Basin-scale community assembly of particle-associated and free-living bacteria in the Pacific Ocean

Bacterial attachment to particulate organic matter is vital for understanding microbial roles in pelagic ecosystems. However, knowledge of how particle-associated (PA) and free-living (FL) bacteria assemble across large spatial scales remains limited. Here, basin-scale distribution patterns of PA and FL bacteria were examined using systematically collected samples from four transpacific cruises across the Pacific Ocean. Community structures were first separated by depth (surface, 0-200 m vs. deep, ≥250 m), lifestyle, and biogeographic region, with distinct dominant lineages in each layer and lifestyle. Patterns of α-diversity along latitude and depth were different between PA and FL communities, and the vertical pattern of FL bacteria was consistent with the area effect hypothesis, as α-diversity positively correlated with water-mass size. Analyses of community assembly processes indicated that species sorting was prominent in both PA and FL bacteria in epipelagic and bathypelagic environments, while mass effect also contributed in epipelagic environments. These findings provide a basin-scale perspective on the ecological and evolutionary dynamics of bacterioplankton lifestyles in the Pacific Ocean, highlighting that basin-scale assembly processes and their controlling factors differ fundamentally between PA and FL bacterioplankton communities.