Analysis of microbial biofilm community composition within constructed wetlands
Faulwetter, Jennifer Lynn
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Constructed wetlands (CWs) are ecologically-based water treatment systems that provide cost-effective amelioration of waterborne pollutants. Fundamental understanding of removal mechanisms, especially microbial processes, limits greater usage of constructed wetlands as a wastewater treatment system. The influence of plant species selection, season, and organic load rate on pollutant removal was previously linked to the redox condition of the sub-surface wetland environment. The goal of this research was to determine which of these environmental variables (including spatial location within the CW) influenced the dominant microbial populations and/or the activity of various sub-populations. Once identified, a constructed wetland might be optimized for growth of microorganisms involved in removal of a specific pollutant. To assess environmental factors, microbial population samples were taken in six locations (effluent, 3 root and 2 gravel areas) within replicate unplanted microcosms and wetland microcosms planted with Deschampsia cespitosa or Leymus cinereus during the summer (24°C) and winter (4°C) seasons. Microcosms were fed a synthetic domestic wastewater in 20-day batches for at least 12 months prior to sampling. The most recent techniques in molecular biology including denaturing gradient gel electrophoresis (DGGE) and quantitative PCR were utilized and included treatment with and without propidium monoazide (PMA) to distinguish between "live" and "dead" microbial communities. Primer sets targeted the entire bacterial community (16S rDNA) and two functional groups, nitrifying bacteria (amoA gene) and sulfate reducing bacteria (dsrB gene). Results indicated that overall microbial community structure (16S rDNA) was affected by general location within the microcosm (effluent, root, gravel) as well the plant species present. Specific microbial groups appeared to be affected differently with relative gene quantities of sulfate reducing bacteria and nitrifying bacteria being influenced by a combined effect of plant species and season. For dsrB, D. cespitosa had the lowest relative gene quantities overall. Both genes were more abundant in the summer season, indicating seasonal importance. Location within the microcosms was also important, with anoxic environments (column bottom) being more important for dsrB presence and a diverse population of cultivated sulfate reducers. The roots were an important location for both microbial diversity and activity for all genes investigated.