Structure and function of microbial communities in constructed wetlands - influence of environmental parameters and pesticides on denitrifying bacteria

Abstract: This thesis addresses the interactions and relationships between natural aquatic bacterial communities, environmental parameters, anthropogenic chemicals and the denitrification pathway in the habitat of agricultural constructed wetlands. The main aim was to gain fundamental knowledge of the drivers behind the processes of the denitrification (i.e. nitrogen removal) in constructed wetlands, hence, the structure and function of the denitrifying bacterial community as efficient nitrogen removal in wetlands will decrease the risk of eutrophication of freshwaters and oceans. Programmes for restoring and recreating wetlands in agricultural areas have been initiated throughout the world. Aquatic environments in these areas are also exposed to pollution from e.g. pesticides, in fact, wetland are also constructed with the purpose of reducing transport of pesticides. However, little is known whether the wetland may fulfil both purposes simultaneously. Hence, may constructed wetlands maintain a high denitrification efficiency even during pesticide exposure? Both structure and function of the eubacterial and the denitrifying bacterial communities were analysed, but focus has been put on the denitrifying bacteria. Structural endpoints of the bacterial communities, as diversity and heterogeneity were analysed using molecular fingerprinting. Potential denitrification and leucine incorporation (i.e. bacterial growth) were measured as functional endpoints, when assessing the effects of pesticide exposure on constructed wetland bacterial communities. These structural and functional endpoints were measured without any treatments as well as measured after pesticide exposure. The results showed that structural endpoints of eubacterial (16S rRNA gene) and denitrifying bacterial community (nirK, nirS and nosZ) varied between the studied constructed wetlands, and their communities were influenced by environmental parameters. The enzyme gene nirS showed higher community heterogeneity than both nirK and nosZ, while the enzyme gene nirK had the highest diversity based on structure and richness. Exposure to environmental concentrations of pesticides affected structure (16S rRNA gene but not nosZ) and function (potential denitrification rate) of the constructed wetland bacterial community, however there were few indications of direct toxic effects. Using leucine incorporation as an endpoint of bacterial activity and growth community was a quicker and more sensitive method to detect toxicity of fungicides exposure on bacterial communities than measuring potential denitrification, and clear concentration-response relationships were easily generated that could be standardized for community level risk assessments of pesticide exposure to aquatic environments.