Periodic Reporting for period 1 - SPECADIS (Speciation and bioavailability of heavy metal cadmium (Cd) in the soil-plant system: a novel approach combining stable isotope geochemistry and experimental spectroscopy.)
Berichtszeitraum: 2019-02-04 bis 2021-02-03
The environmental pollution by ecotoxic heavy metals (e.g. mercury, lead) is a problem of increasing significance for ecological, evolutionary, nutritional, and, ultimately, human health reasons. Among these elements, cadmium (Cd), a transition heavy metal, is of special concern due to its relatively high mobility in soils and soil/plant systems and its acute toxicity toward almost all forms of life, even at low concentrations. According to the World Health Organization, human exposure to cadmium is a major public health concern. In Europe, a significant part of the population is already overexposed to that metal via the consumption of potatoes and wheat-derived products. To limit Cd contamination in plants and food products, and enhance food safety, it is primordial to understand how Cd is transferred from the soil to the plant and what controls its bioavailability. Previous studies demonstrated that the mobility and bioavailability of Cd in the soil/plant system – thus its toxicity in this system – depends ultimately on its chemical speciation. In this context, the development of direct, experimental characterization of trace elements speciation by X-ray absorption spectroscopy has brought key information on heavy metal behavior in the critical zone and keep on unravelling what controls their bioavailability. However, applying this cutting-edge methodology to the experimental determination of Cd speciation in biological samples remains challenging: in most organisms, due to Cd acute toxicity, Cd concentration lies below the technical detection limit of most X-ray absorption beamlines. That is why, at the starting date of the SPECADIS project, Cd speciation determination had only been carried out on Cd-hyperaccumulating and/or Cd-tolerant systems.
The overarching goal of SPECADIS was to bypass these technical difficulties to measure Cd speciation in natural plant samples by developing an indirect tracer of Cd speciation relevant to the soil/plant context: Cd stable isotopes.
In addition, SPECADIS aimed at investigating the uptake mechanisms and fate of cadmium in plants, and bringing new constraints on Cd toxicity/detoxification in plants that feed the world.
The overarching goal of SPECADIS was to bypass these technical difficulties to measure Cd speciation in natural plant samples by developing an indirect tracer of Cd speciation relevant to the soil/plant context: Cd stable isotopes.
In addition, SPECADIS aimed at investigating the uptake mechanisms and fate of cadmium in plants, and bringing new constraints on Cd toxicity/detoxification in plants that feed the world.
First, we grew plants from the Solanum family – Solanum nigrum, also known as black nightshade, that can accumulate cadmium; and S. melongena, known as eggplant, that is not tolerant towards Cd – in a controlled growth chamber. Plants were cultivated on a standard clayey loam soil doped with cadmium: 4 different Cd concentrations were investigated (0, 25, 50 and 100 mg Cd per kg of soil). For each soil concentration, 7 pots of two seedlings were prepared for S. nigrum and S. melongena, to have enough material to perform the SPECADIS project tasks. (i) have replicates to statistically investigate the amount of Cd in the plant parts; (ii) have samples for X-ray absorption measurements and (iii) have samples for Cd isotope composition measurements.
In parallel, the fellow applied for beamtime at several synchrotron facilities: the Swiss Light Source in Switzerland (obtained 15 8-hours shifts), the Diamond Light Source in the U.K. (12 shifts) and the Soleil Synchrotron in France (15 shifts).
For the X-ray absorption spectroscopy measurements, the samples were prepared frozen in liquid nitrogen, and kept at a temperature of 80°C, to prevent changes in Cd speciation between harvest and measurements.
During the 3 synchrotron sessions, cadmium speciation was determined in plant parts from S. nigrum (roots, stem, young and old leaves) and S. melongena (roots, stem, fresh and dead leaves) grown on the 25, 50 and 100 mg.kg-1 Cd-doped soil and on the soil used for their culture. We found that all the cadmium in the soils (pre and post culture) is adsorbed on iron oxyhydroxides such as goethite. Upon roots uptake, Cd undergoes a major change in speciation. For both plants, we found that sulphur plays a major role: in roots from S. nigrum and S. melongena, Cd is mainly bound to organic, sulphur-bearing thiol ligands. By comparing speciation results in fresh and dead leaves, we hypothesized that Cd is actively stored bound to S ligands in the vacuoles. Our results also show that, in S. nigrum leaves, a part of Cd is bound to inorganic sulphur. The SPECADIS results deepened our understanding of Cd detoxification mechanisms in Solanaceae.
Last, the fellow developed and implemented the measurements of cadmium stable isotopes on the multi-collector Inductively coupled plasma mass spectrometer from the CEREGE host department, and she measured the Cd isotope compositions of the plant samples previously analysed by X-ray absorption spectroscopy. The results were compared to the Cd speciation in the plant parts, and the potential of Cd stable isotopes as tracers of Cd speciation in the soil/plant system was assessed. Cadmium isotope compositions in the plant parts also provided information on the translocation of Cd from the roots to the leaves.
The work performed in SPECADIS has led to 1 published journal article, and two manuscripts in preparation. The results were presented at 2 international conferences, as well as 1 public outreach event. So far, 1 article and 1 dataset were deposited in open access repositories.
In parallel, the fellow applied for beamtime at several synchrotron facilities: the Swiss Light Source in Switzerland (obtained 15 8-hours shifts), the Diamond Light Source in the U.K. (12 shifts) and the Soleil Synchrotron in France (15 shifts).
For the X-ray absorption spectroscopy measurements, the samples were prepared frozen in liquid nitrogen, and kept at a temperature of 80°C, to prevent changes in Cd speciation between harvest and measurements.
During the 3 synchrotron sessions, cadmium speciation was determined in plant parts from S. nigrum (roots, stem, young and old leaves) and S. melongena (roots, stem, fresh and dead leaves) grown on the 25, 50 and 100 mg.kg-1 Cd-doped soil and on the soil used for their culture. We found that all the cadmium in the soils (pre and post culture) is adsorbed on iron oxyhydroxides such as goethite. Upon roots uptake, Cd undergoes a major change in speciation. For both plants, we found that sulphur plays a major role: in roots from S. nigrum and S. melongena, Cd is mainly bound to organic, sulphur-bearing thiol ligands. By comparing speciation results in fresh and dead leaves, we hypothesized that Cd is actively stored bound to S ligands in the vacuoles. Our results also show that, in S. nigrum leaves, a part of Cd is bound to inorganic sulphur. The SPECADIS results deepened our understanding of Cd detoxification mechanisms in Solanaceae.
Last, the fellow developed and implemented the measurements of cadmium stable isotopes on the multi-collector Inductively coupled plasma mass spectrometer from the CEREGE host department, and she measured the Cd isotope compositions of the plant samples previously analysed by X-ray absorption spectroscopy. The results were compared to the Cd speciation in the plant parts, and the potential of Cd stable isotopes as tracers of Cd speciation in the soil/plant system was assessed. Cadmium isotope compositions in the plant parts also provided information on the translocation of Cd from the roots to the leaves.
The work performed in SPECADIS has led to 1 published journal article, and two manuscripts in preparation. The results were presented at 2 international conferences, as well as 1 public outreach event. So far, 1 article and 1 dataset were deposited in open access repositories.
The SPECADIS project has contributed to deepening our understanding of cadmium fate in plants from the Solanum family, by providing the first comprehensive Cd solid speciation dataset on separate plant parts – roots, stem, leaves – and soils. SPECADIS contributed to a better understanding of Cd detoxification mechanisms in this plant family, that includes several edible products – potatoes, bell peppers, tomatoes, eggplants… - which is of importance regarding food safety issues.
The SPECADIS project enhanced our understanding of the links between Cd speciation in the soil/plant system and Cd stable isotope composition and opened new applications of Cd isotope geochemistry to investigate changes of Cd atomic environment in plants, even at low concentrations.
The SPECADIS project has been a tremendous opportunity for the fellow to operate a thematical shift towards environmental sciences, learn new skills, collaborate with great scientists, and gain visibility and credibility. It has boosted her academic career, and offered her the possibility to become a permanent researcher working at host department CEREGE (European Centre for Research and Teaching in Environmental Geosciences, Aix-en-Provence, France).
She will build up on SPECADIS’ results and further her research on Cd in agricultural soil and food products, and will investigate the fate of Cd from phosphate fertilizers using SPECADIS innovative methodology.
The SPECADIS project enhanced our understanding of the links between Cd speciation in the soil/plant system and Cd stable isotope composition and opened new applications of Cd isotope geochemistry to investigate changes of Cd atomic environment in plants, even at low concentrations.
The SPECADIS project has been a tremendous opportunity for the fellow to operate a thematical shift towards environmental sciences, learn new skills, collaborate with great scientists, and gain visibility and credibility. It has boosted her academic career, and offered her the possibility to become a permanent researcher working at host department CEREGE (European Centre for Research and Teaching in Environmental Geosciences, Aix-en-Provence, France).
She will build up on SPECADIS’ results and further her research on Cd in agricultural soil and food products, and will investigate the fate of Cd from phosphate fertilizers using SPECADIS innovative methodology.