Senecio conrathii N.E.Br. (Asteraceae), a new hyperaccumulator of nickel from serpentinite outcrops of the Barberton Greenstone Belt, South Africa

The Influence of Serpentine Soil on the Early Development of a Non-Serpentine African Thistle, Berkheya radula (Harv.) De Wild

Serpentine soils are rich in heavy metals and poor in nutrients, limiting plant species’ performance and survival. Nevertheless, specificities of such limitations as well as adaptability features required for thriving in serpentine environments are barely known. The Barberton Greenstone Belt in South Africa is an example of an area containing serpentine soil with adapted vegetation. In this study, a pot experiment was performed to compare development features (i.e., germination rates, leaf count, leaf length, biomass and photosynthetic capacity) during the early development of the non-serpentine species Berkheya radula, a genus consisting of known metal hyperaccumulators from serpentine areas in South Africa. B. radula was grown in serpentine soils taken from the Barberton region. B. radula leaves had heavy metals in concentrations that confirmed the species as a phytoextractor. There were trends for enhanced productivity and photosynthesis in the serpentine treatments compared to the control. Leaf count, leaf length, electron transport efficiency (ψ_Eo/(1 − ψ_Eo), density of reaction centers and PI_ABS,total were significantly and positively correlated with at least one of the heavy metals in the leaves. Germination rates were positively influenced by K, whereas biomass and the density of reaction centers were negatively affected by Ca and P, and only Ca, respectively. The heavy metals Zn, Ni and Co were positively correlated with each other, whereas they were negatively correlated with the macronutrients K, Ca and P. The latter correlated positively with each other, confirming higher fertility of the control soil. Our study suggests that B. radula exhibits metallophyte characteristics (i.e., preadapted), despite not naturally occurring on metal-enriched soil, and this provides evidence that the potential for bioaccumulation and phytoremediation is shared between serpentine and non-serpentine species in this genus.

Influence of land use and topography on distribution and bioaccumulation of potentially toxic metals in soil and plant leaves: A case study from Sekhukhuneland, South Africa

Potentially toxic metal (PTM) enrichment of the soil-plant system in ultramafic and mining regions is a global concern as it affects the food chain. With expanding mining industry, it is important to assess if anthropogenic factors (i.e., land use practices) have a greater influence in this regard compared to natural factors (i.e., topography). Localities in Sekhukhuneland, South Africa, were selected along an altitudinal gradient (i.e., topography: upper slope, footslope, valley and valley bottom) and a land use profile (i.e., rangelands, gardens, tailings and wastelands) to investigate the distribution of Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Sr and Zn of natural (i.e., ultramafic geology) and anthropogenic (i.e., mining) origin in surface soil and plant leaf tissue. Plant life form was considered as an additional factor to evaluate PTM accumulation in leaves. Findings revealed a wider distribution range for Cr and Ni in the surface soil. Co, Cu, Mg, Mo, Sr and Zn were accumulated (bioaccumulation factor, BAF > 1) in leaf tissue of 74% of the evaluated plants of which 83% were indigenous. Grasses, forbs, dwarf shrubs and shrubs showed the highest accumulation levels. Despite an observed trend in the distribution of PTMs in soils and plant leaves along the altitudinal gradient, no significant differences were determined among the topographic positions. Land use practices, however, differed significantly indicating anthropogenic interference as a predominant determinant of PTM enrichment of soil-plant systems. Metal tolerant dominant plants in Sekhukhuneland could be classified as metallophytes. Indigenous species, accumulators and excluders, showed prospects for phytoremediation and rehabilitation of metal contaminated sites, respectively. Concentrations of Cr and Co in food and medicinal plant leaves exceeded the international permissible limits, which highlighted the necessity to estimate human health risks for PTMs in metalliferous sites.

The Wasp as a Terrestrial Indicator of Environmental Metal Composition: Evidence from Zimbabwe

We explored metal concentrations in wasps from 4 sites near Harare, Zimbabwe, on a 106 km west-east transect. We found elevated concentrations at 2 presumed-polluted sites (a platinum [Pt] mine and a known polluted lake) located near a metal-enriched geological feature (the Great Dyke). A site in urban Harare and a nature reserve served as reference. Only wasps from the 2 presumed-polluted sites had quantifiable Pt. For Cr, Ni, Mg, Se, Fe, Mn, and V, we report the highest concentrations in wasps yet published. Wasps from the presumed-polluted sites had significantly higher concentrations of most metals when compared with wasps from the reference sites, suggesting pollution as a source. Geology, however, differs between the sites. It is probable, therefore, that both geology and pollution contributed to the differences in metal concentrations. Because of its long and narrow dimensions (550 km long and 4-11 km wide), the Great Dyke offers opportunities for comparative studies. Because wasps form a complex part of the food web and ecology, studies on the transfer of metals to wasps' predators are needed, especially given that some birds specialize in feeding on hymenopterans. The rich diversity of wasps (>145 000 species worldwide) occupying multiple different trophic levels is a good indicator, and wasps have a rich potential to join other invertebrates as terrestrial indicators. Environ Toxicol Chem 2021;40:1726-1739. © 2021 SETAC.

Ecophysiology of nickel hyperaccumulating plants from South Africa - from ultramafic soil and mycorrhiza to plants and insects

An overview of 30 years of studies related to South African nickel hyperaccumulators is presented. Only five species have so far been identified as Ni hyperaccumulator plants among very rich and diversified South African flora. All of them occur on soils derived from ultramafic (serpentine) rocks and belong to the family Asteraceae: Berkheya coddii Roessler, Berkheya zeyheri subsp. rehmannii var. rogersiana, Berkheya nivea, Senecio coronatus, Senecio anomalochrous. Several techniques and methods were used to investigate ecophysiological aspects of the Ni hyperaccumulation phenomenon, from basic field and laboratory studies, to advanced instrumental methods. Analysis of elemental distribution in plant parts showed that in most cases the hyperaccumulated metal was stored in physiologically inactive tissues such as the foliar epidermis. However, an exception is Berkheya coddii, which has a distinctly different pattern of Ni distribution in leaves, with the highest concentration in the mesophyll. Such a distribution suggests that different physiological mechanisms are involved in the Ni transport, storage location and detoxification, compared to other hyperaccumulator species. Berkheya coddii is a plant with high potential for phytoremediation and phytomining due to its large biomass and potentially high Ni yield, that can reach 7.6% of Ni in dry mass of leaves. Senecio coronatus is the only known hyperaccumulator with two genotypes, hyperaccumulating and non-hyperaccumulating, growing on Ni-enriched/metalliferous soil. Detailed ultrastructural studies were undertaken to characterize specialized groups of cells in the root cortex of Ni-hyperaccumulating genotype, that are not known from any other hyperaccumulator. The occurrence of arbuscular mycorrhiza (AM) in Ni-hyperaccumulating plants was found for the first time in South African hyperaccumulator plants, and this type of symbiosis has been proved obligatory in all of them. There is a significant influence of mycorrhiza on the concentration and distribution of several elements. Three highly specialized herbivore insects feeding only on Ni hyperaccumulator plants were identified: Chrysolina clathrata (formerly Chrysolina pardalina), Epilachna nylanderi and Stenoscepa sp. The Ni-elimination strategies of these specialised insects have been established. Microbiological studies have revealed several genera of fungi and bacteria isolated from B. coddii leaves as well as presence of specialised, Ni-resistant yeasts in the C. clathrata gut. Understanding ecophysiological response to harsh environment broadens our knowledge and can have practical applications in cleaning polluted environments through phytomining/agromining. Finally, conservation aspects are also discussed and lines for future research are proposed.

Towards an Understanding of the Molecular Basis of Nickel Hyperaccumulation in Plants

Metal hyperaccumulation is a rare and fascinating phenomenon, whereby plants actively accumulate high concentrations of metal ions in their above-ground tissues. Enhanced uptake and root-to-shoot translocation of specific metal ions coupled with an increased capacity for detoxification and sequestration of these ions are thought to constitute the physiological basis of the hyperaccumulation phenotype. Nickel hyperaccumulators were the first to be discovered and are the most numerous, accounting for some seventy-five percent of all known hyperaccumulators. However, our understanding of the molecular basis of the physiological processes underpinning Ni hyperaccumulation has lagged behind that of Zn and Cd hyperaccumulation, in large part due to a lack of genomic resources for Ni hyperaccumulators. The advent of RNA-Seq technology, which allows both transcriptome assembly and profiling of global gene expression without the need for a reference genome, has offered a new route for the analysis of Ni hyperaccumulators, and several such studies have recently been reported. Here we review the current state of our understanding of the molecular basis of Ni hyperaccumulation in plants, with an emphasis on insights gained from recent RNA-Seq experiments, highlight commonalities and differences between Ni hyperaccumulators, and suggest potential future avenues of research in this field.