Abstract
Crude oil is composed of saturated and aromatic hydrocarbons, polar compounds, resins, and asphaltenes. Aromatic hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) are the most important drivers of hydrocarbon toxicity. Some sulfate-reducing bacteria (SRB) have the ability to degrade these toxic hydrocarbons and play a significant role in the remediation of oil spills. For a better understanding of SRB populations responsible for hydrocarbon degradation in coastal sediment, a microcosm experiment was conducted using oxygen and hydrogen sulfide microelectrode profiles and high-throughput 16S rRNA and dissimilatory (bi)sulfite reductase (dsrB) gene sequencing. Oxygen penetration depth and hydrogen sulfide concentrations were used to compare sediment columns as well as diversity analysis, similarity profile tests (SIMPROF), PICRUSt analysis of 16S rRNA phylotypes, and gene abundance (qPCR). Additionally, microbial communities in coastal sediment five years after the Deepwater Horizon oil spill were investigated through 16S rRNA gene sequencing. The subtidal sediment microbial communities from the Chandeleur Islands, LA were previously exposed to the Deepwater Horizon oil spill in 2010, while the sediments of Estero Bay, FL served as controls where a major oil spill event has never occurred. Except for one Estero Bay sample collected in 2015, oxygen penetration in all sediment columns decreased and oxygen concentrations decreased within the water column with exposure to the water accommodated fraction of crude oil (WAF). Hydrogen sulfide concentrations varied with sediment source. 16S rRNA gene amplicon sequencing showed that Estero Bay sediments were dominated by Desulfosporosinus sp. (34% of all sequences in a 2013 sediment sample) and Desulfotalea sp. (51% of all sequences in a 2015 sediment sample), SRB members of Clostridia, and Deltaproteobacteria, respectively, and a Chandeleur Island sediment sample was dominated by unclassified Clostridiales and Marinilabilia sp (55% and 19% of all sequences, respectively), fermentative anaerobes that are members of Clostridia and Bacteroidia respectively. There were no significant differences in the relative abundance of any hydrocarbon degradation genes analyzed using PICRUSt. WAF exposure diminished the diversity of 16S rRNA and dsrB phylotypes in most WAF exposed sediment columns. SIMPROF analysis showed that the communities of all oiled sediment columns were significantly different from their control counterparts for 16S rRNA and dsrB gene amplicon sequencing (p<0.05). The dsrB gene sequencing showed shifts in the abundance of members of Desulfobacteraceae, Desulfobulbaceae, and Syntrophaceae with exposure to WAF as shown using SIMPER analysis. For qPCR, exposure to WAF did not have a significant effect on either gene copy number. Exposure to WAF created an anaerobic environment reflected in both bacterial community structure and oxygen profiles. Our study showed rapid impact (over 22 days of WAF exposure) with community shifts to members of Desulfobulbaceae and taxa previously reported with hydrocarbon contamination. The Deepwater Horizon oil spill had no impact on bacterial community biodiversity 5 years following the spill. This study showed that SRB communities can rapidly respond to WAF exposure and that bacterial communities five years after the Deepwater Horizon oil spill managed to recover or reach a new stable state.
