Relationships between microbial community structure and hydrochemistry in a landfill leachate-polluted aquifer

Knowledge about the relationship between microbial community structure and hydrogeochemistry (e.g., pollution, redox and degradation processes) in lan dfill leachate-polluted aquifers is required to develop tools for predictin g and monitoring natural attenuation. In this study analyses of pollutant a nd redox chemistry were conducted in parallel with culture-independent prof iling of microbial communities present in a well-defined aquifer (Banisveld , The Netherlands). Degradation of organic contaminants occurred under iron -reducing conditions in the plume of pollution, while upstream of the landf ill and above the plume denitrification was the dominant redox process. Ben eath the plume iron reduction occurred. Numerical comparison of 16S ribosom al DNA (rDNA)based denaturing gradient gel electrophoresis (DGGE) profiles of Bacteria and Archaea in 29 groundwater samples revealed a clear differen ce between the microbial community structures inside and outside the contam inant plume. A similar relationship was not evident in sediment samples. DG GE data were supported by sequencing cloned 16S rDNA. Upstream of the landf ill members of the beta subclass of the class Proteobacteria (beta -proteob acteria) dominated. This group was not encountered beneath the landfill, wh ere gram-positive bacteria dominated. Further downstream the contribution o f gram-positive bacteria to the clone library decreased, while the contribu tion of delta -proteobacteria strongly increased and beta -proteobacteria r eappeared. The beta -proteobacteria (Acidovorax, Rhodoferax) differed consi derably from those found upstream (Gallionella, Azoarcus). Direct compariso ns of cloned 16S rDNA with bands in DGGE profiles revealed that the data fr om each analysis were comparable. A relationship was observed between the d ominant redox processes and the bacteria identified. In the iron-reducing p lume members of the family Geobacteraceae made a strong contribution to the microbial communities. Because the only known aromatic hydrocarbon-degradi ng, iron-reducing bacteria are Geobacter spp., their occurrence in landfill leachate-contaminated aquifers deserves more detailed consideration.