Effect of selenium on arsenic metabolism in Cylindrotheca fusiformis

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Abstract
Effects of selenium (Se) on the kinetics of incorporation and excretion of 74As have been studied by use of a marine diatom, Cylindrotheca fusiformis. For the incorporation experiment, the cells were incubated with carrier‐free 74As in a phosphate‐free normal medium, and collected sequentially over 45 h. For the excretion experiment, the cells were transferred to a normal medium and collected sequentially over 95 h. The cell components were fractionated into watersoluble, insoluble and lipid fractions by TLC. 74As was mostly incorporated into the water‐soluble fraction and less than 10% of it into the insoluble and lipid fractions. Within a few hours considerable amounts of 74As were found in those fractios, but after that the rate of accumulation decreased. 74As was excreted into the medium rapidly at the beginning, but the rate of excretion slowed down gradually. In the cells, the amount of As in the insoluble fraction decreased more rapidly than that in the lipid and water‐soluble fractions. The time course changes of 74As in the three fractions showed some oscillation of larger amplitude of variation at first and later approached a constant value. Selenium in the medium showed some effects o the rate of arsenic incorporation as well as on the ratio of the three fractions. In the excretion process, selenium changed the amplitude of the variation and the ratio of 74As in the three fractions at the later times.

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