Characterisation of the Diatom Photophobic Response to High Irradiance

It has previously been shown that high intensity light irradiation at the leading tip of a moving benthic diatom can trigger reversals of cell direction. In this paper we have begun to characterise this response, determining the irradiance threshold and exposure durations needed to stimulate a response, as well as the amount of time that the effect of irradiation persists in the cell. Using irradiation levels of ca. 5 × 105 μmol m−2 s−1 (at 470 nm), we found that for Craticula cuspidata a reversal response requires a minimum exposure duration of 10 ms; exposures ≥ 100 ms stimulated 95–100% of cells to change direction. Using a single uninterrupted 2 s irradiation, cells show a threshold of response at ca. 1 × 103 μmol m−2 s−1. Using two irradiation pulses in rapid succession, we determined that cells can resolve individual light pulses as closely spaced as 1 ms. Similarly, when using longer total exposures broken up into a number of shorter light pulses, cell responses diminish rapidly if the pulses are separated by ≥ 1 ms. The threshold stimulation of cells appears to require a relatively consistent total number of photons as the irradiation duration for cell stimulation is inversely proportional to the irradiance used. Cell irradiations also cause a persistence of effect on the cells for about 60 s, during which the response of cells to subsequent irradiations are reduced.