Impact of Oil on Bacterial Community Structure in Bioturbated Sediments

<div><p>Oil spills threaten coastlines where biological processes supply essential ecosystem services. Therefore, it is crucial to understand how oil influences the microbial communities in sediments that play key roles in ecosystem functioning. Ecosystems such as sediments are characterized by intensive bioturbation due to burrowing macrofauna that may modify the microbial metabolisms. It is thus essential to consider the bioturbation when determining the impact of oil on microbial communities. In this study, an experimental laboratory device maintaining pristine collected mudflat sediments in microcosms closer to true environmental conditions – with tidal cycles and natural seawater – was used to simulate an oil spill under bioturbation conditions. Different conditions were applied to the microcosms including an addition of: standardized oil (Blend Arabian Light crude oil, 25.6 mg.g⁻¹ wet sediment), the common burrowing organism <i>Hediste (Nereis) diversicolor</i> and both the oil and <i>H. diversicolor</i>. The addition of <i>H. diversicolor</i> and its associated bioturbation did not affect the removal of petroleum hydrocarbons. After 270 days, 60% of hydrocarbons had been removed in all microcosms irrespective of the <i>H. diversicolor</i> addition. However, 16S-rRNA gene and 16S-cDNA T-RFLP and RT-PCR-amplicon libraries analysis showed an effect of the condition on the bacterial community structure, composition, and dynamics, supported by PerMANOVA analysis. The 16S-cDNA libraries from microcosms where <i>H. diversicolor</i> was added (oiled and un-oiled) showed a marked dominance of sequences related to <i>Gammaproteobacteria</i>. However, in the oiled-library sequences associated to <i>Deltaproteobacteria</i> and <i>Bacteroidetes</i> were also highly represented. The 16S-cDNA libraries from oiled-microcosms (with and without <i>H. diversicolor</i> addition) revealed two distinct microbial communities characterized by different phylotypes associated to known hydrocarbonoclastic bacteria and dominated by <i>Gammaproteobacteria</i> and <i>Deltaproteobacteria</i>. In the oiled-microcosms, the addition of <i>H. diversicolor</i> reduced the phylotype-richness, sequences associated to <i>Actinobacteria</i>, <i>Firmicutes</i> and <i>Plantomycetes</i> were not detected. These observations highlight the influence of the bioturbation on the bacterial community structure without affecting the biodegradation capacities.</p>