Ecological Considerations of Diatom Cell Motility. I. Characterization of Motility and Adhesion in Four Diatom Species1

To better determine the ecological role of motility in pennate diatoms, we quantitatively characterized several motility and adhesion properties of four species of motile pennate diatoms(Craticulasp.,Pinnulariasp.,Nitzschiasp., andStauroneissp.)isolated from the same freshwater pond. Using computer‐assisted video microscopy, we measured speed, size/shape, functional adhesion, path curvature, and light sensitivity for these species, each of which shows a distinctive set of motile behaviors. The average speeds ofStauroneis, Pinnularia, Nitzschia, andCraticulacells are 4.6, 5.3, 10.4, and 10.0 μm · s−1, respectively.CraticulaandNitzschiacells move in a relatively straight path (<4 degrees rotation per 100 μm movement),Stauroneisexhibits minor rotation (about 7 degrees per 100 μm movement), andPinnulariarotates considerably during movement (about 22 degrees per 100 μm moved). Functional adhesion (as measured by the release rate of attached cells from the underside of an inverted coverslip) shows a half time for cell release of approximately 50 min forCraticula, 192 min forPinnularia, and >1 day forNitzschiaandStauroneis.Direction reversal at light/dark boundaries, which appears to be the main contributor to diatom Phototaxis, is most responsive forCraticula, Pinnularia, andNitzschiaat wavelengths around 500 nm.CraticulaandNitzschiacells are the most sensitive in the photophobic response, with over 60% of these cells responding to a 30‐1x light/dark boundary at 500 nm, whereasPinnulariacells are only moderately responsive at this irradiance, showing a maximal response of approximately 30% of cells at 450 nm.Stauroneiscells, in contrast, had a maximal photosensitive response at 700 nm, suggesting that this cell type may use a different response mechanism than the other three cell types. In addition,CraticulaandPinnulariashow a net movement out of the light spot when illuminated at 650 nm, whereasStauroneisshows a net movement out of the light spot when illuminated at 450 nm. Such quantitative characterizations of species‐specific responses to environmental stimuli should give us a firm foundation for future studies analyzing the behavior of interspecies diatom competition for limited light or nutrient resources.