How chronic PAH pollution impacts coastal benthic communties

In recent years, the effects and microbial responses to oil spills have garnered much interest. However, estimates suggest oil spills to be significantly less important than chronic pollution. In marine sediments, acute polyaromatic hydrocarbon (PAH) pollution has been shown to lead to a decrease in α-diversity, as a result of the increase in hydrocarbonoclastic microbes. However, few studies have focused on the repercussions of chronic pollution. In their paper, Jeanbille et al. (2016) examined the effect of chronic PAH pollution in coastal sediments in the North Atlantic and the Mediterranean Sea. The community structures of Bacteria, Archaea and Eukarya were mapped across both the regional and the continental scale. Moreover, the impact of sediment contamination by PAHs on predicted prokaryotic metagenomic functions was also investigated. 

The sediment samples were collected from the top (1-2 cm) of the surface layer using a sampling box corer at 46 sites across the North Atlantic and the Mediterranean Sea. The sites were characterized by site observation and proximity to potential PAH pollution as either PAH-contaminated or uncontaminated prior to sampling. Predictions of metagenomic functions for Bacteria and Archaea were made using the PICRUSt database.

The analysis of the environmental data showed salinity to be correlated with latitude. Average temperate, sediment particle size (PSD) and the percentage of total organic carbon were also higher in the Mediterranean samples. In both regions, the mean PAH concentration was significantly higher in the contaminated sites. Nevertheless, the difference between contaminated and uncontaminated sites was larger in the Mediterranean samples. In most sites (44), the origin of the pollution was identified as being from combustion processes while the remaining sites (14) were most likely contaminated from both combustion and direct input. 

The variance in α-diversity was not significantly explained by PAH abundance. Moreover, the differences in β-diversity were mainly influenced by habitat and PAH pollution didn’t influence prokaryotic communities on a continental scale. Neither did the analysis of predicted metagenomic function show differences in the abundance of PAH degradation pathways between regions or Bacteria and Archea. 

In contrast to acute pollution, the authors showed microbial communities were not impacted by chronic pollution. In general, α-diversity is not indicative of stress under chronic pollution as the increase in tolerant species can lead to recovery. However, β-diversity differed on regional and continental scale. Compared to environmental selection, PAH pollution had little effect on microbial communities on a continental scale. On a smaller scale, the influence of environmental selection was reduced and the range of PAH concentrations was greater. Thus, the pollution had a greater impact on the benthic communities. Nevertheless, the authors acknowledge most of the variance to be unexplained. The divergence between continental and regional impact, indicates that results may not generally be transferable across different spatial scales. Furthermore, the changes in bacterial community did not affect predicted metagenomic functions and therefore not influence fundamental ecological processes. 

Out of the three domains, Eukarya were the most influenced by the chronic pollution. Macro-Eukarya richness decreased, while Dinophyceae dominated contaminated samples along with crenoarcheotic Archaea. The authors suggest these phyla could serve as potential biomarkers for PAH contamination. 

In conclusion, the coastal benthic communities seemed remarkably resilient to chronic PAH pollution. However, the study could have benefited from more sample sites, especially in the Eastern Mediterranean and the North Sea. Personally; I think that the regional breakdown should have been finer in order to identify sites that are particularly impacted. I would also be interested in the potential repercussions additive effects of different pollutants (e.g. plastic) have on the resilience of benthic microbial communities. 

Reviewed Paper:

Jeanbille, M., Gury, J., Duran, R., Tronczynski, J., Ghiglione, J. F., Agogué, H., ... & Auguet, J. C. (2016). Chronic polyaromatic hydrocarbon (PAH) contamination is a marginal driver for community diversity and prokaryotic predicted functioning in coastal sediments. Frontiers in microbiology, 7. Link: http://journal.frontiersin.org/article/10.3389/fmicb.2016.01303/full