-length alcohols caused non-specific inhibition and also inhibited hydroxylamine oxidation. Ethanol was tolerated by 'Ca. N. franklandus' C13 at a higher threshold concentration than other chain-length alcohols, with 80 mM ethanol being required for complete inhibition of ammonia oxidation., (© The Author(s) 2023. Published by Oxford University Press on behalf of FEMS.)

Ammonia-oxidising archaea (AOA) are environmentally important microorganisms involved in the biogeochemical cycling of nitrogen. Routine cultivation of AOA is exclusively performed in liquid cultures and reports on their growth on solid medium are scarce. The ability to grow AOA on solid medium would be beneficial for not only the purification of enrichment cultures but also for developing genetic tools. The aim of this study was to develop a reliable method for growing individual colonies from AOA cultures on solid medium. Three phylogenetically distinct AOA strains were tested: 'Candidatus Nitrosocosmicus franklandus C13', Nitrososphaera viennensis EN76 and 'Candidatus Nitrosotalea sinensis Nd2'. Of the gelling agents tested, agar and Bacto-agar severely inhibited growth of all three strains. In contrast, both 'Ca. N. franklandus C13' and N. viennensis EN76 tolerated Phytagel™ while the acidophilic 'Ca. N. sinensis Nd2' was completely inhibited. Based on these observations, we developed a Liquid-Solid (LS) method that involves immobilising cells in Phytagel™ and overlaying with liquid medium. This approach resulted in the development of visible distinct colonies from 'Ca. N. franklandus C13' and N. viennensis EN76 cultures and lays the groundwork for the genetic manipulation of this group of microorganisms., (© The Author(s) 2022. Published by Oxford University Press on behalf of FEMS.)

Ammonia oxidation is a fundamental core process in the global biogeochemical nitrogen cycle. Oxidation of ammonia (NH3) to nitrite (NO2 -) is the first and rate-limiting step in nitrification and is carried out by distinct groups of microorganisms. Ammonia oxidation is essential for nutrient turnover in most terrestrial, aquatic and engineered ecosystems and plays a major role, both directly and indirectly, in greenhouse gas production and environmental damage. Although ammonia oxidation has been studied for over a century, this research field has been galvanised in the past decade by the surprising discoveries of novel ammonia oxidising microorganisms. This review reflects on the ammonia oxidation research to date and discusses the major gaps remaining in our knowledge of the biology of ammonia oxidation.