Suburban-urban differences in coarse and fine atmospheric particulate matter with key chemical compositions influencing bacterial communities in Toyama and Yokohama, Japan

Atmospheric particulate matter (PM) contains diverse chemical compositions and airborne microorganisms; but few studies have examined how regional differences, particle sizes, and chemical compositions jointly influence microbial communities. In this study, coarse (SPM-PM_2.5) and fine (PM_2.5) particles were collected from suburban and urban, with bacterial communities and 48 chemical compositions. The results showed that PM mass concentrations in Yokohama (urban, SPM-PM_2.5 = 5.6 μg/m³; PM_2.5 = 11.8 μg/m³) were higher than those in Toyama (suburban, SPM-PM_2.5 = 3.8 μg/m³; PM_2.5 = 9.4 μg/m³), but values remained below Japan's environmental standard. Water-soluble inorganic ions (WSII) and Carbonaceous species (CS) were the dominant chemical compositions, accounting for 65.6 %-72.4 % of the total PM mass. Potential ecological risk (RI) reached a "serious" level at both sites (Toyama: 4839.6; Yokohama: 5443.4), with Sb as the dominant contributor (E_i = 701.1 and 832.7, respectively), indicating that metal-enriched fine particles may exert ecological pressure even when PM mass remains within regulatory limits. Microbial community analysis indicated a predominance of plant-associated bacteria such as Methylobacterium (18.9 %) and Sphingomonas (7.8 %) in Toyama. Yokohama, by contrast, were dominated by human-associated microorganisms including Corynebacterium (9.5 %) and Streptococcus (6.4 %). Co-occurrence network and redundancy analysis (RDA) further indicated that Yokohama's PM_2.5 network had 2.8 times higher modularity than Toyama's and was mainly driven by As, Pb (representing combustion and industrial source elements, CSE), and elemental carbon (EC), while Toyama was primarily influenced by Fe, K, and Rb (representing sea salt and crustal source elements, SCE). These findings highlights how land use patterns control the sources and composition of airborne microorganisms in atmospheric particles.