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 mu M and 6.6 mu M 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 mu M 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. (C) 2009 Elsevier Ltd. All rights reserved.