Effects of nickel on the chloroplasts of the duckweeds Spirodela polyrhiza and Lemna minor and their possible use in biomonitoring and phytoremediation

Toxic effects of Ni2+ on the chloroplasts of the two duckweed species Spirodela polyrhiza, clone SJ and Lemna minor, clone St were investigated according to the ISO 20079 protocol. Ni2+ induced a transition from chloroplasts to chloro-amyloplasts and amylo-chloroplasts, but not to gerontoplasts, as shown by electron microscopy. The contents of the chlorophylls a and b decreased strongly, whereas that of carotenoids remained approximately constant. Most striking was, however, the accumulation of transitory starch. Bell-shaped dose–response curves showed that Spirodela and Lemna amassed maximum starch contents of approximately 10% and 7%, respectively, on a fresh weight basis. Because Ni2+ in the concentrations applied does not stimulate photosynthesis, the Ni2+-induced starch accumulation indicates that the export of carbohydrates out of the plastids decreased, most probably due to the lower demand of the rest of the cells as a result of the Ni2+-dependent inhibition of growth. The half-maximal concentrations for inhibition of the fresh weight increase over the 7-day test period were 3.7 μM and 6.6 μM for Spirodela and Lemna, respectively: Spirodela was thus somewhat more sensitive to the heavy metal. Both species accumulated approximately 3 g of Ni2+ per kg of dry weight after application of 100 μM NiCl2. Because of their high sensitivity to phytotoxic effects, however, Spirodela and Lemna do not appear to be particularly suitable for phytoremediation of Ni2+-contaminated waste water. The high sensitivity to Ni2+ makes them instead a suitable system for ecotoxicological testing in accordance with the ISO 20079 protocol.