Swimming behavior of photobiomodulated Paramecium tetraurelia cells during NIR laser light irradiation

We study the swimming behavior of Paramecium tetraurelia cells using an optical setup that allows the recording of the collective response during irradiation with near-infrared (NIR) laser light. We developed an image-analysis and predictive-tracking algorithm to identify and monitor the paths of individual cells within the field of view. A parameter based on the velocities in consecutive frames is proposed in order to distinguish between forward swimming and avoiding reaction swimming modes. Moreover, the data obtained from the algorithm is used to extract collective parameters such as swimming mode ratio, linear velocity, and avoiding reaction duration. We observed a complex and rich response of cells to NIR irradiation, which showed variation depending on the optical power used. During the first milliseconds of laser stimulus, the proportion of cells undergoing avoidance reaction increases. After a few seconds, the number of cells detected in the avoiding reaction swimming mode decreases, and the linear velocity increases steadily. Numerical simulations suggest that temperature increments under the experimental conditions are insufficient to trigger heat-mediated action potentials in the membrane of the studied cells. • Tracking algorithm developed to monitor Paramecium cells during NIR laser exposure. • Cells exhibited behavioral changes, including shifts in swimming mode and velocity. • Numerical simulations suggest negligible thermal effects during NIR laser exposure. • We believe that non-thermal mechanisms are triggering action potentials in the cells. • This methodology can be used for dynamic cell monitoring and multi-cell analysis.