Theses and Dissertations

Issuing Body

Mississippi State University


Williams, Mark A.

Committee Member

Cox, Michael S.

Committee Member

Peterson, Daniel G.

Committee Member

Schultz, Tor P.

Committee Member

Lu, Shi-En

Date of Degree


Document Type

Dissertation - Open Access



Degree Name

Doctor of Philosophy


College of Agriculture and Life Sciences


Department of Plant and Soil Sciences


Understanding the response of soil microbial communities to various environmental stresses is of current interest, because of their pivotal role in nutrient cycling, soil organic matter mineralization and influence on plant growth. Determining the affect of several biotic and abiotic factors on soil microbial communities is the overall objective of the study. The specific goals are to determine 1) the response of microbial communities to water deficit in soil and 2) how the presence of a rich biotic community determines the direction of microbial community development in cultures. Both goals are novel and unique contributions to understanding microbial ecology in soil. Dynamics in water potentials due to drying and rewetting of soil impose significant physiological challenges to soil microorganisms. To cope with these fluctuations, many microorganisms alter the chemistry and concentration of their cytoplasmic contents. The aim of this research is to understand how the microbial biomass and their cytoplasm change in response to water potential deficits under in situ soil conditions. To address this objective we characterized intracellular and extracellular metabolites in moist, dry and salt stressed soils. Our results provided the first direct evidence that microbial communities in soil in situ utilize sugars and sugar alcohols to cope with low water potential. While the cultivation and isolation of microorganisms is essential to completely explore their physiology and ecology, 99% of soil microbes resist growing in cultures. Presence of very unnatural conditions in the culture plates was considered as main reason for low cultivability. Thus, a culture-based study was conducted whereby microorganisms were grown in association with their native habitat with an objective of mimicking native conditions to promote the growth of previously uncultivated microorganisms. Moreover, the importance of biotic communities (microbe-microbe) and abiotic soil effects were assessed on bacterial growth. Our results strongly indicate that the presence of living microbial community in the vicinity of the target culture resulted in the cultivation of novel members of rare bacterial taxa from phyla Verrucomicrobia, Bacteroidetes, Proteobacteria, and Planctomycetes. These results emphasize the need to develop new culturing methods to tap the hidden microbial potential for emerging anthropogenic needs.