Bacterial contribution to nitrogen processing in the atmosphere

Abstract. This study investigates potential microbial interactions with nitrogen compounds in the atmosphere, with a focus on inorganic forms (mainly NH₄⁺, NO₃^-, and N₂). The reanalysis of metagenomes and metatranscriptomes from cloud-free and cloudy collected at the mountain site of puy de Dôme (1465 m asl, France) indicate equivalent representation of genes involved in organic and inorganic nitrogen utilization processes. Glutamate metabolism and denitrification (in particular nitrite reduction) contributed most (70 %) of the microbial sequences of genes and transcripts linked to nitrogen utilization pathways. Other prevalent processes included assimilatory and dissimilatory nitrate reduction, and nitrogen fixation, with the latter being overexpressed in particular during clear atmospheric conditions. The screening of bacteria isolates revealed that 15 % of them carry the biomarker gene for biological N₂ fixation (nifH). In addition, laboratory incubations of rainwater points towards the processing of NH₄⁺. The decay rate of NH₄⁺ concentration correlated positively with the relative abundance of Sphingomonadales, and negatively with that of Burkholderiales. The latter may rather obtain nitrogen from N₂ and organic forms. Overall, these results demonstrate multiple potential microbiological roles in the processing of inorganic nitrogen in the atmosphere, in relation with atmospheric conditions and microbial diversity. This opens up new perspectives in our understanding of biogeochemical cycles and chemical processing in the atmosphere, as well as microbial functioning in this major part of the Earth system.