Abnormal concentrations of Cu-Co in Haumaniastrum katangense, Haumaniastrum robertii and Aeolanthus biformifolius: contamination or hyperaccumulation?

The Central African Copperbelt of the DR Congo and Zambia hosts more than 30 known Cu-Co hyperaccumulator plant species. These plants can accumulate extraordinarily high concentrations of Cu and Co in their living tissues without showing any signs of toxicity. Haumaniastrum robertii is the most extreme Co hyperaccumulator (able to accumulate up to 1 wt% Co), whereas Aeolanthus biformifolius is the most extreme Cu hyperaccumulator (with up to 1 wt% Cu). The phenomenon of Cu-Co hyperaccumulator plants was studied intensively in the 1970s through to the 1990s, but doubts arose regarding earlier observations due to surficial contamination of plant material with mineral particles. This study set out to determine whether such extraneous contamination could be observed on herbarium specimens of Haumaniastrum robertii and Aeolanthus biformifolius using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). Further, synchrotron X-ray absorption spectroscopy (XAS) was used to identify the chemical forms of Cu and Co in newly collected Haumaniastrum katangense plant material from the DR Congo. The results show that surficial contamination is not the cause for abnormal Cu-Co concentrations in the plant material, but rather that Cu-Co enrichment is endogenous. The chemical form of Cu and Co (complexation with carboxylic acids) provides additional evidence that genuine hyperaccumulation, and not soil mineral contamination, is responsible for extreme tissue concentrations of Cu and Co in Haumaniastrum katangense.