ABSTRACT
Denitrification is one of the most studied respiratory pathways because of its ecological importance, since it allows the return of fixed nitrogen to the atmosphere through the reduction of nitrates and nitrites of soils and water. However, there is little knowledge about this route in the case of extreme environments, which are characterized by very high or low pH, extreme temperatures, high salt concentrations, and so forth. This chapter shows how denitrification has special peculiarities in these ecosystems, since it is a process strongly affected by the environmental conditions: generally, the denitrification ratio is lower and greenhouse gas emissions, such as nitric and nitrous oxides, are higher. Likewise, the composition of the microbial community changes, with archaea being the predominant microbial population. Therefore, the haloarchaea are taken as the model group of denitrifiers in extreme environments to describe the four enzymes catalyzing the process: nitrate, nitrite, nitric oxide, and nitrous oxide reductases, All these show different characteristics of their bacterial counterparts, such as the number of subunits or the subcellular localization. Despite advances in the knowledge of this route in extreme environments, the true impact of denitrification in these ecosystems still has to be determined due to the difficulty to measure in situ the denitrification and the lack of many biochemical studies. These studies are yet to come, and they should focus on nitric oxide and nitrous oxide reductases, which have not yet been characterized in haloarchaea.
