(234n) Development of a Pore-Scale Transport Assay for Protist-Facilitated Transport of Plant Growth-Promoting Bacteria

The rhizosphere, the area of soil that surrounds plant roots, is critical to the health of soil and the productivity of crops. Microbes active in the rhizosphere can promote plant growth by fixing atmospheric N₂ to bioavailable nitrogen forms, protecting plants from pathogens, and promoting the retention of soil moisture. Bacterial inoculants and other biological additives are a growing billion-dollar industry being used more often despite the well-known difficulty in transporting live bacteria through soil. Prior work in our lab has demonstrated that naturally-occurring soil protists can actively transport live bacteria and other micro- or nano-scale agricultural payloads. The objective of this project is to screen the particle-transport capabilities of a group of soil protists. Here, a high-throughput microfluidic screening assay was developed. The microfluidic device was comprised of an input well connected by a micro-structured region to a target well. The micro-structured region featured a pseudo-2D representation of the physical microstructure of a sandy loam soil. The target well contained a protist chemoattractant immobilized in a hydrogel plug. Devices were constructed out of polydimenthyl siloxane (PDMS) using photolithography and soft lithography, and were operated under quiescent (no-flow) conditions. Preliminary results show a wide range of soil protists are well-adapted to navigating porous media, and will take up a micro-scale payload. Ultimate performance of protist-facilitated transport as a sustainable agriculture biotechnology is a function of many factors including ease of protist culturing and storage, the rate and extent of swimming motility through porous media, the extent of chemotaxis for root-targeting functionality, payload uptake and egestion rates, and carrying capacity.