The recent article by Mayer and Holmstrup (Passive Dosing of Soil Invertebrates with Polycyclic Aromatic Hydrocarbons: Limited Chemical Activity Explains Toxicity Cutoff, Environmental Science and Technology 2008 42:7516-7521) is a important step forward for soil ecotoxicology.  Poly Aromatic Hydrocarbons (PAH) are found primarily in pollutants such as a diesel, fuel oil, creosote, etc.  These PAHs are toxic and for higher organisms, such as humans, are largely a concern because they can cause cancer.  However, for invertebrates and other organisms that inhabit the soil, the primary mode of toxic action is ‘narcosis’, or basically the disruption of the lipid membrane.  For narcotic chemicals, what is important is not so much; the ‘identity’ of the chemical but rather how much of the chemical has accumulated in an organism.

In the research article by Mayer and Holmstrup, they wanted to evaluate if they could predict how toxic a narcotic chemical would be by basic chemical properties.  To do this, they dosed a common soil invertebrate, a springtail called Folsomia candida, with a novel testing method that allowed them to provide an infinite source of PAHs to the springtail.  What they found was that the toxicity of the chemical could be predicted by the melting point of the chemical but not the octanol/water partitioning coefficient.  This has two important implications.

First, as noted by the authors, this will allow them to calculate the toxicity of a PAH mixture to an invertebrate.  The reason being that toxicity to the springtail was a function of the chemical activity of the PAH.  When chemical activity reached 0.058, 50% of the springtails died.  For chemicals with a narcotic mode of action, it doesn’t matter if there is one chemical with a chemical activity of 0.058 or 10 different chemicals each with a chemical activity of 0.0058, 50% of the springtails will die.

More controversially the results from this work suggest that aging may not substantially reduce toxicity for soil invertebrates.  In the test system, the springtails were directly exposed to the PAHs with the springtails either directly absorbing the PAHs or inhaling the PAHs; there was no need for the PAHs to be ingested in drinking water.  Thus, the reduction of PAH dissolution into drinking water that occurs as pollutants age, may have no effect on PAH toxicity for invertebrates.

The second significant implication from the work by Mayer and Holmstrup is that typical fugacity based multimedia models may not be accurate for soil invertebrates.  Currently, fugacity models consider biotic accumulation of pollutants to be a function of the octanol:water partitioning coefficient and the amount of lipid present in an organism.  In this scenario, organisms are modelled as ‘bags of octanol’ and the amount of contaminant present in the organisms is a function of how much a chemical likes octanol compared to water.  However, the results from Mayer and Holmstrup indicate that for springtails, it is chemical activity (which can be modelled from melting temperature) that determined the amount of PAHs accumulated by the springtails.