Baseline estimates for cell-to-cell interactions in marine and terrestrial habitats

Microbial taxa in the environment do not exist in isolation but are highly connected to other microbial cells in the exchange of nutrients, toxins, and mobile genetic elements. The presence of auxotrophy in many taxa means it is essential that cells can bridge the spatial distances between them, but the exact distances are largely unknown. What is the number of cells that a microbe can realistically expect to interact with in natural environments? Here we use order-of-magnitude calculations to estimate a typical distance between cells, based on empirical estimates of cell abundance for terrestrial and marine habitats. Our estimates reveal that 30%-80% of the global microbial biomass occurs in spatially structured habitats, with expected cell-to-cell distances ranging from 10s of microns in the soil to ~200 microns for planktonic cells in the ocean water column. Despite these short distances, only a small fraction of the molecules secreted by one cell reaches the other cell due to the three-dimensional diffusion of molecules. In a careful comparison, we estimate the spatial distances that cells can bridge purely by the diffusion of nutrients and with additional cell motility. Overall, our work provides an estimate for cell distances that helps to better contrast surface and subsurface habitats and formulate a baseline expectation for the conditions that microbial communities are experiencing in nature.