the enzymes and the genes responsible for them. However, there is only one very well characterized aerobic pathway for alkanes. Considering that there are many for the PAHs, there is likely to be others for the alkanes as well; but even in the aerobic realm, our information is limited.
Another issue with respect to the ability of organisms to use different electron acceptors has been addressed by Dr. Jerry Kukor, who studied groundwater from aquifers at three different sites contaminated with benzene, toluene, and xylenes. Data from the three sites indicated that the oxygen was present in significantly lower concentrations compared with the pristine site; furthermore, the levels of oxygen in the contaminant plume is also lower than in the pristine site. In the plume, the potential for both aerobic and anaerobic activity exists. Using the organisms isolated from this site, we can see that low oxygen with nitrate supported much better degradation of toluene than oxygen without nitrate. Here is a hybrid system in which the oxygen is necessary for these organisms because all are aerobic organisms in terms of the mechanisms they use for the degradation process; however, their activity is boosted because they can use nitrate as well as oxygen for respiratory purposes.