Bacterial Hives in the Ocean

Marine bacteria play a key role in re-mineralizing the organic carbon in marine snow. The relative importance of chemotaxis and activity-flow coupling in context of sedimentation and how bacteria intercept and escape sinking snow in the ocean remains poorly understood. Here we study the dynamics of bacteria around marine snow aggregates in the California currents on board the research vessel Kilo Moana, using a custom microfluidic setting with high resolution microscopy. We expose marine snow aggregates to bacteria collected at different depths in the ocean and resolve the swimming trajectories of individual bacteria around the aggregate. We find that bacteria display rich dynamical behaviors that includes tracer-like dynamics, active hopping and pluming. We construct a minimal model of chemotaxis of bacteria including coupling with the Stokes flow around the aggregate and conceptualize our observations using this framework.