Diversity and ecology of NrfA-dependent ammonifying microorganisms.

Non-fermentative nitrite reduction to ammonium, resulting in the release of ammonium in the environment, can be performed by a variety of enzymes found among bacteria and archaea. NrfA-dependent ammonifiers are phylogenetically diverse and are present in terrestrial and aquatic environments, and for the latter especially in sediments. NrfA-dependent ammonifiers often carry genes involved in denitrification and redox reactions with sulfur or iron compounds, suggesting that they play a role in multiple biogeochemical cycles. In most aquatic and terrestrial environments, denitrifiers dominate over nitrate ammonifiers, and denitrification is the dominating process of nitrate reduction. However, nitrate ammonification rates can be significant under certain redox conditions. Nitrate ammonifiers are a taxonomically diverse group of microorganisms that reduce nitrate to ammonium, which is released, and thereby contribute to the retention of nitrogen in ecosystems. Despite their importance for understanding the fate of nitrate, they remain a largely overlooked group in the nitrogen cycle. Here, we present the latest advances on free-living microorganisms using NrfA to reduce nitrite during ammonification. We describe their diversity and ecology in terrestrial and aquatic environments, as well as the environmental factors influencing the competition for nitrate with denitrifiers that reduce nitrate to gaseous nitrogen species, including the greenhouse gas nitrous oxide (N 2 O). We further review the capacity of ammonifiers for other redox reactions, showing that they likely play multiple roles in the cycling of elements. [ABSTRACT FROM AUTHOR]