Microbial community composition in polycyclic aromatic hydrocarbon contaminated sediments

Background/Question/Methods

Contaminated river sediments are a common worldwide problem. In the Mahoning River, Northeast Ohio, polycyclic aromatic hydrocarbons (PAHs) occur in the riverbank sediments along 51 km of the riparian corridor. These contaminated sediments are the main factor limiting aquatic life of the river and microbial inhabitants have the potential to degrade these compounds. In this study, we used molecular techniques to investigate the microbial ecology of these sediments. Our goals were to: i) characterize the geochemistry, ii) analyze bacterial community structure and diversity, and iii) to evaluate seasonal changes in bacterial communities. Information on microbial dynamics within these sediments will be useful in developing remediation approaches.

Sediment was collected during winter, summer and fall of 2011 using stainless steel cores. Total bacteria were enumerated by staining with 4',6-diamidino-2-phenylindole, dihydrochloride. DNA was extracted for Terminal Restriction Fragment Length Polymorphism (T-RFLP) analysis and to construct bacterial clone libraries. Geochemical data including PAHs, sulfate, carbon & nitrogen, organic matter, moisture, and pH were determined by gas chromatography mass spectrometry, ion chromatography, CN analyzer and standard methods, respectively.

Results/Conclusions

Total PAHs ranged from 20 to 100 mg/kg dry mass. Organic matter content (~15%), moisture content (36- 61%) and pH (7.4- 7.8) were uniform among seasons. Sulfate concentrations were much higher during winter and fall (100 to 1400 μg/g) than in summer (4 to 23 μg/g). High carbon to nitrogen ratios (~55:1) were constant among dates suggesting a nitrogen limiting system.

Sediments contained an average of ~3.0E+7 bacteria per gram. T-RFLP analysis, using HaeIII as the restriction enzyme, revealed that seasonality shaped the microbial community. Preliminary cloning and sequencing yielded members of the !- Proteobacteria, including Fe and Mn-reducing bacteria (Geobacter and Leptothrix sp.) and methanogens (Smithella and Syntropus sp.). Interestingly thermophilic bacteria (Thiobacillus aquaesulis and Thermotogae sp.) were also found. Nonmetric multidimensional scaling analyses showed that bacterial communities were segregated spatially and by season, indicating that microbial communities responded to environmental parameters. The presence of anaerobes and Fe and Mn-reducing bacteria is currently being explored to address the potential role of these organisms in anaerobic oxidation of PAHs. In summary, these highly contaminated sediments had a diverse and abundant bacterial community whose composition varied temporally and was correlated with physicochemical conditions.