Theses and Dissertations at Montana State University (MSU)
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Item Response of soil bacterial communities to cropping systems, temporal changes, and environmental conditions in the northern Great Plains(Montana State University - Bozeman, College of Agriculture, 2021) Ouverson, Laura Tindall; Chairperson, Graduate Committee: Fabian D. MenalledSoil bacterial communities are essential components of the soil ecosystem that support crop production. However, agriculture in semiarid drylands and their associated soil bacterial communities face increasingly warmer and drier conditions due to climate change. Two complementary studies were conducted to assess the response of soil bacterial communities to cropping systems, temporal changes, and soil temperature and moisture conditions in semiarid, dryland agricultural systems of the Northern Great Plains. The first study focused on soil bacterial community response to crop phase in contrasting cropping systems (chemical inputs and no-till, USDA-certified organic tilled, and USDA-certified organic sheep grazed) over a growing season. Organic grazed management supported more diverse bacterial communities than chemical no-till, though diversity in all systems decreased over the growing season. Organic grazed bacterial communities were distinct from those in the organic tilled and chemical no-till systems. An interaction between cropping system and crop phase affected community dissimilarity, indicating that overarching management systems and environmental conditions are influential on soil bacterial communities. The second study evaluated soil bacterial communities in a winter wheat - cover crop or fallow rotation. Observations were conducted in the summer fallow and two cover crop mixtures differing by species composition and phenologies, terminated by three different methods (chemical, grazing, or haying), and subjected to either induced warmer/drier or ambient soil conditions. Only the presence and composition of cover crops affected bacterial community dissimilarity, where mid-season soil bacterial communities were distinct from early season and fallow communities. Bacterial communities responded to an interaction between the presence and composition of cover crops and environmental conditions, but not termination. No treatment effects were observed in bacterial communities in 2019, which could be attributed to above average rainfall. The results of these studies suggest cover crop mixtures including species tolerant to warmer and drier conditions can foster diverse soil bacterial communities compared to fallow soils. Overall, these studies contribute to a better understanding of how soil bacterial communities respond to soil health building practices in the Northern Great Plains. Cropping systems can foster unique soil bacterial communities, but these effects may be moderated by environmental and temporal conditions.Item Introducing the ArsR regulated arsenic stimulon(Montana State University - Bozeman, College of Agriculture, 2017) Saley, Tara Carolyne; Chairperson, Graduate Committee: Timothy McDermottThe United States EPA ranks arsenic as the number one environmental toxin. Since microorganisms are significant drivers of arsenic toxicity and mobility in nature, it is important to understand how microbes detect and react to arsenic. The microbial arsenic resistance operon (ars) is critical for sensing arsenic in the environment and controlling the cellular response to this toxin. The ars operon is minimally comprised of arsRBC, which codes for an ArsR transcriptional repressor, arsenite effluxer, and an arsenate reductase, respectively, with the operon negatively regulated by the transcriptional repressor, ArsR. Our model organism Agrobacterium tumefaciens 5A carries two ars operons, with each containing two arsR genes. We conducted an RNASeq study to examine the regulatory roles of the encoded four ArsR regulatory proteins as a function of +/- arsenite. We report that the regulatory influence of the ArsR proteins extends well beyond the ars operon, with both activation and repression effects. In addition to the expected arsenic resistance response, many cellular functions were impacted, including: phosphate acquisition/metabolism, sugar transport, chemotaxis, copper tolerance, and iron homeostasis. Each of the ArsR proteins uniquely influenced different sets of genes and an arsR regulatory hierarchy was observed, wherein ArsR1 is auto regulatory and negatively regulates arsR4, ArsR4 activates arsR2, and ArsR2 negatively regulates arsR3. ArsR3 is the least active with respect to number of genes regulated. To summarize, this study provides a more complete understanding of how microbial gene expression and biogeochemical cycling may be influenced by arsenic in the environment.Item Bacterioplakton dynamics in stratified lakes of the Taylor Valley, Antarctica during the transition to polar night(Montana State University - Bozeman, College of Agriculture, 2010) Vick, Trista Juliana; Chairperson, Graduate Committee: John C. Priscu.Limnological research on the lakes of the McMurdo Dry Valleys (MCM), Antarctica, is typically carried out during the austral spring-summer (October January) when logistical support is readily available; the current study marks the first sampling effort during the summer-fall transition (January-April). Sampling during the darkness of winter is logistically difficult and expensive, and my study is an important step towards understanding the year-round ecology of the dry valley lakes. Bacterial productivity, measured as protein synthesis and DNA replication, and bacterial cell numbers were measured 10-12 times between October 2007 and April 2008 in Lakes Fryxell (FRX) and the east and west lobes of Lake Bonney (ELB and WLB). Lake Fryxell was the most productive (bacterial) lake on average by an order of magnitude (average = 1.24 mg C m -²d -¹; range = 0.00 to 3.29 mg C m -²d -¹), and also contained the greatest bacterial biomass (~10 ⁶ cells ml -¹) by 1 to 3 orders of magnitude. If bacterial production were directly linked to organic carbon supplied by photosynthetic primary production, a decrease in bacterial production would be expected during the sunset; however, no statistically significant change in bacterial production (a=0.05) was observed during the summer-fall transition. A distinct decoupling of bacterial protein production and DNA replication was detected in FRX and ELB of the lakes as the season progressed, and was present in WLB throughout the season, indicating either a shift towards a lower growth-rate in response to decreasing light or nutrient supply, or a mechanism for dealing with the perennially low temperatures, low light, and nutrient poor conditions in the lakes. Overall, it appears that bacterial communities remain active during the darkness of winter, when the lakes enter a period of "net heterotrophy", which cannot be sustained unless the carbon balance of the TV lakes is reset by climatic events.