Donna L. Jacob

CONFLICTING PROCESSES IN THE WETLAND PLANT RHIZOSPHERE: METAL RETENTION OR MOBILIZATION?

Increasingly wetlands are used for treatment of metal-contaminated water or as a cover over metal-enriched mine tailings. Natural wetlands may also be contaminated with metals from anthropogenic sources. While wetland conditions tend to be favorable for immobilization of metals, wetland plants could influence metal mobility through redox and pH processes in the rhizosphere. Our current knowledge of these processes is reviewed, focusing on the question of whether the advantages of growing wetland plants in metal-contaminated sediments outweigh the disadvantages. Wetland plants alter the redox conditions, pH and organic matter content of sediments and so affect the chemical speciation and mobility of metals. Metals may be mobilized or immobilized, depending on the actual combination of factors, and it is extremely difficult to predict which effects plants will actually have on metal mobility under a given set of conditions. However, while the effects of plants can extend several tens of centimeters into the sediments, there are no reports suggesting large-scale mobilization of metals by wetland plants.

UPTAKE AND TRANSLOCATION OF TI FROM NANOPARTICLES IN CROPS AND WETLAND PLANTS

Bioavailability of engineered metal nanoparticles affects uptake in plants, impacts on ecosystems, and phytoremediation. We studied uptake and translocation of Ti in plants when the main source of this metal was TiO2 nanoparticles. Two crops (Phaseolus vulgaris (bean) and Triticum aestivum (wheat)), a wetland species (Rumex crispus, curly dock), and the floating aquatic plant (Elodea canadensis, Canadian waterweed), were grown in nutrient solutions with TiO2 nanoparticles (0, 6, 18 mmol Ti L−1 for P. vulgaris, T. aestivum, and R. crispus; and 0 and 12 mmol Ti L−1 for E. canadensis). Also examined in E. canadensis was the influence of TiO2 nanoparticles upon the uptake of Fe, Mn, and Mg, and the influence of P on Ti uptake. For the rooted plants, exposure to TiO2 nanoparticles did not affect biomass production, but significantly increased root Ti sorption and uptake. R. crispus showed translocation of Ti into the shoots. E. canadensis also showed significant uptake of Ti, P in the nutrient solution significantly decreased Ti uptake, and the uptake patterns of Mn and Mg were altered. Ti from nano-Ti was bioavailable to plants, thus showing the potential for cycling in ecosystems and for phytoremediation, particularly where water is the main carrier.

Biogeochemistry of Metals in the Rhizosphere of Wetland Plants — An Explanation for “Innate” Metal Tolerance?

Publisher Summary Wetland plant rhizospheres are often aerobic and oxidized, while bulk soil is anaerobic and chemically reduced. As a result, metals tend to be immobile in the bulk soil but are mobilized by plant-induced changes in the rhizosphere. This leads to enrichment of metals near the roots and enhanced availability for uptake by plants. The apparent metal tolerance of wetland plants compared to dryland plants, without the development of separate metal-tolerant ecotypes, is because of the relatively high exposure of plant roots to metals under the soil conditions prevailing in wetlands. To live in the anaerobic soil conditions, wetland plants have developed root morphology different from that of dryland plants. Many species have porous roots for the supply of oxygen for root respiration, often forming a specialized tissue known as aerenchyma or air tissue. This supply of oxygen is thought to be more than sufficient for root respiration, and excess oxygen may leak into the rhizosphere.

Cadmium and associated metals in soils and sediments of wetlands across the Northern Plains, USA

Cadmium, present locally in naturally high concentrations in the Northern Plains of the United States, is of concern because of its toxicity, carcinogenic properties, and potential for trophic transfer. Reports of natural concentrations in soils are dominated by dryland soils with agricultural land uses, but much less is known about cadmium in wetlands. Four wetland categories - prairie potholes, shallow lakes, riparian wetlands, and river sediments - were sampled comprising more than 300 wetlands across four states, the majority in North Dakota. Cd, Zn, P, and other elements were analyzed by ICP-MS, in addition to pH and organic matter (as loss-on-ignition). The overall cadmium content was similar to the general concentrations in the areas soils, but distinct patterns occurred within categories. Cd in wetland soils is associated with underlying geology and hydrology, but also strongly with concentrations of P and Zn, suggesting a link with agricultural land use surrounding the wetlands.

Mine Area Remediation

The history of mining extends over thousands of years, but remediation of mine areas began only about a century ago. The relatively recent interest in remediation stems from the realization that healthy ecosystems are essential to human well-being. Mining typically has serious, negative effects on the ecology, at all levels of organization, from microbial to landscape. It may be desirable to restore the ecology and associated ecosystem services of mine areas to be similar to that of the original land prior to mining. However, this is typically not possible – even after the most successful remediation efforts, the structure of the substrate and the ecosystems supported by it are altered and biodiversity may be lower. In some cases it may be more strategic to remediate the mine waste by creating an alternative ecosystem. For example, under comparable conditions on mine wastes, wetlands often support greater biodiversity than drylands. Wetlands also tend to provide better stabilization of mine wastes than drylands and may therefore provide good alternatives for remediation, even if no wetlands existed before mining commenced. Sometimes, remediation of mine areas also provides an opportunity to compensate for ecological degradation elsewhere by providing newly created habitat.

Phyto (In)Stabilization of Elements

The effects of plants (corn, soybean, and sunflower) and fertilizer on mobility of more than 60 elements were assessed in a greenhouse experiment. Unplanted columns with the same soil served as controls. Half the columns received fertilizer and all columns were watered at the same rate. At the end of the experiment, the columns were watered to mimic a rainstorm event such that water drained from the bases of the columns, which was collected and analyzed for element content. Soil from between the roots of the plants was also collected and the water-extractable fraction determined. It was expected that (1) more mobile elements, as measured by water extraction, would be leached from the soils at a higher rate compared to less mobile elements, (2) plants would immobilize most elements, but that some would be immobilized, and (3) that this would depend on plant species. The results led to the following conclusions: plants cause metal mobility to vary over a wide range for a specific soil and do mobilize some elements (e.g., Th) while immobilizing others (e.g., U). The effects depended on plant species for some elements. Water-extractable fractions of elements do not predict mobility.

Interdependence of Genotype and Growing Site on Seed Mineral Compositions in Common Bean

Essential minerals are considered as key determinants of optimum health and nutritive quality of common bean seed. This study aimed to identify genetically stable essential minerals in common bean. Eleven diverse common bean genotypes were grown in three distinct growing environments and 17 essential minerals were analyzed by Inductively Coupled Plasma-Optical Emission Spectroscopy. Genetic control of mineral composition in common bean seed was demonstrated by large (p<0.01) genotypic differences in Ca and Sr contents and moderate genotypic difference was observed in Fe content. Significant influence of genotype and environments (G×E) interaction was observed in the content of all minerals. The ratios between genetic and environmental variances and between genetic and G×E variances indicated the greater influence and stability of genetic factor on the concentration of Ca and Sr in common bean seed. Significant positive correlations among important minerals such as Zn with S, P, Fe and Na and Cu with K, Mg, Ni, P were identified. The stability of genetic effects on Ca and Sr concentration in common bean has been identified in this study. Calcium is one of the most important minerals which regulates many cellular processes and has important structural roles in living organisms. Further studies to characterize Ca physiology in common bean may identify genetic or biochemical markers to expedite breeding common bean with enhanced Ca concentration.