Scale-free Vertical Tracking Microscopy: Towards Bridging Scales in Biological Oceanography

Many cells and organisms in the ocean spend their entire lives freely-suspended or swimming, rarely encountering any substrate. Marine microscale plankton epitomize this lifestyle, where single cells to small organisms can travel distances many orders of magnitude their own sizes vertically in the water column, constituting the largest daily bio-mass migration on our planet. Such migrations, together with the sinking of dead and decaying particles, are key components of vertical material fluxes in the ocean and drive planetary-scale biogeochemical processes like carbon sequestration and ecological processes like species dispersal. Mechanistically studying such multi-scale processes in the lab, although crucial, presents a significant challenge: how to image single organisms or cells, at microscale resolution, while allowing them to freely move hundreds of meters in the vertical direction? Here we present a solution in the form of a scale-free, vertical tracking microscope, based on a circular "hydrodynamic-treadmill" with no bounds for motion along the axis of gravity. Our method demonstrates a new paradigm for multi-scale measurement where microscale characteristics like cell-state and organismal behavior can be directly connected to macroscale outcomes like depth in the water column. Using this method to bridge spatial-scales, we measured diel vertical migrations of plankton, while for the first time concurrently resolving the microscale behavioral processes and flows at the scale of individual organisms. We also measured the evolution of “marine-snow” particles sinking over tens of meters over several hours while resolving their microscale constituents and transport processes over the scale of microns and milliseconds. Using our platform, we further demonstrate ‘depth-patterned’ virtual-reality environments for novel quantitative behavioral analyses of microscale plankton. Our method enables ecologically relevant phenomena in the ocean to be explored with physiological resolution of their microscale constituents. We anticipate that the measurement paradigm enabled by our instrument will open up new avenues for understanding key processes in our oceans.