HIGH ENERGY IRRADIATION AT THE LEADING TIP OF MOVING DIATOMS CAUSES A RAPID CHANGE OF CELL DIRECTION

Abstract

Previous investigations have suggested that the phototactic behaviour of diatoms, as observed by the accumulation of diatoms in low/moderate intensity light spots and avoidance of high intensity light, is primarily due to a change in cell direction at light/dark boundaries. In order to better determine the nature of this direction reversal and the manner in which it is generated, we used a high energy microbeam coupled to a monochrometer to irradiate specific areas of moving diatoms. When the leading end of moving Craticula cuspidata (Kützing) D.G. Mann cells were irradiated with a 1–3 s exposure of high irradiance (about 500 to 2000 μmol photons m−2 s−1), they quickly reversed their direction (100% of cells tested), while unirradiated control cells rarely changed direction. Similar irradiation of the trailing half of the cell caused no change in direction, but greatly reduced the effect of a subsequent irradiation of the leading half. Using a monochrometer to vary the wavelength of irradiation and a shutter to adjust the exposure time, the irradiation time needed to induce a direction change in 50% of the cells (I50) was determined for each of five wavelengths. Concurrent measurement of the energy output of the light source at each wavelength showed the most efficient wavelength to be 500 nm, close to the wavelength previously determined to be most effective in inducing a direction change at low light level light/dark boundaries in C. cuspidata. By adjusting the area of irradiation to a small spot size, we compared the effectiveness of irradiating cells at the tip, middle, and edge of the leading half of the cell. While irradiations at the tip of moving cells still resulted in 100% of cells changing direction, irradiating the edge (where the chloroplast was located) or the middle of the leading half (near the cytoplasmic storage granules) had greatly reduced effect. Such evidence suggests that phototaxis in C. cuspidata requires a photodetection system that is most sensitive to approximately 500 nm light and is located at the tips of the cells.