Understanding key biophysical phenomena in the ocean often requires one to simultaneously focus on microscale entities, such as motile plankton and sedimenting particles, while maintaining the macroscale context of vertical transport in a highly stratified environment. This poses a conundrum: How to measure single organisms, at microscale resolution, in the lab, while allowing them to freely move hundreds of meters in the vertical direction? We present a solution in the form of a scale-free, vertical tracking microscope based on a circular "hydrodynamic-treadmill".Our technology allows us to transcend physiological and ecological scales, tracking organisms from marine zooplankton to single-cells over vertical scales of meters while resolving microflows and behavioral processes. We demonstrate measurements of sinking particles, including marine snow as they sediment tens of meters while capturing sub-particle-scale phenomena. We also demonstrate depth-patterned virtual-reality environments for novel behavioral analyses of microscale plankton. This technique offers a new experimental paradigm in microscale ocean biophysics by combining physiological-scale imaging with free movement in an ecological-scale patterned environment.One sentence summary: Scale-free vertical tracking microscopy captures, for the first time, untethered behavioral dynamics at cellular resolution for marine plankton.Our oceans represent the largest habitable ecosystem on the planet. With an average 1 depth of 4 kilometers, this unique ecosystem is highly vertically stratified with physical pa-2 rameters such as light, temperature, salinity and pressure varying dramatically as a function 3 of depth [1]. For example, only the first 200 meters of the ocean receives all the sunlight, 4 while the deeper parts of the ocean are effectively dark. For every 10 meters in depth, the 5 pressure increases by 1 atmosphere. Despite being only a few hundredths of the biomass 6 of terrestrial ecosystems [2], the oceans are responsible for half of the carbon fixed on our 7 planet [3]. Remarkably, this primary production in the ocean comes mostly from minuscule 8 plankton [4], the majority of whom are invisible to the naked eye. Although we have known 9 since the work of Haeckel [5] that the ocean abounds with microscopic plankton, only re-10 cently have we begun to realize their critical role in our planetary cycles [4, 6]. Despite their 11 importance, understanding the key biophysical mechanisms at the scale of planktonic single 12 cells and organisms, which help them navigate the ocean's complex vertical landscape, and in 13 turn influence planetary-scale processes, remains a major hurdle in biological oceanography. 14 A significant challenge in studying these biophysical processes is bridging the vast length 15 scales (from microns to kilometers) and time scales (from millisecond to days). Conven-16 tionally, vertical fluxes in the ocean are measured using sedimentation traps and sampling 17 at different depths [7]. Although crucial, the data is expensive t...
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