Effects of Quantum Dot on Microbial CommunitiesEPA Grant Number: R833858
Title: Effects of Quantum Dot on Microbial Communities
Investigators: Alvarez, Pedro J. , Colvin, Vicki L. , Mahendra, Shaily
Institution: Rice University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 2008 through September 30, 2011
Project Amount: $399,889
RFA: Exploratory Research: Nanotechnology Research Grants Investigating Fate, Transport, Transformation, and Exposure of Engineered Nanomaterials: A Joint Research Solicitation - EPA, NSF, & DOE (2007) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Quantum dots (QDs) are being increasingly used in a wide variety of commercial products and applications such as biomedical imaging, targeted gene and drug delivery, solid state lighting, and solar cells. QDs are reported to be safe for their intended use, but they contain hazardous constituents that could be released as they weather in the environment, and their behavior and potential impact to human and ecosystem health as are not yet known. This project will characterize weathering of QDs and its effects on indigenous bacteria, which serve as models of cell toxicity and indicators of ecosystem health. Specific tasks include (1) quantification of the heavy metal release (rate and extent) during weathering of QDs, (2) characterization of QD-bacterial interactions, and (3) evaluation of the impact of QDs on the composition and selected environmental services of microbial communities.
QD particles consist of metallic core/shell capped with organic functional groups. We hypothesize that the toxic effects of QDs on microorganisms are primarily due to the heavy metal, and weathering will increase the toxicity of the QDs. Further, QD pollution will disturb the composition and function of microbial communities. For example, progressive weathering and heavy metal release of QDs will select for bacteria carrying metal-resistance, and fortuitously, antibiotic-resistance genes, because these genes are generally co-harbored in bacteria. Similarly, QDs will impact viability and enzymatic activity of bacteria catalyzing critical steps in global biogeochemical cycles, such as the nitrogen cycle.
To elucidate the toxic effects caused by QDs and their weathering products, we will first measure abiotic weathering rates of QDs of various compositions and surface coatings under conditions likely to be encountered in the environment. Data will be fitted to empirical dissolution models. Toxicity caused to bacterial populations at various stages of weathering will be investigated using respirometry and cell viability assays, and the correlation of toxic effects to bioavailable heavy metal concentrations will be investigated. In the second task, bioaccumulation and effects of QDs and their weathering products will be studied using physiologically diverse bacteria. Microbial impacts will be quantified as a function of accumulation and distribution of QDs, and cell growth and enzymatic activities. Finally, the impact of QD weathering products on environmentally-relevant microbial communities will be investigated. Molecular tools such as quantitative PCR and functional microarrays will be used to characterize changes in microbial communities. Gene expression will be studied in the presence of QDs, with focus on genes for involved in nitrification, denitrification, N2 fixation, heavy metal resistance, and antibiotic resistance.
Although QDs currently being synthesized and used are considered stable while capped, extreme conditions sometimes encountered even in the intended applications can cause weathering of QDs and release of toxic heavy metals. Microorganisms are at the foundation of all ecosystems, and play key roles in global biogeochemical cycles. Consequently, understanding their interactions with engineered nanomaterials such as QDs is important to enable their sustainable use in medical, electronic, and environmental applications. At the conclusion of this project, we will report (1) a model for QD weathering which can provide a measure of the timescale of heavy metal release, and the persistence of QDs in the environment, (2) a definition of the equivalent dose between QDs and bulk heavy metals with respect to their effects on bacteria, and (3) insights into the effects of chronic QD exposure on microbial community function, in particular, antibiotic-resistance propagation and the nitrogen cycling processes. This will benefit risk assessment and management efforts, and may contribute to the development of environmentally safe and durable QDs.