Effects of Dispersant on the Petroleum Hydrocarbon Biodegradation and Microbial Communities in Seawater from the Baltic Sea and Norwegian Sea

Abstract

Dispersants have been used in several oil spill accidents, but little information is available on their effectiveness in Baltic Sea conditions with low salinity and cold seawater. This study investigated the effects of dispersant use on petroleum hydrocarbon biodegradation rates and bacterial community structures. Microcosm experiments were conducted at 5 °C for 12 days with North Sea crude oil and dispersant Finasol 51 with open sea Gulf of Bothnia and coastal Gulf of Finland and Norwegian Sea seawater. Petroleum hydrocarbon concentrations were analysed with GC-FID. Bacterial community structures were studied using 16S rDNA gene amplicon sequencing, and the abundance of genes involved in hydrocarbon degradation with quantitative PCR. The highest oil degradation gene abundances and oil removal were observed in microcosms with coastal seawater from the Gulf of Bothnia and Gulf of Finland, respectively, and the lowest in the seawater from the Norwegian Sea. Dispersant usage caused apparent effects on bacterial communities in all treatments; however, the dispersant's effect on the biodegradation rate was unclear due to uncertainties with chemical analysis and variation in oil concentrations used in the experiments.

Keywords: Baltic Sea; Norwegian Sea; bacterial community; dispersant; oil biodegradation; oil spill.

Figure 1

Map of sampling sites. Sampling locations in Tvärminne Zoological Station, UBS6 and Narvik are indicated with black circles.

Figure 2

Petroleum hydrocarbon (C₁₀-C₄₀) concentrations in microcosm experiments with different seawaters (A–D). Different dilutions used with WAF (without dispersant) and CE-WAF (with dispersant) are presented below the graph; also note that figures have different scales. Shown are mean values (n = 3), and error bars indicate standard deviation. Control samples consisting only of seawater analysed at the beginning of each experiment were always below the limit of detection (<100 µg/L).

Figure 3

Comparison of gas chromatograms from the control experiment for WAF (without dispersant, dilution 1:1) and CE-WAF (with dispersant, dilution 1:50) at the end of the 12 d experiment. Black color indicates normal 12 d WAF or CE-WAF sample, and blue color indicates abiotic control.

Figure 4

Gene copy numbers of alkB, PAH-RHDα (sum of gram-negative and gram-positive gene copies) and 16S rRNA genes for different experiments. Shown are mean values (n = 3), and error bars indicate standard deviation.

Figure 5

Principal coordinates analysis (PCoA) of microbial communities comparing different microcosm experiments. PCoA was based on a Bray–Curtis distance matrix using genera data.

Figure 6

PCoA of microbial communities comparing high and low oil level Gulf of Finland experiments. PCoA was based on a Bray–Curtis distance matrix using genera data.

Figure 7

PCoA of microbial communities in the Norwegian Sea experiment. PCoA was based on a Bray–Curtis distance matrix using genera data.

Figure 8

PCoA of microbial communities at the genus level in the Gulf of Bothnia experiment. PCoA was based on a Bray–Curtis distance matrix using genera data.

Figure 9

Linear discriminant analysis effect size (LEfSe) analysis of microbial communities comparing seawater from different experiments.

Figure 10

Heatmap of the top 20 most abundant bacterial genera in different experiments and treatments (average of three replicates). NS indicates the Norwegian Sea, GoB indicates the Gulf of Bothnia and GoF indicates the Gulf of Finland.

Figure 11

The relative abundance of the top 20 most abundant bacterial taxa at the genus level. Each bar in the figure is the average abundance calculated from three replicates for different experiments (Gulf of Bothnia, Norwegian Sea and Gulf of Finland with high and low oil concentration seawater).