The effects of continuous illumination on the function and structure of Micrasterias torreyi Bail. (Conjugatophyceae)

Abstract. The division rate of Micrasterias torreyi cells grown under continuous illumination first accelerated but soon slowed down, and the cells lost their ability to divide after about 1 month. During the treatment the cells became pale green, the pyrenoids became fewer in number and defects appeared in the chloroplasts. After 1 month, the cells also soon died, even when subjected to intermittent illumination. The most striking structural alterations were found in the chloroplasts: the starch granules lost their typical structure, the lamellae were damaged and numerous electron dense precipitates appeared in the chloroplasts. The precipitates were similar to those formed in cells treated with supraoptimal external calcium concentrations and X-ray microanalysis showed that the precipitates were rich in calcium in both cases. The results suggest that light controls and activates the Ca²⁺ uptake in the plasma membrane as well as in the chloroplast envelope, that the large sized chloroplasts of Micrasterias are effective in regulation of cytoplasmic Ca²⁺ concentration, and that the injuries caused by continuous illumination may be largely due to the accumulation of Ca²⁺ in the chloroplasts.