|Lerch, Robert - Bob|
Submitted to: American Chemical Society Symposium Series
Publication Type: Book / Chapter
Publication Acceptance Date: 7/1/2011
Publication Date: 12/21/2011
Citation: Lin, C., Thompson, B.M., Hsieh, H., Lerch, R.N. 2011. Introduction of atrazine degrader to enhance rhizodegradation of atrazine. In: Goh, K., Bret, B., Potter, T.L., Gan, J, editors. Pesticide Mitigation Strategies for Surface Water Quality. Washington, DC: American Chemical Society. p. 139-154. Interpretive Summary: The corn herbicide atrazine has been widely used for weed control in U.S. corn production for decades. Public health and ecological concerns have been raised because of contamination of surface and ground water by atrazine and its breakdown products, which may be toxic to humans and aquatic life. Bioremediation is a strategy for cleaning up contaminated soils using micro-organisms, fungi, green plants or their enzymes. In an effort to achieve complete atrazine degradation in soil, ARS and University of Missouri researchers have begun to investigate the potential of adding a bacterium, Pseudomonas sp. strain ADP, to soils to enhance atrazine degradation. This bacterium contains a series of genes known to be responsible for complete degradation of atrazine. In previous research, we developed methods to extract and quanitfy these genes from soil. A key challenge when introducing bacteria into contaminated soil is the need to determine if the added bacteria (and their genes) are persisting long enough to degrade the contaminant. In this research, we tested the ability of a switchgrass rhizosphere (root zone soil) to sustain one of the degrading genes, atzA, in soil compared to bulk soil with no plants. The results showed that switchgrass rhizospheres sustained a higher number of degrading genes compared to the control for about 3 weeks; the numbers were similar thereafter. When the degrading bacteria was added to either treatment, it degraded >50 percent of the atrazine to carbon dioxide within seven days. The sustained gene numbers and high degradation rates observed indicated that the Pseudomonas sp. strain ADP bacteria would be an effective addition to grass buffers for enhancing their degradation of atrazine and reducing stream and ground water contamination. The approach used in this research will benefit environmental scientists as this newly developed bioremediation technique will result in decreased atrazine contamination of water resources and enhance the efficacy of existing grass buffers and waterways.
Technical Abstract: Introducing atrazine (ATR) degraders into riparian vegetative buffer strips (VBS) can be a promising bioremediation approach to accelerate the degradation of ATR and its degradation products deposited into VBS by surface runoff. A growth chamber study was conducted to investigated the synergistic effect of introducing ATR degrader Pseudomonas sp. ADP into switchgrass (Panicum virgatum) rhizospheres on ATR degradation. The results suggested that the introduction of the Pseudomonas sp. ADP into rhizospheres rapidly enhanced the rates of ATR degradation. More than 99% of applied 14C-ATR was degraded within first 72 hours of inoculation and 54.5 percent of applied 14C-ATR was mineralized to CO2. Hydroxylated metabolites, including hydroxyatrazine and desethylhydroxyatrazine, were the major degradation products in the inoculated treatments, while N-dealkylated metabolites (desethylatrazine) were the major degradation products in the un-inoculated treatments. Switchgrass rhizospheres sustained the copy number of atzA at higher levels than the control (bulk soil). In the presence of switchgrass, atzA copy number was stimulated for the first two weeks, but steadily decreased until leveling out from days 24-37. The lack of complete mineralization may be attributed to the loss of atzA gene copy number over time which signified a loss of ATR-degrading potential in the rhizosphere. The addition of ATR-degrading bacteria into switchgrass VBS has the potential to sustain enhanced degradation that can effectively remove entrapped ATR, preventing contamination of surface and ground waters.