Bioremediation: good or bad?

A review of : Dell’Anno, A., Beolchini, F., Gabellini, M., Rocchetti, L., Pusceddu, A., Danovaro, R. 2009. Bioremediation of petroleum hydrocarbons in anoxic marine sediments: consequences on the speciation of heavy metals. Marine Pollution Bulletin. 58, pp. 1808-1814.

Contamination of marine sediments by petroleum hydrocarbons is prevalent along coastal regions and there is concern over the detrimental consequences this may have to the health of the surrounding ecosystem. Bioremediation is an environmentally friendly technique to breakdown harmful chemicals in the environment; this is done via the addition of microbes (bio augmentation) or increasing the biomass/ activity of hydrocarbon degrading microbes (bio stimulation).

Most research has been centred on degrading the hydrocarbon under aerobic conditions as microbial oxygenesis plays a key part in the degradation of hydrocarbons, however there is increasing evidence that degradation of hydrocarbons can take place under anoxic conditions as well. This opens up new opportunities for the treatment of contaminated sediment where O2 cannot be used as an electron acceptor. In anoxic sediments the primary electron acceptors are Mn or Fe, therefore degradation of hydrocarbons in anoxic sediments may only occur if the reduces are sulphate, Mn or Fe. As sulphate is highly available in coastal sediments while Fe is sparse in the heavily contaminated sediments degradation under sulphate reducing conditions is probably the most effective treatment.

Coastal sediments are subjected to a high amount of anthropogenic inputs and not only have high concentrations of hydrocarbons but can also contain heavy metals, the fate of these in the environment is determined by microbial mediated processes. Bioremediation changes the way that the microbes function in the environment, by either introducing new microbes (bio augmentation) or increasing the biomass or metabolism of the microbes already present. Therefore bioremediation may cause changes in the speciation of heavy metals in the environment, this is something that will need to be assessed as a risk factor when determining if bioremediation is to be used in contaminated sediments. This study experimented using bio stimulation and bio augmentation of marine anoxic sediments contaminated by both hydrocarbons and heavy metals in order to test the hypothesis that increased biodegradation of hydrocarbons in contaminated sediments results in changes in the speciation of heavy metals and alters their mobility and bioavailability.

Three different treatments were used in the experiment, the addition of sodium acetate to represent bio stimulation, the addition of sulphate reducing bacteria to represent bio augmentation and the addition of both sodium acetate and sulphate reducing bacteria.

The results found that the addition of sulphate reducing bacteria resulted in a 54% reduction in the total hydrocarbon concentration in the sediment samples after 60 days. Hydrocarbon degradation by natural attenuation (by bacteria intrinsic to the sediment sample) was also compared to the reduction of hydrocarbons by bio stimulation and bio augmentation. There was a 24% decrease in the total amount of hydrocarbons in the sediment after 30 days; however there was no further decrease after these 30 days.

All treatments in the experiment increased the metabolic activity of the microbes in the experiment; this can be seen through the increased PH of the sediment as a result of increasing anaerobic respiration and also the reduction in redox potential as a result of active bacterial mediated electron flow. These findings suggest that the addition of sodium acetate or sulphur reducing bacteria may be an effective treatment in reducing petroleum hydrocarbons in the sediment, as all treatments experienced increased metabolism and also a significant decrease in hydrocarbon concentration.

In regards to the effect of the increased metabolism on the fate of heavy metals in the sediment it was found that in the treatment which produced the highest reduction in hydrocarbons there was an increase in Cu, Pb and Zn concentrations. A PERMANOVA and regression was performed to determine if microbial activity was the cause in the changes in heavy metal concentrations, the results suggested that metal speciation was largely dependent on microbial growth and activities. Although the paper doesn’t come to this conclusion, from what was mentioned earlier in the paper this is an important finding as the availability and therefore toxicity of the metals is determined on what chemical form they are in (speciation), therefore if the microbial activity is affecting their speciation this may result in the metals becoming more or less available and effect their potential toxicity.

I felt that the paper was well written although sometimes it did jump around a little bit and didn’t flow as well as I thought it could. This is some interesting research though, what I thought would be a good advancement of this paper is to see if the increase in the heavy metals is of a concentration which is known to cause detrimental biological effects. As bioremediation would be a good way in which to decrease the levels of hydrocarbons, however understanding if the effect on the speciation of heavy metals will affect the organisms in the environment is important, as this method may not be used because of the “risks associated with it” when in fact it the heavy metals may not be of a concentration that would cause significant detrimental effects.