Abstract
The Kuroshio-Oyashio Extension (KOE) region is a highly variable region in the North Pacific Ocean, characterized by strong environmental gradients and multi-scale oceanographic processes. However, the fine-scale impact of these currents and associated water masses on microbial communities remains poorly understood. Here, high-resolution samples from 18 to 24 layers were collected along a transect in the KOE region in 2021, with 16S rRNA gene amplicon sequencing and environmental parameter measurements conducted to investigate the Kuroshio-Oyashio influence on microbial communities. Strong regional and vertical variations in environmental parameters and microbial communities were observed, with main horizontal regional differentiations confined to the upper 500 m. Photoautotrophic and oligotrophic taxa (e.g., SAR11 clade and Cyanobacteria) were enriched in warm, oligotrophic Kuroshio region, whereas the cold nutrient-rich Oyashio and confluence regions supported higher microbial abundance, diversity, and complex microbial interactions. Consistently, heterotrophic bacteria (1.00 × 10⁶-1.17 × 10⁹ cells L⁻¹) were more abundant in the upper 55 m of the Oyashio and confluence regions than in the Kuroshio region. Below the thermocline (~500 m), community composition was primarily structured by depth, indicating a diminishing Kuroshio-Oyashio current influence. Three main water masses (subtropical mode water [STMW], central mode water, and North Pacific intermediate water [NPIW]) with distinct microbial communities were identified, explaining ~11% of microbial variation beyond depth and geography, with biomarker taxa identified (e.g., Actinomarinales for STMW, Nitrosopumilales for NPIW). This study reveals the extent of Kuroshio-Oyashio influence on microbial communities and highlights the integrated impacts of large-scale currents and fine-scale water masses on shaping microbial biogeography in the KOE region.IMPORTANCEThe convergence of the Kuroshio and Oyashio currents shapes high microbial diversity, as well as complex microbial-mediated biogeochemical processes. However, investigations into the microbial distribution patterns in relation to these current systems remain limited in spatial resolution. This study with high-resolution samples reveals the extent of Kuroshio-Oyashio influence on microbial communities and advances the understanding of how multi-scale oceanographic processes influence microbial biogeographical patterns. It provides a fine-scale perspective for exploring microbial distribution and assembly in highly dynamic oceanic environments.