Adaptive locomotion of living organisms contributes to their competitive abilities and helps maintain their fitness in diverse environments. To date, however, our understanding of searching behavior and its ultimate cause remains poorly understood in ecology and biology. Here, we investigate motion patterns of biofilm-inhabiting marine raphid diatom Navicula arenaria var. rostellata in two-dimensional space. We report that individual Navicula cells display a “circular-run-and-reversal” movement behavior at different concentrations of dissolved silicic acid (dSi). We show that gliding motions of cells can be predicted accurately with a Langevin model. Our experimental results are consistent with an optimal foraging strategy in which both circular-run and reversal behaviors are important ingredients. Our theoretical results suggest that the evolving movement behaviors of diatoms may be driven by optimization of searching ability, and predicted behavioral parameters coincide with the experimental observations. These optimized movement behaviors are an evolutionarily stable strategy to cope with environmental complexity.