Exometabolomic exploration of culturable airborne microorganisms from an urban atmosphere

Abstract. The interactions of metabolically active atmospheric microorganisms with cloud organic matter can alter the atmospheric carbon cycle. Upon deposition, atmospheric microorganisms can influence microbial communities in surface Earth systems. However, the metabolic activities of cultivable atmospheric microorganisms in settled habitats remain less understood. Here, we investigated exometabolites produced by typical bacterial and fungal species isolated from the urban atmosphere to elucidate their biogeochemical roles. Molecular compositions of exometabolites were analyzed using ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry. Annotation through the Kyoto Encyclopedia of Genes and Genomes database helped identify metabolic processes. Results showed that bacterial and fungal strains produce exometabolites with lower H/C and higher O/C ratios than consumed and resistant compounds. CHON compounds constituted over 50 % of the identified formulas of exometabolites. Bacterial exometabolites contained more abundant CHONS compounds (25.2 %), while fungal exometabolites were rich in CHO compounds (31.7 %). These microbial exometabolites predominantly comprised aliphatic/peptide-like and carboxyl-rich alicyclic molecules (CRAM-like). Significant variations in metabolites were observed among different strains. Bacteria showed a performance for amino acid synthesis, while fungi were more active in transcription and expression processes. Lipid metabolism, amino acid metabolism, and carbohydrate metabolism varied widely among bacterial strains, while fungi exhibited marked differences in carbohydrate metabolism and secondary metabolism. This comprehensive examination of metabolite characteristics at the molecular level for typical culturable airborne microorganisms enhances our understanding of their potential metabolic activities at air-land/water interfaces. These insights are pivotal for assessing the biogeochemical impacts of atmospheric microorganisms following their deposition.