Microbial community responses to repeated S-triazine exposure and carbon and nitrogen availability: Impacts on community structure and xenobiotic degradation

Contamination of surface and groundwater by chemicals resulting from non-point sources is of great concern. Thorough understanding of the conditions that govern the fate of compounds within the environment is critical for predicting the risk of contaminants to ecosystem health. Little is known regarding the relationship between chemical exposure and degradation rate when the primary driving force for catabolism is nutrient elements other than C. Atrazine was chosen as a model compound for these studies because it is a relatively poor source of C and energy, but can serve as an N-source for some soil microorganisms. In this work, culture-independent methods, whole-soil FAME and denaturing gradient gel electrophoresis (DGGE), were used to relate shifts in the microbial community structure of soils with differing s-triazine exposure histories, to atrazine mineralization rate and the density of atrazine-degrading microorganisms in response to repeated s-ariazine exposure and nutrient availability. These studies showed an enhancement in atrazine mineralization, along with an increase in the density of atrazine-degrading microorganisms following laboratory acclimation, but the presence of inorganic-N led to an inhibition in the mineralization of atrazine and a decrease in the degrading population density. The FAME and DGGE data showed that the H- and NH-soils developed remarkably different microbial community compositions following repeated exposure to atrazine despite having a similar mineralization capacity. These results suggest that different microbial populations were selected within the two soils that had the capacity to mineralize atrazine. To examine the diversity of the microbial community selected through conventional enrichment techniques the acclimated soils were used as inoculum for enrichment cultures, and DGGE was used to compare the microbial community fingerprint present in the enrichment cultures with those obtained in the corresponding soil DGGE. The DGGE fingerprints for the enrichment cultures revealed different patterns from those obtained from the laboratory acclimated soils. The results suggest that microorganisms obtained by enrichment techniques are not phylogenetically related to dominant community members mediating degradation of atrazine in these soils.