Soft Matter and Complex Systems Seminar

Microbial interactions critically depend on the rate of physical cell-cell or cell-resource encounters: these microscale processes control the rate of many fundamental ecological functions. In the ocean, a prominent example is the encounters among phytoplankton that lead to the formation of marine snow following a phytoplankton bloom. Another example is bacterial encounters with sinking marine snow particles that bacteria degrade, exerting an important control on carbon export from the upper ocean (the ‘biological pump’). Microscale encounters are nearly always modeled as encounters between inanimate spheres, borrowing from physical models of gases, coagulating colloids, and rain formation. However, these physics-based approaches fail to account for important traits of microorganisms, for example, cell elongation. Yet, certain traits of microorganisms combined with environmental conditions significantly impact many types of encounters in the ocean. For example, cell shape, in conjunction with buoyancy and turbulence, can increase encounter rates and thus speed up the formation of marine snow by elongated phytoplankton nearly ten-fold: this result provides a mechanistic explanation for the rapid clearance of blooms of elongated phytoplankton species. Similarly, encounters between bacteria and sinking marine snow depend on cell shape, motility, and fluid shear, and these factors can alter the encounter rate by orders of magnitude. Finally, microbes can harness encounters - Trichodesmium, a key marine nitrogen fixer, uses smart reversals to convert random encounters between cells into organized active aggregates. I will outline how more realistic models of encounters and aggregation at the microscale can contribute to our understanding of fundamental ecological processes controlled by microbes.