Plant Science & Landscape Architecture Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2797
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Item Kinetics of Tetrachloroethene-Respiring Dehalobacter and Dehalococcoides Strains and Their Effects on Competition for Growth Substrates(2010) Lai, Yenjung; Becker, Jennifer G; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE) are common groundwater contaminants. Reductive dechlorination of PCE and TCE at contaminated sites is commonly carried out by dehalorespiring bacteria that utilize these compounds as terminal electron acceptors, but often results in the accumulation of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC), rather than non-toxic ethene. This project focused on evaluating how interactions among dehalorespiring populations that may utilize the same electron acceptors, electron donors and/or carbon source may affect the extent of PCE dechlorination in situ. These interactions may be particularly important if both Dehalococcoides ethenogenes (Dhc. ethenogenes) and Dehalobacter restrictus (Dhb. restrictus) are present because these bacteria utilize the same electron donor (H2) and both respire PCE and TCE. However, unlike Dhc. ethenogenes, Dhb. restrictus cannot dechlorinate PCE beyond cDCE. Therefore, the outcome of the population interactions may determine the extent of detoxification achieved. Monod kinetic parameter estimates that describe chlorinated ethene and electron donor utilization by Dhc. ethenogenes and Dhb. restrictus at non-inhibitory substrate concentrations were obtained in batch assays. Substrate inhibition effects on both populations were also evaluated. Highly chlorinated ethenes negatively impacted dechlorination of the lesser chlorinated ethenes in both populations. In Dhc. ethenogenes, cometabolic transformation of VC was also inhibited by the presence of other chlorinated ethenes. PCE and TCE dechlorination by Dhb. restrictus was strongly inhibited by VC. The microbial interactions between Dhc. ethenogenes and Dhb. restrictus was investigated using reactors and mathematical models under engineered bioremediation and natural attenuation conditions. Under engineered bioremediation conditions, Dhc. ethenogenes became the dominant population, and the modeling predictions suggested that the inhibition of Dhb. restrictus by VC was a key factor in determining this outcome. Dechlorination rates by Dhb. restrictus appeared to be affected very little by low acetate concentrations under natural attenuation conditions, giving it an advantage over Dhc. ethenogenes, which requires relatively high acetate concentrations. This study highlighted that substrate interactions among dehalorespiring bacteria can influence their performance and contaminant fate under common bioremediation scenarios. A better understanding of the factors affecting the outcomes of these microbial interactions was achieved, which should aid in the design of successful bioremediation strategies.Item Microbial Ecology and Horticultural Sustainability of Organically and Conventionally Managed Apples(2008) Ottesen, Andrea; Walsh, Christopher S; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Objectives: Organically and conventionally managed apple trees (Malus domestica Borkh) were evaluated for three growing seasons (2005-2007) to examine the impact of organic and conventional pesticide applications on the microbial ecology of phyllosphere and soil microflora. An important objective was to establish if organic or conventional selection pressures contribute to an increased presence of enteric pathogens in phyllosphere microflora. The horticultural and economic sustainability of the organic crop was also compared to the conventional crop with regard to fruit yield and input costs. Methods: Microbial populations from phyllosphere and soil environments of apple trees were evaluated using clone libraries of 16S rRNA gene fragments. Clones were sequenced and software was used to assess diversity indices, identify shared similarities and compute statistical differences between communities. These measurements were subsequently used to examine treatment effects on the microbial libraries. Phyllosphere Results: Eight bacterial phyla and 14 classes were found in this environment. A statistically significant difference between organically and conventionally managed phyllosphere bacterial microbial communities was observed at four of six sampling time points. Unique phylotypes were found associated with each management treatment but no increased human health risk could be associated with either treatment with regard to enteric pathogens. Soil Results: Seventeen bacterial phyla spanning twenty-two classes, and two archaeal phyla spanning eight classes, were seen in the 16S rRNA gene libraries of organic and conventional soil samples. The organic and conventional soil libraries were statistically different from each other although the sampling depth was not sufficient to make definitive inference about this environment. Horticultural Results: Fruit yields from organically managed apple trees were from one half to one third of the yields from conventionally managed trees. Based on input costs, organic fruit was about twice as expensive to produce. Asian pears (Prunus serotina) were also included in this horticultural analysis and showed greater field tolerance as an organic specialty niche crop than apples.Item Efficiency and Ecological Risks of Reducing Soil pH during Thlaspi caerulescens Phytoextraction of Cadmium and Zinc(2004-11-29) Wang, Shengchun; Angle, Jay S; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The major aims of this research were to determine whether reducing soil pH can enhance phytoextraction and to examine the ecological risks of reducing pH. Two soils differing in Cd and Zn concentrations were used and adjusted to 5 or 6 different pH levels ranging from 7.27 to 4.74 and seeded with a hyperaccumulator of Cd and Zn, Thlaspi caerulescens. Plants were harvested after six months, the pH were restored to above 6.5, incubated for 6 months. Soils were analyzed for biological activities and microbial population changes after both pH adjustments. Reducing pH significantly (p=0.05) enhanced plant metal uptake. For the high metal soil, plant grew best at the lowest pH treatment (4.74) and the highest metal concentration was at the second lowest pH treatment (5.27). For the low metal soil, due to low pH induced Al and Mn toxicity, plant growth and metal uptake were highest at the intermediate pH level (6.07). Metal sequential extraction results further verified that reducing pH redistributed Cd and Zn among five fractions. The most soluble metal form (F1) was greatly increased. In addition, T. caerulescens was able to differentially utilize Cd in all 5 fractions while it could only access Zn from the F1 and F2 pools. Reducing soil pH significantly reduced a number of soil biological activities and shifted the community structure at different levels. Generally, soil biological activities were more sensitive than soil microbial populations to pH change. Good indicators of soil pH status were acid phosphatase activity, alkaline phosphatase activity, acid to alkaline phosphatase activity ratio, arylsulphatase, nitrification potential, soil microbial biomass C and N, and population of rhizobium. After raising pH to > 6.5, negatively impacted soil parameters were partially restored to original levels. Soil biological activities showed lower recovery than soil microbial populations. The threshold pHs were 6.1 and 5.3 for low and high metal soils, respectively. Above this value, most soil biological activities and all microbial populations returned to background levels within a short period.