Light-induced and apoptosis-like cell death in the unicellular eukaryote, Blepharisma japonicum.

The unicellular eukaryote, Blepharisma japonicum, is a light-sensitive ciliated protozoa. It possesses a photoreceptor pigment called blepharismin that plays critical roles in defensive behavior against predators and step-up photophobic response. In addition, the pigment generates reactive oxygen species such as singlet oxygen and hydroxyl radicals which contribute to photodynamic action. Previous studies reported that intense light (>300W m(-2)) induced rapid photodynamic killing (necrosis) characterized by cell swelling and plasma efflux, while moderate light (3-30W m(-2)) only induced pigment extrusion and photooxidation. We have found that moderate light (5W m(-2)) induced apoptosis-like cell death. Microscopically it was found that >3h of moderate light irradiation induced macronuclear condensation and plasma efflux without cell swelling. Single cell gel electrophoresis assay showed that DNA fragmentation occurred between 1 and 3h of irradiation, and the condensed macronuclei contained quite fragmented DNA. Macronuclear DNA extracted from light-irradiated cells contained DNA fragments of 180-200 and 360-400bp, which were seen as apoptosis ladders.