Elemental distribution and chemical speciation of copper and cobalt in three metallophytes from the copper-cobalt belt in Northern Zambia.

Three metallophyte species, Persicaria capitata, P. puncata (Polygonaceae), Conyza cordata (Asteraceae) from mineral wastes in the Zambian copper-cobalt belt were studied. This study focused on the elemental distribution in the roots, stems and leaves, using a range of techniques: micro-PIXE, SEM-EDS synchrotron XFM and XAS. The species differed in their responses to growing on Co-Cu-enriched soils: Persicaria puncata is a Co hyperaccumulator (up to 1060 µg g-1 in leaves), while Persicaria capitata and Conyza cordata are Co-excluders. All three species are Cu-accumulators. The highest concentrations of Cu-Co are in the epidermal cells, whereas in Persicaria puncata Co was also enriched in the phloem. The Co coordination chemistry shows that an aqueous Co(ii)-tartrate complex was the predominant component identified in all plants and tissues, along with a minor component of a Co(iii) compound with oxygen donor ligands. For Cu, there was considerable variation in the Cu speciation in the various tissues and across the three species. In contrast to hyperaccumulator plants, excluder and accumulator type plants have received far less attention. This study highlights the different biopathways of transition elements (Cu, Co) in hyper-tolerant plant species showing different responses to metalliferous environments.

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.

Copper and cobalt accumulation in plants: a critical assessment of the current state of knowledge.

This review synthesizes contemporary understanding of copper-cobalt (Cu-Co) tolerance and accumulation in plants. Accumulation of foliar Cu and Co to > 300 µg g-1 is exceptionally rare globally, and known principally from the Copperbelt of Central Africa. Cobalt accumulation is also observed in a limited number of nickel (Ni) hyperaccumulator plants occurring on ultramafic soils around the world. None of the putative Cu or Co hyperaccumulator plants appears to comply with the fundamental principle of hyperaccumulation, as foliar Cu-Co accumulation is strongly dose-dependent. Abnormally high plant tissue Cu concentrations occur only when plants are exposed to high soil Cu with a low root to shoot translocation factor. Most Cu-tolerant plants are Excluders sensu Baker and therefore setting nominal threshold values for Cu hyperaccumulation is not informative. Abnormal accumulation of Co occurs under similar circumstances in the Copperbelt of Central Africa as well as sporadically in Ni hyperaccumulator plants on ultramafic soils; however, Co-tolerant plants behave physiologically as Indicators sensu Baker. Practical application of Cu-Co accumulator plants in phytomining is limited due to their dose-dependent accumulation characteristics, although for Co field trials may be warranted on highly Co-contaminated mineral wastes because of its relatively high metal value.

Potential of copper-tolerant grasses to implement phytostabilisation strategies on polluted soils in South D. R. Congo : Poaceae candidates for phytostabilisation.

Phytostabilisation (i.e. using plants to immobilise contaminants) represents a well-known technology to hamper heavy metal spread across landscapes. Southeastern D.R. Congo, Microchloa altera, a tolerant grass from the copper hills, was recently identified as a candidate species to stabilise copper in the soil. More than 50 grasses compose this flora, which may be studied to implement phytostabilisation strategies. However, little is known about their phenology, tolerance, reproductive strategy or demography. The present study aims to characterize the Poaceae that may be used in phytostabilisation purposes based on the following criteria: their ecological distribution, seed production at two times, abundance, soil coverage and the germination percentage of their seeds. We selected seven perennial Poaceae that occur on the copper hills. Their ecological distributions (i.e. species response curves) have been modelled along copper or cobalt gradients with generalised additive models using logic link based on 172 presence-absence samples on three sites. For other variables, a total of 69 quadrats (1 m(2)) were randomly placed across three sites and habitats. For each species, we compared the number of inflorescence-bearing stems (IBS) by plot, the percentage of cover, the number of seeds by IBS and the estimated number of seeds by plot between sites and habitat. Three species (Andropogon schirensis, Eragrostis racemosa and Loudetia simplex) were very interesting for phytostabilisation programs. They produced a large quantity of seeds and had the highest percentage of cover. However, A. schirensis and L. simplex presented significant variations in the number of seeds and the percentage of cover according to site.