Microflora of 2,4-dichlorophenoxyacetic acid degrading biofilms on gas permeable membranes

Bacteria with specific metabolic capabilities are required for the degradation of industrial wastewater. In reactors with suspended biomass these organisms may easily be washed out. Reactors with immobilized biomass appear to be better suited to retain those organisms in the system. In order to monitor immobilization efficiency of such a biofilm reactor, the composition of the biofilm grown in the reactor has to be examined. In this study a Membrane Biofilm Reactor (MBR) was inoculated with Alcaligenes eutrophus JMP 134 to yield 2,4-dichlorophenoxyacetic acid (2,4-D) degrading biofilms. In a MBR the biofilm is supplied with oxygen through a membrane from the gas compartment and with substrate from the bulk liquid. In situ hybridization of cross sections of the biofilm with 16S rRNA-targeted oligonucleotide probes revealed the spatial distribution of bacterial cells in the biofilm. An oligonucleotide probe specific for the 2,4-degrading strain A. eutrophus JMP 134 was developed based on comparative sequence analysis. A. eutrophus JMP 134 cells were hardly found directly attached to the membrane, but clusters of them colonized e. g. the testaceous amoebae in the layer close to the membrane. The biofilm itself consisted of three different layers. The bottom layer was characterized by clusters of testaceous amoebae covered with bacterial cells from all groups examined. The base biofilm layer contained organisms of the beta-subclass and - in most cases - fungi. The surface layer exhibited again a higher diversity of bacterial cells and some testaceous amoebae. The overall composition of the biofilm was characterized by a dominance of organisms of the beta-subclass of Proteobacteria. Cells of A. eutrophus JMP 134 were found in all three layers, but in different morphological shapes.