Step-up photophobic responses of the unicellular alga Haematococcus pluvialis and their interpretation in terms of photoreceptive apparatus characteristics

This paper reports a systematic study of the step-up photophobic responses exhibited by the unicellular alga Haematococcus pluvialis when stimulated unidirectionally or bidirectionally. The stimulus-response curves, obtained at four different wavelengths, are interpreted in terms of the structure of the photoreceptive apparatus. In addition, an indirect method to obtain information about the stigma and photoreceptor(s) is reported. This method is based on the interpolation of experimental with simulated data. Our results confirm the widely accepted view that the photoreceptors are located in the stigma region, and suggest the presence of two photoreceptors.