Yellow plasmodia of the acellular slime mould Physarum polycephalum irradiated with white light of 70 mmol m^-2 s^-1 fluence rate showed two distinct changes in their colour depending on the irradiation period:
1) depigmentation of plasmodia (photobleaching) - within a short-term (hours) exposure, and
2) plasmodia taking on an orange colour as a result of a long period (a few days) continuous exposure to radiation.
Photobleached plasmodia transferred to the dark resynthetized the pigments and slowly, after about 10 hrs become yellowish and than, regained their yellow colour. General features of these pigments (extraction, solubility, absorption spectra) were comparable to those isolated from non-irradiated organisms.
The pigments isolated from plasmodia subjected to a long period of irradiation differed qualitatively from those produced in the dark grown cultures. The general features of these pigments, e. g. solubility, absorption spectra, point to their being of the carotenoid type.