Raina Margaret Maier
Publications
The uptake of arsenic by plants from contaminated soils presents a health hazard that may affect home gardeners neighboring contaminated environments. A controlled greenhouse study was conducted in parallel with a co-created citizen science program (home garden experiment) to characterize the uptake of arsenic by common homegrown vegetables near the Iron King Mine and Humboldt Smelter Superfund site in southern Arizona. The greenhouse and home garden arsenic soil concentrations varied considerably, ranging from 2.35 to 533 mg kg(-1). In the greenhouse experiment four vegetables were grown in three different soil treatments and in the home garden experiment a total of 63 home garden produce samples were obtained from 19 properties neighboring the site. All vegetables accumulated arsenic in both the greenhouse and home garden experiments, ranging from 0.01 to 23.0 mg kg(-1) dry weight. Bioconcentration factors were determined and show that arsenic uptake decreased in the order: Asteraceae>Brassicaceae>Amaranthaceae>Cucurbitaceae>Liliaceae>Solanaceae>Fabaceae. Certain members of the Asteraceae and Brassicaceae plant families have been previously identified as hyperaccumulator plants, and it can be inferred that members of these families have genetic and physiological capacity to accumulate, translocate, and resist high amounts of metals. Additionally, a significant linear correlation was observed between the amount of arsenic that accumulated in the edible portion of the plant and the arsenic soil concentration for the Asteraceae, Brassicaceae, Amaranthaceae, and Fabaceae families. The results suggest that home gardeners neighboring mining operations or mine tailings with elevated arsenic levels should be made aware that arsenic can accumulate considerably in certain vegetables, and in particular, it is recommended that gardeners limit consumption of vegetables from the Asteraceae and Brassicaceae plant families.
Abstract:
Successful remediation of soils co-contaminated with organics and metals may require a combination of technologies. This research addresses the organic component within co-contaminated sites. It is well known that metal contaminants in soil can partially or completely inhibit normal helerotrophic microbial activity and hence prevent in situ biodegradation of organics. Previous work has shown that a rhamnolipid biosurfactant can complex metals such as lead and cadmium. It has also been demonstrated, in pure culture, that rhamnolipid can mitigate metal inhibition during the degradation of naphthalene. The goal of this study was to investigate whether rhamnolipid could reduce the toxicity of a model metal, cadmium, to indigenous soil populations in two different soils, Brazito and Gila, during the mineralization of phenanthrene. Results show that cadmium inhibited phenanthrene mineralization in both soils at bioavailable cadmium concentrations as low as 27 μM. This inhibition was reduced by the addition of rhamnolipid. Since rhamnolipid is degraded by soil populations, a rhamnolipid pulsing strategy was used to maintain a constant level of rhamnolipid in the system. Using this strategy, phenanthrene mineralization levels comparable to the control (0 mM Cd/0 mM rhamnolipid) were achieved in the presence of toxic cadmium concentrations. This research demonstrates that pulsed application of rhamnolipid may allow bioremedialion of organic contaminants in sites that are co-contaminated with organics and metals.
Compost-assisted phytostabilization has recently emerged as a robust alternative for reclamation of metalliferous mine tailings. Previous studies suggest that root-associated microbes may be important for facilitating plant establishment on the tailings, yet little is known about the long-term dynamics of microbial communities during reclamation. A mechanistic understanding of microbial community dynamics in tailings ecosystems undergoing remediation is critical because these dynamics profoundly influence both the biogeochemical weathering of tailings and the sustainability of a plant cover. Here we monitor the dynamics of soil microbial communities (i.e. bacteria, fungi, archaea) during a 12-month mesocosm study that included 4 treatments: 2 unplanted controls (unamended and compost-amended tailings) and 2 compost-amended seeded tailings treatments. Bacterial, fungal and archaeal communities responded distinctively to the revegetation process and concurrent changes in environmental conditions and pore water chemistry. Compost addition significantly increased microbial diversity and had an immediate and relatively long-lasting buffering-effect on pH, allowing plants to germinate and thrive during the early stages of the experiment. However, the compost buffering capacity diminished after six months and acidification took over as the major factor affecting plant survival and microbial community structure. Immediate changes in bacterial communities were observed following plant establishment, whereas fungal communities showed a delayed response that apparently correlated with the pH decline. Fluctuations in cobalt pore water concentrations, in particular, had a significant effect on the structure of all three microbial groups, which may be linked to the role of cobalt in metal detoxification pathways. The present study represents, to our knowledge, the first documentation of the dynamics of the three major microbial groups during revegetation of compost-amended, metalliferous mine tailings. (C) 2014 Elsevier B.V. All rights reserved.
Rhamnolipids are amphiphilic glycolipids biosynthesized by bacteria that, due to their low toxicity and biodegradability, are potential replacements for synthetic surfactants. The previously limited access to pure materials at the gram scale has hindered extensive characterization of rhamnolipid structure-performance behavior. Here, we present an efficient and versatile synthetic methodology from which four diastereomers of the most common monorhamnolipid, α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate, are prepared and subsequently characterized. Exploration of their behavior at the air-water interface is reported and analyzed in terms of the absolute configuration of the lipid tail carbinols at pH 4.0 and 8.0. All diastereomers exhibit a minimum surface tension of about 28 mN/m without a significant difference between the protonated (nonionic) or deprotonated (anionic) states. At pH 4.0 (nonionic), all diastereomers have a critical micelle concentration (CMC) in the micromolar range. At pH 8.0 (anionic), CMC values for the (R,R), (S,S), and (S,R) diastereomers are approximately an order of magnitude higher than in their nonionic states, whereas the (R,S) diastereomer exhibits a CMC about five times larger.
The evolution of solution aggregates of the anionic form of the native monorhamnolipid (mRL) mixture produced by Pseudomonas aeruginosa ATCC 9027 is explored at pH 8.0 using both experimental and computational approaches. Experiments utilizing surface tension measurements, dynamic light scattering, and both steady-state and time-resolved fluorescence spectroscopy reveal solution aggregation properties. All-atom molecular dynamics simulations on self-assemblies of the most abundant monorhamnolipid molecule, l-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10), in its anionic state explore the formation of aggregates and the role of hydrogen bonding, substantiating the experimental results. At pH 8.0, at concentrations above the critical aggregation concentration of 201 μM but below ∼7.5 mM, small premicelles exist in solution; above ∼7.5 mM, micelles with hydrodynamic radii of ∼2.5 nm dominate, although two discrete populations of larger lamellar aggregates (hydrodynamic radii of ∼10 and 90 nm) are also present in solution in much smaller number densities. The critical aggregation number for the micelles is determined to be ∼26 monomers/micelle using fluorescence quenching measurements, with micelles gradually increasing in size with monorhamnolipid concentration. Molecular dynamics simulations on systems with between 10 and 100 molecules of Rha-C10-C10 indicate the presence of stable premicelles of seven monomers with the most prevalent micelle being ∼25 monomers and relatively spherical. A range of slightly larger micelles of comparable stability can also exist that become increasing elliptical with increasing monomer number. Intermolecular hydrogen bonding is shown to play a significant role in stabilization of these aggregates. In total, the computational results are in excellent agreement with the experimental results.