Photodynamically induced chemoresponses of the colorless flagellate,Astasia longa,in the presence of riboflavin

In the presence of the photosensitizer riboflavin, Astasia accumulates in illuminated fields at high fluence rates. The quenchers of riboflavin excited states, NaN₃ and KI, abolish the photodynamic effect of riboflavin. Crocetin, a ¹O₂ quencher, does not influence the photodynamic action of riboflavin while 1,4-benzoquinone very strongly depresses its effect. This indicates a type I pathway forming H₂O₂ as a photoproduct. The photodynamic effect is abolished by the addition of 10^-5 M catalase which breaks down H₂O₂. Astasia shows chemoaccumulations around the opening of a capillary filled either with riboflavin (under high intensity irradiation) or H₂O₂ which proves the hypothesis that the photobehavioral response in the presence of riboflavin is based on a chemoresponse toward H₂O₂, produced when the dye is irradiated. The accumulation around the capillary opening is not due to a direct chemotactic movement of cells but rather to a chemophobic response which prevents the cells from swimming away from the chemoattractant.