A recent article by Dr. Gan’s group in Environmental Science and Technology (DOI: 10.1021/es802966z) investigates how available phenanthreneis to bacteria. What the group did was measure phenanthrene degradation and also calculate the amount of phenanthrene freely dissolved in soil pore water. To measure the amount of phenanthrene in pore water, they used solid phase microextraction fibers. Importantly they checked many of the assumptions about SPMEs and their ability to measure freely dissolved phenanthrene. Once they did this, they found that the freely dissolved concentration of phenanthrene could only predict the initialy 144 h of degradation but after that was not predictive at all. This suggests that microorganisms can access phenanthrene directly from soil without the phenanthrene dissolving into pore water. This is not a brand new observation by Dr. Gan’s group has been able nicely pin it down and also investigate the influence of other soil factors like organic carbon and surface area on bacterial use of phenanthrene.
Why do we care about this? The concept of chemical activity versus bioaccessibility hinges on this very same issue. Chemical activity can be linked to the freely dissolved chemical concentration in pore water. In contrast, bioaccessibility is an estimate of how much of a contaminant an organism can access. One good rule of thumb put forward by Reichenberg and Mayer (2006) is that bioaccessibility is required when organisms can specifically access a chemical whereas chemical activity is more important for passive uptake systems. Typically for bacteria we think of them as passive systems for polyaromatic hydrocarbons. The toxicity of PAHs to organisms is often thought to be due to non-polar narcosis in which the PAHs disrupt the lipid membrane (nitrifiers are an important exception to this). Based on this recent publication by Dr. Gan, we are going to need to rethink this idea. If microorganisms can specifically uptake the toxicants, then we need some sort of estimate of bioaccessibilty for microorganisms. This will allow us to modify site specific clean up scenarios to protect biogeochemical cycling at impacted sites.