Most of earth is covered by water, and it is essential to keep these ocean and freshwater ecosystems healthy. To do so, scientists must understand how they stay clean and in balance. A critical part of healthy bodies of water are microscopic organisms, which process waste and keep water clean by eating bacteria and debris. The feeding rate of these tiny organisms (how much water they clean per time) determines the impact they have on the health of their environments. Therefore, understanding this feeding rate is important both to predicting how bodies of water will react to change (e.g., pollution, sewage leaks, climate change) and determining how to promote the recovery of unhealthy bodies of water. One type of microscopic organism that is important for clean water and is ubiquitous in freshwater and marine environments is the surface-attached protist. These organisms live in dense aggregations attached to under-water surfaces, such as rocks, plants, and sinking debris and create feeding currents that draws their food to them. The investigators will use a novel holographic microscope to make the first three-dimensional measurements of feeding currents of these organisms in realistic conditions. The findings will be used to validate existing mathematical models of the feeding rates of these microscopic organisms and will inform the development of strategies to clean bodies of water and improve wastewater treatment plant performance. This project will also provide interdisciplinary opportunities for undergraduate and graduate students, including those from groups that are traditionally underrepresented in the STEM workforce. <br/><br/><br/>Microscopic sessile suspension feeders (MSSFs) are surface-attached protists that are a ubiquitous and critical component in aquatic ecosystems, performing the vital ecological function of removing bacteria and contaminants and serving as a key trophic intermediate between algae/bacteria and higher eukaryotic taxa. Previous studies of the feeding activity of these surface-attached protists have been limited to confined conditions that distort the flow and 2D measurements do not capture the non-axisymmetric nature of the current. Further, theory predicts that feeding is restricted by closed eddies in the flow caused by the surface of attachment, and that the impact of these eddies on feeding rate depends strongly on the angle of the MSSF with respect to the surface. The researchers will use Vorticella, a sessile ciliate, to determine whether food uptake of MSSFs is limited by the hydrodynamics of feeding attached to surfaces. Using a novel 3D holographic microscope, they will determine: (1) how the 3D feeding flow field of a single MSSF changes with body orientation of the organism relative to the surface, (2) how the dynamics of body orientation for Vorticella in culture compares to those in nature, (3) how feeding flows and body orientations affect food uptake, and (4) how the feeding flows are affected by ambient environmental flows. The measurements will be used to validate hydrodynamic models of MSSFs that can predict clearance rates. The resulting mechanistic understanding of microscopic interactions will ultimately build a foundation to scale up to community- and ecosystem-level processes to inform efforts to protect the health of aquatic ecosystems.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.