Projekte

Although arsenic is a common pollutant worldwide, many questions about As metabolism in non-hyperaccumulator plants remain. Concentration and tissue dependent speciation and distribution of arsenic was analysed in the aquatic plant Ceratophyllum demersum to understand arsenic metabolism in non-hyperaccumulator plants. Speciation was analysed chromatographically (HPLC-(ICP-MS)-(ESI-MS)) in whole-plant extracts and by tissue-resolution confocal X-ray absorption spectroscopy (µ XANES) in intact shock-frozen hydrated leaves, which were also used for analysing cellular element distribution through X-ray fluorescence (µ XRF). Chromatography revealed up to 20 As-containing species binding >60% of accumulated As. Of these, eight were identified as thiol-bound (phytochelatins; PCs, glutathione; GSH and cystein) species including three newly identified complexes: Cys-As(III)-PC2, Cys-As-(GS)2 and GS-As(III)-desgly-PC2 complex. Confocal µ XANES showed As(V), As(III), As-(GS)3 and As-PCs with varying ratios in various tissues. The epidermis of mature leaves contained the highest proportion of thiol- (mostly PC-) bound As, while in younger leaves a lower proportion of As was thiol-bound. At higher As concentrations, the percentage of unbound As(III) increased in the vein and mesophyll of young mature leaves. At the same time, µ XRF showed an increase of total As in the vein and mesophyll but not in the epidermis of young mature leaves while it was reverse for Zn distribution. Thus, As toxicity was correlated with a change in As distribution pattern and As species, rather than general increase in many tissues.

Even essential trace elements are phytotoxic over a certain threshold. In this study, we investigated whether heavy metal concentrations were responsible for the nearly complete lack of submerged macrophytes in an oligotrophic lake in Germany. We cultivated the rootless aquatic model plant Ceratophyllum demersum under environmentally relevant conditions like sinusoidal light and temperature cycles and a low plant biomass to water volume ratio. Experiments lasted for six weeks and were analysed by detailed measurements of photosynthetic biophysics, pigment content and hydrogen peroxide production. We established that individually non-toxic cadmium (3 nM) and slightly toxic nickel (300 nM) concentrations became highly toxic when applied together in soft water, severely inhibiting photosynthetic light reactions. Toxicity was further enhanced by phosphate limitation (75 nM) in soft water as present in many freshwater habitats. In the investigated lake, however, high water hardness limited the toxicity of these metal concentrations, thus the inhibition of macrophytic growth in the lake must have additional reasons. The results showed that synergistic heavy metal toxicity may change ecosystems in many more cases than estimated so far.

Hyperaccumulators are being intensely investigated. They are not only interesting in scientific context due to their “strange” behavior in terms of dealing with high concentrations of metals, but also because of their use in phytoremediation and phytomining, for which understanding the mechanisms of hyperaccumulation is crucial. Hyperaccumulators naturally use metal accumulation as a defense against herbivores and pathogens, and therefore deal with accumulated metals in very specific ways of complexation and compartmentation, different from non-hyperaccumulator plants and also non-hyperaccumulated metals. For example, in contrast to non-hyperaccumulators, in hyperaccumulators even the classical phytochelatin-inducing metal, cadmium, is predominantly not bound by such sulfur ligands, but only by weak oxygen ligands. This applies to all hyperaccumulated metals investigated so far, as well as hyperaccumulation of the metalloid arsenic. Stronger ligands, as they have been shown to complex metals in non-hyperaccumulators, are in hyperaccumulators used for transient binding during transport to the storage sites (e.g., nicotianamine) and possibly for export of Cu in Cd/Zn hyperaccumulators [metallothioneins (MTs)]. This confirmed that enhanced active metal transport, and not metal complexation, is the key mechanism of hyperaccumulation. Hyperaccumulators tolerate the high amount of accumulated heavy metals by sequestering them into vacuoles, usually in large storage cells of the epidermis. This is mediated by strongly elevated expression of specific transport proteins in various tissues from metal uptake in the shoots up to the storage sites in the leaf epidermis. However, this mechanism seems to be very metal specific. Non-hyperaccumulated metals in hyperaccumulators seem to be dealt with like in non-hyperaccumulator plants, i.e., detoxified by binding to strong ligands such as MTs.