Comparative analysis of light-stimulated motility responses in three diatom species

Diatoms have long been known to be photosensitive, altering the direction of their movement in response to changes in ambient light conditions detected at the tips of the cells. In order to better understand the light conditions responsible for triggering positive photophobic (out-of-light) and negative photophobic (into-light) responses of diatoms, cells from three species of diatoms, Craticula cuspidata (Kutzing) D.G. Mann, Stauroneis phoenicenteron (Nitzsch) Ehrenberg, and Pinnularia viridis (Nitzsch) Ehrenberg, were irradiated at their leading or trailing ends during cell movement. The response times for direction changes when cells were irradiated at various irradiance levels and wavelengths were measured to determine the quality of light responsible for eliciting cell direction changes in each of the three species. All three species displayed strong out-of-light responses at the highest irradiances measured. Craticula cuspidata cells displayed negative photophobic sensitivity (into-light) responses in moderate level blue light, while S. phoenicenteron cells showed into-light responses with low-level red light. Pinnularia viridis cells showed less responsiveness to blue and green light than the other two species, and almost no sensitivity to red light. By re-irradiating cells a second time after a previous leading or trailing end irradiation, we observed a 2-3 fold (leading) or 3-8 fold (trailing) motility repression, caused by the initial light exposures, which lasted for approximately 30-60 sec. Irradiating the cells multiple times, upon each direction change, indicated some degree of habituation to irradiation over time. Multiple consecutive irradiations of the trailing end of diatoms resulted in strong repression of any direction change, with cells continuing to move in the same direction for up to 20min; this repression became reduced as the interval time between trailing-end irradiations increased. These results suggest that diatoms display species-specific physiological responses to light irradiations that may help them to appropriately respond to ambient light conditions, and better organize and succeed within larger algal or multi-species diatom assemblages.