Endogenous respiration rates within Pseudomonas putida biofilms growing on toluene and ethanol as sole external carbon source were evaluated using oxygen microelectrodes in a Flat Plate Vapour Phase Biological Reactor (FPVPBR). The endogenous respiration was experimentally evaluated in Pseudomonas putida 54G biofilm cultures growing on toluene and ethanol, by measuring the flux of oxygen through the liquid-biofilm interface in absence of any external carbon source. Two fluorescent probes, 2,4-diamidino-2-phenylindole (DAPI) and 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) were used to evaluate the number of total and respiring cells within the biofilm, respectively. Biofilm samples were also analysed for viable and VOC-culturable cells by spread plating on non-selective and selective media, respectively.
Results suggest that endogenous respiration rate may contribute significantly to the oxygen requirements of the overall process and, consequently, affect the performance of the VPBR degrading VOCs. In the time the biofilm growing process at expenses of VOCs may deviate towards serious cryptic growth and endogenous metabolism. The endogenous respiration within the biofilm is a function of the VOC concentration and in last term of the injury phenomena that may act as regulatory mechanism of the biofilm thickness and density, which might justify the absence of plugging phenomena in VPBR. As the VOC concentration increases, the injury increases, meaning increasing intracellular material leakage and injured cell death resulting in lysis products. These observations can justify the unusually long operation periods characterizing VOC vapours biofiltration units.