Calcium-sequestering vesicles and contractile flagellar roots

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Abstract
The intracellular localization of calcium during contraction of striated flagellar roots of the green alga Tetrasehnis subcordiformis was investigated using calcium pyroantimonate and calcium oxalate cytochemistry and energy-dispersive X-ray microanalysis. Vesicles (0·25-0·50 μm 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.