Atmospheric deposition of anthropogenic particulate matter (PM) is increasingly recognized as a significant external nutrient source in marine ecosystems, influencing marine biogeochemical cycles. However, the detailed impact of PM input on microbial community dynamics and metabolic functions and remains poorly understood. In this study, by combining on-board microcosm experiments with flow cytometry, radioactive isotope tracing, amplicon-sequencing, and metatranscriptome analysis, we investigated the responses of abundance, production, diversity and community composition of both marine particle-associated and free-living bacteria, analyzing changes of their metabolic functions in the ocean basin of the South China Sea. Our results showed that PM deposition increased nutrient heterogeneity, facilitating nutrient utilization by marine bacteria. Copiotrophic, particle-associated bacteria exhibited a notable increase in abundance and contributed more to bacterial production than their free-living counterparts, despite an overall decline in total bacterial abundance. Free-living bacteria displayed increased diversity, with a community shift from oligotrophs to copiotrophs, while particle-associated bacteria experienced reduced diversity and a greater, non-directional shift in community composition, indicating different assembly mechanisms for the two groups in response to external nutrient inputs. The addition of PM reduced cyanobacterial abundance and photosynthetic activity due to the toxic effects of heavy metals, as indicated by the upregulation of genes related to reactive oxygen species elimination and detoxification of heavy metals. Furthermore, PM deposition stimulated the growth of oil-degrading bacterial taxa and enhanced expression of genes involved in material cycling, such as alkaline phosphatase, iron transport, denitrification, and nitrogen fixation. Pathways related to bacterial chemotaxis, flagellum assembly, and biofilm formation were also upregulated. These findings emphasize the complex and differential responses of bacterial communities to atmospheric deposition, revealing the ecological importance of PM in shaping marine microbial dynamics and its potential influence on biogeochemical cycles.

atmospheric deposition,bacterial community composition,metabolic function,bacterial abundance,bacterial production