Using biodegradation kinetics to measure availability of aged phenanthrene to bacteria inoculated into soil

The rate of biodegradation of pollutants in soil can be limited by the pollutant's availability to microorganisms. We have developed a bioassay for the availability of phenanthrene to bacteria that degrade phenanthrene in soil. The assay uses a soil in which phenanthrene is degraded very slowly. The rate of phenanthrene mineralization in this soil may be increased substantially through bioaugmentation with a bacterial inoculum. By delaying inoculation, it is possible to manipulate the time phenanthrene is present in soil before accelerated biodegradation begins. A phenanthrene concentration much lower than the affinity constant of the inoculum is added; thus, biodegradation kinetics approach first order. Because the phenanthrene first- order rate constant for the inoculum is the same regardless of the phenanthrene residence time in soil, the change in phenanthrene availability to the inoculum can be measured over time. The availability of phenanthrene to bacteria declined in a biphasic double exponential pattern with time. The initial rapid decline in availability resembled the change in amount of phenanthrene extracted from soil with hexane-water. However, after phenanthrene had been present in the soil longer than 300 h, the fraction extracted with hexane-water declined faster than the substrate available to the bacterial inoculum, suggesting that the bacteria are able to access a pool of phenanthrene unavailable to hexane.