Arsenic (As) pollution is a serious concern worldwide. Recent studies under environmentally relevant conditions revealed that, in the aquatic plant Ceratophyllum demersum, pigments are the first observable target of toxicity, prior to any effect on photosynthetic parameters or to oxidative stress. Lethal toxicity was initiated by a change of As species and their distribution pattern in various tissues. Here, the localization of As was investigated at the subcellular level through X-ray fluorescence using a submicron beam and a Maia detector. Further, it was possible to obtain useful tissue structural information from the ratio of the tomogram of photon flux behind the sample to the tomogram of Compton scattering. The micro-X-ray fluorescence tomograms showed that As predominantly accumulated in the nucleus of the epidermal cells in young mature leaves exposed to sublethal 1 µM As. This suggests that As may exert toxic effects in the nucleus, for example, by interfering with nucleic acid synthesis by replacing phosphorous with As. At higher cellular concentrations, As was mainly stored in the vacuole, particularly in mature leaves. An analysis of precursors of chlorophyll and degradation metabolites revealed that the observed decrease in chlorophyll concentration was associated with hindered biosynthesis, and was not due to degradation. Coproporphyrinogen III could not be detected after exposure to only 0.5 µM As. Levels of subsequent precursors, for example, protoporphyrin IX, Mg-protoporphyrin, Mg-protoporphyrin methyl ester, and divinyl protochlorophyllide, were significantly decreased at this concentration as well, indicating that the pathway was blocked upstream of tetrapyrrole synthesis.

Centre Algatech Institute of Microbiology Academy of Sciences of the Czech Republic CZ 379 81 Třeboň Czech Republic Institute of Plant Molecular Biology Department of Biophysics and Biochemistry of Plants Biology Centre of the AS CR Branišovská 31 1160 CZ 370 05 České Budějovice Czech Republic

CSIR National Botanical Research Institute Plant Ecology and Environmental Science Division Rana Pratap Marg Lucknow 226 001 India Universität Konstanz Mathematisch Naturwissenschaftliche Sektion Fachbereich Biologie Postfach M665 D 78457 Konstanz Germany

Deutsches Elektronen Synchrotron Photon Science Notkestr 85 22603 Hamburg Germany

Universität Konstanz Mathematisch Naturwissenschaftliche Sektion Fachbereich Biologie Postfach M665 D 78457 Konstanz Germany Institute of Plant Molecular Biology Department of Biophysics and Biochemistry of Plants Biology Centre of the AS CR Branišovská 31 1160 CZ 370 05 České Budějovice Czech Republic University of South Bohemia Faculty of Science Branišovská 31 CZ 370 05 České Budejovice Czech Republic

Universität Konstanz Mathematisch Naturwissenschaftliche Sektion Fachbereich Biologie Postfach M665 D 78457 Konstanz Germany University of South Bohemia Faculty of Science Branišovská 31 CZ 370 05 České Budejovice Czech Republic

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