Abstract
The nitrogen cycle (N-cycle), principally supported by prokaryotes, involves different redox reactions mainly focused on assimilatory purposes or respiratory processes for energy conservation. As the N-cycle has important environmental implications, this biogeochemical cycle has become a major research topic during the last few years. However, although N-cycle metabolic pathways have been studied extensively in Bacteria or Eukarya, relatively little is known in the Archaea. Halophilic Archaea are the predominant microorganisms in hot and hypersaline environments such as salted lakes, hot springs or salted ponds. Consequently, the denitrifying haloarchaea that sustain the nitrogen cycle under these conditions have emerged as an important target for research aimed at understanding microbial life in these extreme environments.The haloarchaeon Haloferax mediterranei was isolated 20 years ago from Santa Pola salted ponds (Alicante, Spain). It was described as a denitrifier and it is also able to grow using NO3-, NO2- or NH4+ as inorganic nitrogen sources. This review summarizes the advances that have been made in understanding the N-cycle in halophilic archaea using Hfx mediterranei as a haloarchaeal model. The results obtained show that this microorganism could be very attractive for bioremediation applications in those areas where high salt, nitrate and nitrite concentrations are found in ground waters and soils.
Highlights
Nitrogen (N) is a major element in all organisms. It accounts for approximately 6% of their dry mass on average and in nature its assimilation is a key process of the nitrogen cycle (N-cycle) carried out by higher plants [1], algae [2], yeast [3], and bacteria [4]
N can be found in different redox states from +5 to -3, but in biological compounds it is almost exclusively present in the most reduced form as a component of the two pre-eminent biological macromolecules: proteins and nucleic acids [5]
The assimilatory pathways of the N-cycle (N2 fixation and nitrate assimilation) generate ammonium that is incorporated into the carbon skeletons to produce amino acids
Summary
Nitrogen (N) is a major element in all organisms. It accounts for approximately 6% of their dry mass on average and in nature its assimilation is a key process of the N-cycle carried out by higher plants [1], algae [2], yeast [3], and bacteria [4]. Based on subunit composition, subcellular location of the active site and nar gene organization, it can be concluded that archaeal Nars are a new type of enzyme with the active site facing the outside and anchored to the membrane by a cytochrome b (as it has been proposed for Hfx mediterranei system) or stabilized by the lipid environment in the membrane as described for the P. aerophilum Nar [57] This system could be an ancient respiratory nitrate reductase, the nitrite formed could http://www.salinesystems.org/content/4/1/9 be assimilated. Only two respiratory nitrite reductases have been characterised from a haloarchaea member; these are the enzymes from Har marismortui and Hfx denitrificans, which contain two Cu centres, and in both cases, the protein is encoded by the nirK gene. Nir: Assimilatory nitrite reductase; Nas: Assimilatory nitrate reductase; Nar: Respiratory nitrate reductase; NIR: Respiratory nitrite reductase; Nor: Nitric oxide reductase; Nos: Nitrous oxide reductase; GS: Glutamine synthetase; GOGAT: Glutamate synthase; GDH: Glutamate dehydrogenase; SHE: Standard hydrogen electrode
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