Ultrastructure of chloroplasts in'Marimo'(Cladophora aegagropila, Chlorophyta), and changes after exposure to light

This study compares the ultrastructure of the inner, dark-habituated cells of the green ‘Cladophora-ball’, or Marimo, to that of similar cells at the surface. Cells not exposed to light possess fewer, but larger and more irregular, chloroplasts than do the outer cells. Unexposed chloroplasts have a pyrenoid matrix lacking starch sheaths and containing unusually thick granal stacks. Despite prolonged exposure to darkness, the chloroplasts contain small starch grains. After exposure to light, such chloroplasts divide, become smaller and take on the appearance of those in the outer layer cells. Within 48 h, all of the chloroplasts develop substantial starch grains and the pyrenoids are surrounded by starch sheaths. This response is consistent with previous reports of the recovery of photosynthetic activity in inner cells exposed to light.