Sessile ciliates attach to substrates and generate feeding currents to capture passing particulates and dissolved nutrients. Optimal ciliary activity that maximizes nutrient flux at the cell surface while minimizing the rate of hydrodynamic energy dissipation is well characterized in uniform nutrient fields. However, it is unclear how ciliary motion should change when nutrients are nonuniform or patchy. To address this question, we modeled the sessile ciliate and feeding currents using the spherical envelope model, and used an unsteady advection-diffusion equation to describe the nutrient scalar field. In the absence of flows, we introduced an analytical solution to the diffusive nutrient uptake in linear concentration gradients and found no advantage over uptake in uniform concentration fields. With ciliary activity driving feeding currents, we used a combination of spectral and finite difference method to solve for the unsteady nutrient concentration. We found that when the axis of symmetry of the ciliary motion is aligned with the concentration gradient, nutrient uptake at the cell surface increases steadily over time, with the highest uptake achieved by the treadmill ciliary motion, which is optimal in uniform fields as well. The associated nutrient uptake in concentration gradients scales with the square root of the product of time and Péclet number. In patchy environments, optimal ciliary activity depends on the nature of the patchiness. Our findings highlight strategies that enable sessile ciliates to thrive in environments with fluctuating nutrient availability.

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• Biological fluid dynamics
• Living matter & active matter
• Low Reynolds number flows