Atomic force microscopy reveals microscale networks and possible symbioses among pelagic marine bacteria

Marine Bacteria and Archaea ('bacteria') interact with upper ocean productivity to fundamentally influence its biogeochemical fate with consequences for ecosystems and global climate. Most bacteria-mediated carbon cycling is due to numerically dominant free-living bacteria, but their adaptive strategies to interact with primary productivity are not fully understood. Using atomic force microscopy (AFM), we made the surprising discovery that a substantial, and variable, fraction (on average 30 +/- 17.8% with a range of 0 to 55%) of 'free-living' bacteria in our samples from California coastal and open ocean environments were, in fact, intimately associated with other bacteria at nanometer to micrometer scales. Twenty-one to 43% of bacteria, including Synechococcus, were conjoint. Such close associations could indicate symbioses; however, they could also be antagonistic, parasitic, neutral, or accidental. Further, a substantial fraction (4 to 55%) of bacteria was connected by pili and gels into cell-cell pairs or occurred in networks of up to 20 cells. We frequently observed nanoparticles associated with the networks, raising the question of their identity and origin (e.g. scavenged from the seawater colloid pool by the networks or produced by the bacteria within the networks). The networks occasionally contained structures that morphologically resembled coccoliths or protist scales. These may impart ballast to sinking particles if the networks coalesce to form larger, sinking, particles. Our finding of abundant bacteria-bacteria associations and possible microenvironment structuring by pelagic bacteria offers a novel context for bacterial ecology and diversity and models of ocean productivity and elemental cycling.