X-ray microanalysis and chlorotetracycline staining of calcium vesicles in the green alga Mougeotia.

Calcium ions have been proposed to play a key role in the sensory transduction of phytochrome-governed chloroplast movement in the green alga Mougeotia. To test this hypothesis, the intracellular pattern of calcium distribution was studied in this alga by two independent techniques, namely, X-ray microanalysis of fixed and of unfixed frozen-hydrated cells, as well as in vivo fluorescence by chlorotetracycline. Both methods of detection reveal a significant compartmentation of calcium in vesicles close to the chloroplast edge and, less frequently, in the cortical cytoplasm. Microfilaments, presumably actin, which could function in driving chloroplast movement, have been observed running between the chloroplast edge and the cortical cytoplasm (Wagner, G., Klein, K. (1978) Photochem. Photobiol. 27, 137). The vesicular calcium concentration is stable and decays only slowly in the absence of extracellular calcium much in the same way as the ability of the chloroplast to perform movements decreases. A functional relationship between vesicular calcium compartmentation and phytochrome-governed chloroplast movement in the green alga Mougeotia seems indicated.