ENVIRONMENTAL-FACTORS INFLUENCING DIATOM CELL MOTILITY

This article describes the effect of various environmental conditions on the speed of freshwater diatoms. The diatoms display a wide distribution of cell speeds, from 0 to >20 mu m.s(-1), as indicated by both the distribution of speeds within a cell population as well as the distribution of 1-s interval speeds of individual cells. Cell length has little or no effect on cell speed, as the speed of large postconjugal populations of Craticula and smaller preconjugal cell populations were not significantly different. The diatoms showed a broad pH tolerance, with Craticula spp and Nitzschia spp. displaying active motility between pH 3 and 12 and pH 4 and 10, respectively, with velocity maxima for both species at approximately pH 7. In contrast to previous reports on marine diatoms, these freshwater diatoms do not require millimolar levels of external calcium for motility, as both Craticula spp. and Nitzschia spp. showed significant motility in distilled water (<0.5 mu M calcium)for up to 5 h. Although addition of greater than or equal to 500 mu M of EGTA inhibited motility, this inhibition did not appear to be due to calcium chelation, as EGTA solutions preincubated with up to 20-fold excess calcium, magnesium, or both still inhibited motility with the same dose-dependent response as EGTA alone. Moreover, significant motility was restored by rinsing EGTA-treated cells in distilled water, Ca-free medium, or regular diatom medium. The calcium channel inhibitors lanthanum and ruthenium red also inhibited motility in a dose-dependent manner, suggesting that regulation of internal calcium stores may be important in motility. Craticula motility also declines rapidly in medium greater than or equal to 50 mOsM while the isotonic concentration appears to be 100-120 mOsM, suggesting that movement may require the plasma membrane to be exerting a force greater than or equal to 50-70 mOsM osmotic pressure (1-1.5 atm) against the cell wall.