High variability of the electron carrier plastocyanin in microalgae

1. The plastocyanin content of Scenedesmus can be dramatically varied with the copper content of the culture medium. Figures as high as 7 mmol plastocyanin/mol chlorophyll are possible. Electron paramagnetic resonance (EPR) spectroscopy has been used to determine this physiological response quantitatively in intact cells having different amounts of plastocyanin. The results obtained by the EPR technique were compared with data on isolated plastocyanin determined either by spectrophotometry or immunoelectrophoresis. Agreement was found for the amount of plastocyanin detected by the first two methods, whereas the last assay yielded data at least 25% higher on the average. Under all culture conditions a copper-free plastocyanin precursor is present. 2. The EPR properties of purified plastocyanin and those of cellular plastocyanin located within the thylakoids are practically identical in terms of g-values, hyperfine splittings, signal linewidths and saturation behavior at temperatures of 12--15 K. Our data indicate that plastocyanin is not present in a membrane-bound form but exists as a single soluble pool. 3. The studies have been extended to the algae Dunaliella parva and Bumilleriopsis filiformis. The first species exhibits a limited variation of plastocyanin with the copper content of the medium. Furthermore, no cytochrome c-553 could be detected in Dunaliella even under conditions of copper deficiency. In contrast, Bumilleriopsis does not contain plastocyanin regardless of the amount of copper offered.