Bacterial-fungal interactions in the air environment of composting facilities.

Bacteria-fungi interactions (BFIs) shape ecosystem processes and influence human health, yet critical gaps in airborne BFIs continue to obscure both the ecological functions and health risks of airborne microbes. In this study, 41 paired air and compost samples were collected from five composting facilities. Bacterial and fungal communities were characterized by Illumina 16S/ITS sequencing, inter- and intra-kingdom links were identified through co-occurrence networks, and a neutral community model was applied to infer assembly processes. Our results showed a significant positive correlation between fungal and bacterial richness in composting facilities, although the number of the operational taxonomic units (OTUs) of bacteria (467-2115) was significantly higher than that of fungi (43-416) (p < 0.001). Both α- and β-diversity analyses indicated that composting exert greater impacts on airborne bacterial than fungal community, and such influence was confined to proximal air environment. Both airborne bacterial (R2 = 0.69, Nm = 3682) and fungal (R2 = 0.819, Nm = 11,331) assembly was mainly driven by stochastic processes. The air-featured BFI patterns may be due to the differences in diversity and influencing factors between bacteria and fungi, as well as different community assembly processes within their respective microbiomes. For the first time we demonstrate that airborne microbial co-occurrences were largely intra-kingdom, whereas inter-kingdom pathogen co-occurrences were frequent in solid samples, indicating that the air itself imposes a biological filter. This finding redefines composting facility exposure assessments by showing that the most ecologically influential and potentially pathogenic interactions occur in the airborne particle fraction, meriting aerosol control strategies.