Calcium-sequestering vesicles and contractile flagellar roots.

The intracellular localization of calcium during contraction of striated flagellar roots of the green alga Tetraselmis subcordiformis was investigated using calcium pyroantimonate and calcium oxalate cytochemistry and energy-dispersive X-ray microanalysis. Vesicles (0.25 - 0.50 microns diameter) located in the anterior-most region of the cell are calcium-sequestering organelles. These vesicles exhibit a consistent and reproducible localization of electron-opaque pyroantimonate and oxalate salts of calcium. Striated contractile flagellar roots (CFR) show a periodic pattern of calcium antimonate (Ca-Sb) deposits following 'calcium shock' stimulation of contraction of this organelle. When CFRs are in a fully extended state they do not show Ca-Sb deposits. Deposits of Ca-Sb precipitate are present throughout the cytosol when CFRs are stimulated to contract. The implications of these findings with respect to the regulation of intracellular levels of 'free' Ca2+ and to the mechanism of CFR contraction are discussed.