Biased Lévy Walk Enables Light Gradient Sensing in <i>Euglena gracilis</i>

We examine the navigation behavior of the photosensitive alga Euglena gracilis in confined environments. Under uniform lighting conditions, E. gracilis exhibits stochastic movements with nearly straight runs interrupted by abrupt directional changes. The lengths of these runs follow a long-tailed distribution typical of a Lévy walk, with scaling exponents that vary with light intensity. In gradient lighting conditions, the cells modulate their run durations-extending them upon detecting an increase in light intensity and shortening them when a decrease is detected. This adjustment effectively biases the Lévy walk, enabling the cells to ascend the spatial light gradient. This behavior mirrors well-known prokaryotic stochastic navigation strategies, such as bacterial chemotaxis, offering a eukaryotic parallel. The experimental observations under varied lighting conditions are consistently replicated through an agent-based model.