• apoptotic-like;
  • cell death;
  • Chlamydomonas;
  • DNA fragmentation;
  • necrosis;
  • UV damage

Chlamydomonas reinhardtii (Ehrenberg) cells exhibited cell death process akin to that of apoptosis when exposed to ultraviolet (UV)-C irradiation (1–100 J/m2). We observed typical hallmarks of apoptosis including cell shrinkage, associated nuclear morphological changes, flipping of phosphatidylserine, and DNA fragmentation detected by the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay and oligonucleosomal DNA laddering assay. Interestingly, fluorescence imaging of DNA changes in UV-C exposed cells, following PicoGreen staining, revealed that extra-nuclear DNA disintegrates before that of nuclear changes, where the latter extensively diffuses out of the nuclear compartment, spreading into the whole cell and reaching the periphery of dying cells. Antibodies against a mammalian caspase-3 shared epitopes with a protein of 28 kDa; whose pattern of expression correlated with the onset of cell death. Moreover, growth experiments indicate that spent medium recovered from UV-C exposed cells exhibit a protective effect against cell killing of fresh cultures of C. reinhardtii cells by UV irradiation. The protective effect of UV-spent medium is not a general growth promotional response on normal cells, but rather, is specific to UV-exposed cells. We propose a model that C. reinhardtii cells exposed to UV elicit apoptotic-like changes, which in turn lead to an adaptive response in neighboring cells against fresh rounds of UV exposure, thereby promoting survival of the cell population.