DDT degradation in flooded soils

Gohil, H., Ogram, A., & Thomas, J. (2014). Stimulation of anaerobic biodegradation of DDT and its metabolites in a muck soil: Laboratory microcosm and mesocosm studies. Biodegradation. https://doi.org/10.1007/s10532-014-9687-0

Why read this article?

The results highlighted some processes and mechanisms of POP degradation in flooded (anoxic) soils and showed what effect electron donors such as lactate have on the degradation of DDT.

The soil-incubations in this study used historically-contaminated soils from wetlands in Florida (Histosol (WRB), organosol (Isbel) or just call it peat or muck soil) which are very high in organic carbon content (45%). The lack of oxygen in the wetlands allows organic carbon to accumulate. Soil characteristics can be found here.

What were its main conclusions?

Microcosms with 10 g of historically contaminated soil were incubated for 2 months without oxygen. Additional (fresh) DDT was applied to the soil at the beginning of the incubation, despite the fact that it contained high amounts of aged DDT residues since DDT has been used extensively in these wetlands for so-called muck farming. Fresh DDT was applied as it was assumed that the aged DDT was not available to microorganisms.

It was found that different electron acceptors such as sulfate, iron, and nitrate reducing conditions, and fermentation made no difference to DDT-degradation.

However, electron donors did have an effect. At the end of the 2-month incubation the lactate treatment had the lowest DDT concentrations at ~ 3.5µg/g drysoil (10nm/g drysoil) soil and was 83% lower compared to the autoclaved control. The hydrogen gas (H₂) treatment seemed also good, i.e. resulted in a trend of low DDT concentrations. Acetate did nothing. All in all, this was an interesting result and was supported by literature as far as I know.

But one of the weirdest results from the microcosm incubation was that the no-DDT control (i.e. a control treatment in which no additional DDT was applied) - resulted in the highest DDT concentrations by far (135µg /g dry soil)!?!?! The authors assumed that DDT increased during the incubation and may have been a result of increased extractability/solubility due to the anoxic conditions of the experiment. If true, then perhaps the increase solubility under anoxic conditions made DDT more extractable/bioavailable but somehow at the same time inhibited its degradation. The authors argued that this could be due to insufficient utilizable carbon and energy sources for the degrading population. But at the same time, the autoclaved control (which resulted in relatively low DDT concentrations), which had fresh DDT added in the beginning somehow still resulted in much lower DDT concentrations which would include a reduction of aged DDT. That means simply adding fresh DDT supercharged degradation of aged DDT too? A bit like positive priming of organic matter. Is that possible? By the way, I am assuming that autoclaving of soils did not kill all organisms and that microbes were active - see limitations below.

The authors went on and did a 6-month incubation in large tanks with 200L of soil and confirmed that lactate improved DDT degradation at the end. The authors also established a connection between an increase of dissolved organic carbon, pH, redox and enhanced solubility of DDT.

This highlighed that anaerobic conditions such as in flooded soils, result in increased alkalinity (as hydrogen ions are consumed in microbial metabolism) which in turn results in increased dissolved organic carbon (due to changes in ph-dependent sorption of organic carbon) which in turn increases DDT solubility. It is known that anoxic conditions improve the degradation of persistent pollutants and this increased solubility could help with this process as it becomes available to microorganisms.

It has to be noted though that one of the products of DDT degradation, DDE, was unaffected throughout the 6 months. And that is one of the big white elephants in the room when it comes to research of persistent pollutants: Their degradation products are equally toxic and persistent as only one or two chlorines are respired and the remaining chlorines are still attached to the compound… and we know hardly anything about these compounds… But in general it makes sense that similar degradation processes would apply.

What were the main limitations?

Overall, the conclusions remained somewhat uncertain due the small number of replicates compared to the high variability of DDT extraction.

Unfortunately no starting or baseline concentrations were provided - so it was unclear if the no-DDT control concentrations (which was so weirdly high) in the microcosms were low at the beginning and increased during the incubation or if they simply happened to by high from the beginning. The latter may be possible if the 10 g of soil came from an odd batch.

Another limitation was that the processes of autoclaving for the autoclaved control may have volatilised and removed some DDT at the start or improved microbial DDT-degradation as the processes is prone to solubilise organic food sources and supercharge microbial growth after a few days (instead of killing it what it is supposed to do). Any comparison to living soil is difficult without a valid non-living soil as control. But I think it is fair to say that it was living soil microorganisms that did the degrading.

What does this mean for POPs?

Adding electron donors such as lactate into flooded and contaminated soils may increase dechlorination rates of chlorinated pollutants.

However, toxic dechlorination products such as DDE may not dechlorinate any further.

Flooding of currently non-flooded contaminated soils may result in a sudden increase of bioavailable contaminants in water.

What do I think about it?

I really enjoyed reading the discussion of this paper as it was helping me to understand some of the fundamental principles of anoxic soil conditions and how they relate to POP degradation.