Progress 10/01/99 to 09/30/04
Outputs
Heavy metals contained in biosolids and urban waste can be toxic to both plants and soil microorganisms and may also be taken up by plants leading to food chain transfer. Among the metals that are of major concern is cadmium, which has no biological function and which is extremely toxic at low concentrations. In research conducted here, the effects of cadmium were examined on soil microbial communities and on gene expression in the model bacterium, Escherichia coli. Exposure of soil microbial communities to a range of cadmium concentrations from 1 to 50 ppm resulted in selective toxicity to different microbial species and reduced the functional redundancy of the microbial community with respect to microorganisms that are capable of growth on different carbon substrates. This decrease in functional redundancy is predicted to result in decreased reliability in the degradation of certain carbon substrates that may accumulate in soils to higher levels than would other
occur in uncontaminated soils. The impact of cadmium was significant even at 1 ppm with further reductions in microbial diversity at each incremental step increase in cadmium. To further evaluate the toxicity of cadmium to microorganisms, cells of E. coli were exposed to 1 ppm cadmium and were analyzed for dynamic changes in their gene expression patterns using a DNA microarray that permited monitoring of 4290 annotated open reading frames and intergenic regions. Results of this study confirmed that Cd toxicity caused profound changes in gene expression in which several stress response systems were induced simultaneously. Changes in gene expression were highly dynamic as the bacterial cells responded to ensuing toxicity which was manifested by a shift to anaerobic metabolism, up-regulation of genes related to energy conservation, and disruption in the synthesis of r-proteins and zinc-binding proteins. The results showed that exposure to Cd caused an arrest in protein biosynthesis, a
shift to anaerobic metabolism, and lowered energy production. Mistaken assembly of r-proteins and Cd-replacement in zinc-binding/cysteine-rich proteins may account for some of the damage caused by Cd. Induction of a complex network of regulatory systems was observed, including those that function for DNA repair, heat shock, oxidative stress, cold shock, osmotic stress, taxis, acid, antibiotic, and Nudix, as well as efflux systems for heavy metals that have previously been shown to facilitate resistance to Cd. Results from this study may be valuable for guiding future research on mechanisms of Cd toxicity, and for stimulating comparative studies on gene expression in other organisms in which Cd causes mutations and cancer.
Impacts
This research has provided fundamental information on the impact of municipal solid waste residues that contain heavy metals on microbial community diversity and function. Detailed studies on the mechanisms of cadmium toxicity revealed global changes in gene expression patterns indicative of direct damage to DNA and the cellular machinery for synthesis of proteins.
Publications
- No publications reported this period
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Progress 01/01/03 to 12/31/03
Outputs
Greenhouse experiments examining uptake and bioavailability of metals from biosolids were completed in 2001 and 2002. Organic acid production by plants and microorganisms was quantified in relation to plant growth stage and the type of biosolid amendment that was added to the soil. The results have been analyzed and journal articles have been submitted for publication. During 2003, we further examined the impact of cadmium, a common metal contaminant of biosolids and other municipal and industrial wastes, as a toxicant to soil microorganisms using Escherichia coli as a model bacterium. Experiments were conducted using a phenotype array to examine changes in functional gene expression. We also completed analysis of the impacts of cadmium on microbial functional redundancy in soils, in which our results show significant impacts of cadmium on microbial diversity at concentrations as low as 1 ppm Cd.
Impacts
This research is providing fundamental information on the impact of municipal solid waste residues that contain heavy metals on microbial community diversity and function.
Publications
- Wang, A., J. Chen, and D.E. Crowley. 2004. Changes in metabolic and structural diversity of a soil bacterial community in response to cadmium toxicity. Biol Fert. Soils. 39
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Progress 01/01/02 to 12/31/02
Outputs
Greenhouse experiments examining uptake and bioavailability of metals from biosolids in progress last year were completed. Organic acid production by plants and microorganisms was quantified in sand media in the presence and absence of corn plants (Zea mays L.) that were grown to maturity. Plant growth stage had no effect on the relative proportions of the organic acids in the organic acid mixtures, but did influence the total quantities of the organic acids that were recovered. In contrast, the source of biosolids (MWRDGC 1975 and 1999, Los Angeles, Riverside, and Milorganite) did not have a significant effect on either the quantity or composition of the organic acids in any of the media. Microbial fermentation products including, lactic, acetic, and butyric acids were predominant in the solutions recovered from the planted media and collectively accounted for 0.65 - 0.75 of the COO- mole fraction. The predominance of organic acids that are microbial fermentation
products suggests that the carbon contained in the plant root exudates and the biosolid amendments was transformed into a mixture of various fermentation products that accumulated in the rhizosphere solution and sand medium as a result of microbial growth and activity. Synthetic root exudates/organic acid mixtures were then used as soil extractants to estimate metal bioavailability to plants. Mathematical models for estimating metal bioavailability to plants from biosolids were validated using data from several independent data sets.
Impacts
This research demonstrates that root exudates and carbon provided by biosolids are rapidly converted to organic acids that accumulate in soils. These organic acids influence trace metal bioavailability and subsequent uptake by plants. Previously methods for predicting metal bioavailability to plants have been unsatisfactory. Methods developed here will improve prediction of metal bioavailability to plants from biosolids.
Publications
- Crowley, D.E. and S. Alvey. 2002. Influence of pH on soil microbial processes. In: Z. Rengel (ed) Handbook of Plant Growth. Marcel Dekker, Inc. 351-382.
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Progress 01/01/01 to 12/31/01
Outputs
Root exudates may influence the availability of metals in biosolid waste by a variety of mechanisms. In this research, a synthetic root exudates mixture was developed as a soil extractant for use in predicting the bioavailability of metals to plants grown in biosolids amended soils. A series of greenhouse experiments were conducted to evaluate the effect of biosolids on the composition of organic acids contained in root exudates from different plants. Lactate, butyrate, and acetate were identified as the predominant organic acids, indicating that microorganisms converted much of the available to carbon to these fermentation products that accumulated in the soil solution. When used as a soil extractant, these organic acids extracted heavy metals (Cu, Ni, Cr, Zn, Cd, Pb) in similar quantities to those that accumulated in the plant tissues. These data suggest that synthetic organic acid mixtures may be used as soil extractants to semiquantitatively predict heavy metal
bioavailability and plant tissue loading in different soils.
Impacts
This research revealed that organic acids that are produced in biosolid amended soils have an important role in mobilizing metals for plant uptake. These effects appear to be uniform for different biosolids. The information will be useful in developing a new soil extraction procedure for predicting metal uptake by plants in biosolid amended soils.
Publications
- No publications reported this period
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Progress 01/02/00 to 12/31/00
Outputs
A greenhouse experiment was conducted to evaluate the effect of biosolids on organic acids produced by microorganisms in the plant rhizosphere, and their effects on the bioavailability of heavy metals (Cu, Ni, Cr, Zn, Cd, Pb). Corn seedlings (4-5 cm) were grown in sand-filled pots amended with five biosolids from various sources at the rate of 20 M g per hectare. The plants were harvested at 4-, 8- and 12 week after planting and samples were analyzed for metals, and organic acids contained in the root exudates were characterized using various analytical techniques. Metal uptake by corn was increased significantly in media amended with biosolids. The most commonly found organic acids, were lactic, acetic, butyric and oxalic acids. The organic acids composition of root exudates of various biosolids, however, did not vary significantly. The results suggest that biosolids from different sources have similar effects on organic acid composition of the rhizosphere soil
solution.
Impacts
This research revealed that organic acids that are produced in biosolid amended soils have an important role in mobilizing metals for plant uptake. These effects appear to be uniform for different biosolids. The information will be useful in developing a new soil extraction procedure for predicting metal uptake by plants in biosolid amended soils.
Publications
- No publications reported this period
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