Short-term effects of diesel oil and dispersant on the Baltic Sea surface water microbiome in coastal areas and open sea

The Baltic Sea is a geologically and biologically unique sea highly vulnerable to environmental hazards, and the most emphasized threat is the risk of oil spills. The microbiology of the Baltic Sea has not been extensively studied, and most studies have focused on bacteria, leaving archaea and fungi to less attention. In addition to the natural microbial communities of different parts of the Baltic Sea, the effects of diesel oil, and dispersants applied in case of an oil spill, on these microbial communities is yet to be elucidated. The focus of this Master’s thesis was to compare the bacterial, archaeal and fungal community compositions of the Baltic Sea surface water at three distinct locations; the open sea, a pristine archipelago, and putatively oil contaminated coastal water at an oil refinery. In addition, the short-term effects of diesel oil and dispersant on the three locations were studied during a 72-hour microcosm experiment. Next-generation Ion Torrent sequencing of bacterial V3–V4 and archaeal V4 regions of 16S rDNA and fungal ITS regions was used for a community composition analysis. Quantitative polymerase chain reaction (qPCR) was applied to determine the changes in the copy numbers of bacterial 16S rRNA genes and two genes associated with microbial hydrocarbon degradation, i.e. ring-hydroxylating dioxygenases (RHD) and alkane hydroxylases (AlkB). Based on the findings, the three sites under investigation harbored differing surface water microbiomes demonstrating differing responses to diesel and dispersant amendments, and furthermore, the results indicate that the putatively oil contaminated coastal site has higher natural petroleum hydrocarbon degradation potential compared to the pristine archipelago and especially the open sea. It is noteworthy, that over 90% of the fungal sequences from the open sea and the pristine archipelago, and over half of the fungal sequences from the putatively oil contaminated coastal site were unidentified even at phylum level. In addition, almost half of the archaeal sequences from the putatively oil contaminated site were unindentified. Assessing the petroleum hydrocarbon degradation potential of the indigenous microbiome in different parts of the Baltic Sea is of great importance, since the data can potentially be utilized when developing suitable biological oil spill response methods as well as predicting the rates of petroleum hydrocarbon degradation in different parts of the Baltic Sea area