Flow physics of nutrient transport drives functional design in ciliates

Nutrient transport is essential for cellular survival, and the evolution from prokaryotic to eukaryotic cells brought significant changes in nutrient acquisition. Ciliates are important aquatic protistan grazers that evolved cilia to generate feeding currents, directing nutrients towards specialized feeding structure. However, the link between extra- and intra-cellular transport and its impact on the functional design of these unicellular eukaryotes are seldom discussed. Here, we analyze cell designs that optimize the physics of fluid and material transport. We employ a mathematical model of nutrient transport; we solve the Stokes equation and the advection-diffusion equation for nutrient distribution around the ciliated cell and systematically explore boundary conditions for the "mouth" and cilia arrangement. Our results demonstrate that optimal feeding efficiency occurs when cilia are positioned adjacent to the feeding apparatus. Importantly, these optimized adjacent ciliary arrangements are tailored to the organism's lifestyle, motile or sessile, driving distinct feeding strategies. Our findings suggest that, unlike in smaller prokaryotes, the limiting factor in unicellular eukaryotes is not the energy required to generate feeding currents, but rather the capacity of their feeding structures and intracellular capacity to process transported nutrients. This work shows the critical role of flow physics in shaping the functional design of eukaryotic cells and provides new avenues for exploring the evolutionary trajectory of predatory and cooperative traits in early eukaryotes.