Cold sediment microbial community shifts in response to crude oil water-accommodated fraction with or without dispersant: a microcosm study.

In cold environments, the low temperature slows down microbial metabolisms, such as the biodegradation processes of hydrocarbons, which are often stimulated by the addition of dispersants in oil spill disasters. In this study, we investigated the effects of hydrocarbon water-accommodated fraction (WAF) prepared with and without dispersant on benthic microbial communities in a microcosm experiment in which hydrocarbon removal was observed. Both WAFs contained similar polycyclic aromatic hydrocarbon (PAH) content. The microcosm experiment, set up with either pristine or contaminated sediments, was conducted for 21 days at 4 °C under WAF and WAF + dispersant conditions. The behavior of bacterial communities in response to WAF and WAF + dispersant was examined at both DNA and RNA levels, revealing the effect of WAF and WAF + dispersant on the resident and active communities respectively. The contaminated sediment showed less taxa responsive to the addition of both WAF and WAF + dispersant than the pristine sediment, indicating the legacy effect by the presence hydrocarbon-degrading and dispersant-resistant taxa inhabiting the contaminated sediment.

Legacy and dispersant influence microbial community dynamics in cold seawater contaminated by crude oil water accommodated fractions.

Dispersants, used for combating oil spills, increase hydrocarbon bioavailability promoting their biodegradation. Oil weathering process introduces harmful soluble hydrocarbons, such as polycyclic aromatic hydrocarbons (PAHs), into the water column, resulting in water-accommodated fraction (WAF). The presence of dispersants can influence the weathering process by increasing PAHs solubility, toxicity and biodegradability. However, little is known on how dispersants affect microbial communities and their degradation capacities, especially in cold environment where low temperature decreases microbial activity and thus hydrocarbon degradation. Here, we investigated the microbial community dynamics in cold water contaminated by WAF prepared from crude oil with or without a commercial dispersant (Finasol OSR52). The WAFs, prepared with Naphthenic North Atlantic crude oil, were used to contaminate seawater from Norwegian cold sites, one oil-contaminated and the other pristine. The WAF-contaminated seawaters were maintained in microcosms at 4 °C for 21 days. The content of PAHs and microbial compositions (16S rRNA gene sequencing) were determined at days 0, 7, 14 and 21. In addition, the 96 h toxicity assay with adult Acartia tonsa revealed WAFs toxicity at days 0 and 21. The toxicity of WAF mixtures, with and without dispersant, against Acartia tonsa was reduced during the experiment, but PAHs removal was not increased. The water from the oil-contaminated site showed the highest PAHs removal revealing legacy effect (presence of microorganisms adapted to PAHs). Additionally, our results reveal: i) microbial community plasticity allowing the adaptation to the presence of PAHs and dispersant, ii) specific bacteria taxa probably involved in PAHs degradation, and iii) dispersants shape the microbial communities dynamics by stimulating potential dispersant-degrading taxa, such as Fusibacter. Thus, our results provide valuable insights on the role of microbial community in determining the fate of water-solubilized hydrocarbon in cold environment while questioning the role of dispersant used for fighting oil spill.

Use of dispersant in mudflat oil-contaminated sediment: behavior and effects of dispersed oil on micro- and macrobenthos.

The present study aimed to examine whether the use of dispersant would be suitable for favoring the hydrocarbon degradation in coastal marine sediments without impacting negatively micro- and macrobenthic organisms. Mudflat sediments, maintained during 286 days in mesocosms designed to simulate natural conditions, were contaminated or not with Ural blend crude oil (REBCO) and treated or not with third-generation dispersant (Finasol OSR52). While the dispersant did not lead to an increase of hydrocarbon biodegradation, its use enables an attenuation of more than 55 % of the sediment concentration of total petroleum hydrocarbons (TPH). Canonical correspondence analysis (CCA) correlating T-RFLP patterns with the hydrocarbon content and bacterial abundance indicated weak differences between the different treatments except for the mesocosm treated with oil and dispersant for which a higher bacterial biomass was observed. The use of the dispersant did not significantly decrease the macrobenthic species richness or macroorganisms' densities in uncontaminated or contaminated conditions. However, even if the structure of the macrobenthic communities was not affected, when used in combination with oil, biological sediment reworking coefficient was negatively impacted. Although the use of the dispersant may be worth considering in order to accelerate the attenuation of hydrocarbon-contaminated mudflat sediments, long-term effects on functional aspects of the benthic system such as bioturbation and bacterial activity should be carefully studied before.

Effect of physical sediments reworking on hydrocarbon degradation and bacterial community structure in marine coastal sediments.

The present study aimed to examine whether the physical reworking of sediments by harrowing would be suitable for favouring the hydrocarbon degradation in coastal marine sediments. Mudflat sediments were maintained in mesocosms under conditions as closer as possible to those prevailing in natural environments with tidal cycles. Sediments were contaminated with Ural blend crude oil, and in half of them, harrowing treatment was applied in order to mimic physical reworking of surface sediments. Hydrocarbon distribution within the sediment and its removal was followed during 286 days. The harrowing treatment allowed hydrocarbon compounds to penetrate the first 6 cm of the sediments, and biodegradation indexes (such as n-C18/phytane) indicated that biodegradation started 90 days before that observed in untreated control mesocosms. However, the harrowing treatment had a severe impact on benthic organisms reducing drastically the macrofaunal abundance and diversity. In the harrowing-treated mesocosms, the bacterial abundance, determined by 16S rRNA gene Q-PCR, was slightly increased; and terminal restriction fragment length polymorphism (T-RFLP) analyses of 16S rRNA genes showed distinct and specific bacterial community structure. Co-occurrence network and canonical correspondence analyses (CCA) based on T-RFLP data indicated the main correlations between bacterial operational taxonomic units (OTUs) as well as the associations between OTUs and hydrocarbon compound contents further supported by clustered correlation (ClusCor) analysis. The analyses highlighted the OTUs constituting the network structural bases involved in hydrocarbon degradation. Negative correlations indicated the possible shifts in bacterial communities that occurred during the ecological succession.