Microbial availability of different forms of phenanthrene in soils

Microbial degradation is the most important removal process for hydrophobic organic compounds (HOCs) in soil or sediment, and chemical availability is often a governing factor. However, the availability of HOCs in the sorbed forms is still a topic of debate. In this study, we applied rigorous kinetics analysis to the relationship between the freely dissolved concentration (Cfree) of phenanthrene (PHE) measured by polydimethylsiloxane (PDMS) fibers and its degradation by a PAH degrading bacterium PYR-1 under a range of soil conditions. In solutions of soils with varying organic carbon (OC) contents, Cfree of PHE decreased from 28.63 +/- 2.15 to 0.79 +/- 0.04 microg L(-1) when the soil OC content changed from 0.23 to 7.1%. Correlation analysis between Cfree and PHE mineralization rates revealed that the bacterium quickly exhausted the PHE pool available for equilibrium distribution, including Cfree and the reversibly sorbed fraction, after which the sequestered pool was utilized. In addition, unlike changes in Cfree, degradation rates of total PHE only varied by a factor of 1.6-2.1 over the same soil OC range. Regression analysis using a multivariate relationship showed that soil OC content and porosity properties such as soil surface area had a compounded effect on the microbial availability of PHE in these soils. The kinetics analysis using Cfree, as proposed in this study, may be applied to other HOCs to gain a better understanding of microbial availability under various conditions.