Microbial Biodegradation of PAH Pollution

A review of: Wang, Y.F and Tam, N.F.Y. (2011) Microbial community dynamics and biodegradation of polycyclic aromatic hydrocarbons in polluted marine sediments in Hong Kong. Marine Pollution Bulletin. 63, 424-430

Pollutants are a significant environmental concern, particularly in marine environments where they can become ubiquitous. They can cause significant shifts in community structures and the example used in this paper are PAHs, polycyclic aromatic hydrocarbons. These are hydrocarbons with two or more fused hydrocarbon rings formed during incomplete combustion of organic materials and are often pollutants as a result of oil spills and leakages. The concerns over PAHs are due to their tendency to be both carcinogens and mutagens, particularly those that have a high molecular weight. Fortunately, some microbial communities found in marine sediments are resistant to and are even able to degrade PAHs.

This study explores the dynamics of microbial populations during PAH biodegradation and investigates how different levels of contamination affect the microbial community structure by artificially spiking samples of marine sediment with a mixture of PAHs (fluorene, phenanthrene, fluoranthene and pyrene) at three different levels (P1 - P3, where P3 is the highest). To achieve this, the population sizes of THB (total heterotrophic bacteria) and PDB (PAH degrading bacteria) were counted using the most probable number method and analysis of ester-linked fatty acid methyl esters (EL-FAME) and DGGE (denaturing gradient gel electrophoresis) were used to look at phenotypic and genotypic succession in the microbial community.

The numbers of THB were highest in the control and lowest in the high level (P3) treatment while the numbers of PDB were highest in the P3 treatment. The proportion of PDB to THB in the mid and high level treatments (P2 and P3) significantly increased with time indicating the predominance of PDB at high stress.

Analysis of DGGE found significantly lower diversity in the high level treatments compared to any of the others, which was not found in the EL-FAME analysis suggesting that the responses of the microbial community to environmental stresses takes place at different levels and that adaptations to the phenotype comes after genotypic selection under extreme environmental stresses.

At high PAH concentration, the PAH resistant or degrading bacteria survive, become dominant and begin biodegradation of PAH. But once the levels gradually decrease with degradation, the non-PAH degraders start to recover and feed on the PAH intermediates. Further supported by the PDB/THB ratio which was found to increase three days after spiking (PDB required time to adapt before biodegradation could take place), reach a peak in the middle of the experiment and start to decline toward the end, suggesting a recovery of the microbial diversity.

EL-FAME illustrated that the ratio of branched fatty acids to MUFA (mono-unsaturated fatty acids) decreased significantly in all level treatments toward the middle of the experiment, as PAH concentrations began to accumulate in the sediment. Branched fatty acids and MUFA are generally considered as biomarkers for gram-positive and gram-negative bacteria respectively so the shift to increased MUFA suggests a shift to gram negative bacteria.

Previous studies had shown that gram-negative bacteria were more frequently selected with low molecular weight (LMW) PAHs and the relative abundance of fatty acids in strains that were capable of degrading PAHs confirm that gram-negative strains have highly abundant MUFA. This shift in the ratio of branched to MUFA could also suggest a bacterial adaptation or response to environmental stress because the decrease in the ratio could help to increase the fluidity of the cell membranes and therefore facilitate the uptake of LMW PAHs.

The authors note in this study about the significantly increased sensitivity of the DGGE method in comparison to EL-FAME and the study in general illustrates the applications of these methods in marine microbiology. I thought this paper had some similarities with what were aiming to achieve in the practical sessions with regard to exploring the effect of sediment contamination on microbial communities and therefore I thought it might be useful to see examples of similar studies. I also found that it was interesting to see how marine microbes were able to adapt to anthropogenic stresses.