Biodegradation of diesel fuel by an Azolla-derived bacterial consortium

The widely distributed water fern Azolla was investigated for use as an amendment in the bioremediation of fuel-contaminated environments. In a field experiment Azolla pinnata as well as Pistia stratiotes and Salvinia molesta were applied to plots containing soil that had been surface-contaminated with diesel fuel (2.4 L m(-2)) and flooded with water. The plants quickly died and bacterial flocs developed around the dead A. pinnata fronds. After 16 weeks, diesel concentrations (as determined by levels of gas chromatography-detectable hydrocarbons) in the plant-added plots were less than half that of the control plot, and concentrations of xylenes and ethylbenzene were 50-100 times lower. In laboratory experiments, a consortium composed of A. pinnata-derived bacteria displayed dense growth in a 4% diesel-containing mineral salts medium and was found to lower the fluorescence from aromatic compounds by approximately 50% after 19d. It is concluded that the observed enhancement of diesel degradation in the plant-added plots was due to the release of bacteria (bioaugmentation) and physiochemical improvement of the plot conditions (biostimulation).