Protist predator exclusion in a soil chip ecosystem impacts bio- and necromass accumulation?

Trophic interactions are key in the shaping of soil microbial communities, and therefore in their central role on soil function and biogeochemical cycles. The physical features of soil pore space greatly impact microbial activity as well as soil trophic interactions. Smaller pore necks (< 2-3 µm) are believed to provide soil bacteria with shelter from larger predators (mainly protists and nematodes). Microfluidic devices are increasingly used to address ecological questions at the microscale because they enable precise and customizable fabrication of pore-space geometries. This allows pore size and architecture to be treated as controlled experimental variables, while simultaneously permitting high-resolution microscopic assessment of microbial growth and activity, as well as spatially resolved chemical analysis. Here, we show current work on the use of microfluidics to manipulate trophic interactions and to assess their role in the cycling of organic matter. We present the use of a multi-depth soil chip where selected pore space areas are reduced to a 1-2 µm height and 1 µm width (predator filters), allowing for refugia with significantly reduced protist abundance. We show that, while most of these predator filters achieve reduced predation pressure by protists in open areas beyond, it is not uncommon for smaller protists to overcome them. In open spaces where protist predation is significantly reduced, preliminary results suggest increased microbial biomass –and necromass– accumulation. Spaces with occluded entries also tend to show a slower colonization rate and seem to favour certain bacterial taxa, based on morphological differences. This work and its future developments contribute to our understanding of how pore architecture shapes trophic interactions at the microscale and how these impact soil organic matter accumulation.