26.2. Xinxin Liu: Microbial ecology in atrazine and terbutryn dissipation in surface soils and subsurface sediments

The worldwide use of triazines as pesticides has resulted in their widespread occurrence in groundwater. Within the northern boreal region in Southern Finland, 30% of groundwater sampling points contained pesticides, and acceptable drinking water limits were exceeded in 11%. Atrazine and its degradation products were among the most common pesticides observed. Chemical pesticide degradation in soil is slower than microbial degradation. Biodegradation often decreases with increasing depth. Nutritional factors and soil physicochemical properties affect microbial pesticide degradation. However, their effects on separate microorganisms, and the genetic basis of these responses, are not well documented. The aim of this thesis was to isolate and characterize atrazine-degrading microorganisms, and to find appropriate microbial, physicochemical, and nutritional demands for pesticide degradation. Microbial community composition was studied in farmland, forested farmland, and primary forest soils by pyrosequencing and in gardens, groundwater deposits, and vadose zone sediments by cultivation on mineral medium with atrazine or terbutryn as the nitrogen source. Atrazine dissipation efficiency was additionally compared under stagnant and circulating water conditions. The dominant phyla that increased in atrazine-treated farmland, gardens, deposits, and sediments were Proteobacteria and Actinobacteria. The overlap in genera was less than in phyla, while the isolated Pseudomonas strains only slightly overlapped between isolates from surface soils and subsurface deposits and sediments. Atrazine dissipation was better in circulating than in stagnant water, and aerobic microbes from genera known to have atrazine degradation genes, all from phyla Proteobacteria and Actinobacteria, were simultaneously enriched,. Based on the results, the application of microbial remediation of atrazine and terbutryn requires special attention to soil physicochemical properties and selection of proper microbial strains.