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Cytological Mechanism Underlying Darkness-Survival of the Unicellular Red Alga Porphyridillm cruentum

Abstract 1. The ultrastructure of the unicellular rhodophyte Porphyridium cntentum Naegeli was examined prior to 8-weeks' placement in total darkness and after subsequent re-exposure to light. The algal cells were contained in inorganic seawater medium incapable of supporting growth in darkness. The "recovery" process of the darkness-inactivated alga was monitored by electron microscopy of culture samples periodically fixed from zero to 72 hours of reinstatement in continuous light. 2. Four types of cells were recognized in the course of this study: (i) nomlal (vegetative) cells from growth in light; (ii) moribund (presumed non-viable) cells from senescence; (iii) donnant (resting) cells from darkness inactivation; (iv) reactivated cells recovering from dormancy on re-exposure to light. Apart from 22% reduction in cell volume from light deprivation, these cell types differed from one another in relatively minor structural details, and there were no indications of wall thickening or other features commonly considered characteristic of spores and cysts. The donnant cells were characterized by poor stainability of all internal membranes, excessive clumping of thylakoids, absence of recognizable pyrenoid, starch bodies, endoplasmic reticulum, dictyosomes, and proliferation of plastoglobuli and cytoplasmic vacuoles. Cells with such properties intermediate between the normal and dormant types were considered reactivated by light, whilst those showing extensive damage (dilation and rupture) of membranes of vital organelles were considered moribund. Phycobilisomes were observed in all cell types. 3. The population emerging from darkness showed 90% dormant and 10% moribund cells. With increasing duration of re-illumination, the dormant cells were rapidly transformed into reactivated types, which achieved population peak at 6 hr and then declined with further transformation to normal cells. At 24-hr illumination, the normal cells were predominant, with total disappearance of donnant cells. The conversion of reactivated to normal cells was virtually complete within 48-72 hr illumination. 4. It is concluded that P. cruentum survives darkness by a process of dormancy involving minimal ultrastructural changes, and the absence of extra wall or sheath formation is inferred to facilitate the remarkably rapid activation of the alga on restoration of light.