The effects of marine oil pollution and remediation strategies on microbial community structure and function

Due to the demand for oil consistently increasing over the past decade (currently approximately 105 million barrels per day), so does the risk of open water oil spills. Thus the need for strong remediation strategies has become much more apparent. The present thesis thus aims to bring to light the impact remediation responses and marine oil pollution on microbial community structures and functionality. As a considerable point of debate chemical dispersants were the main focus of these studies, investigating their effectiveness in enhancing biodegradation processes and their impact on ecological processes such as marine snow formation and microbial community composition. Due to a spill occurring during the writing of this thesis focus on other remediation strategies and the impact of an in-situ spill on microbial community structure and function was also implemented to highlight the effectiveness and usefulness of metagenomic analysis for post spill monitoring in-situ. As an important tool for mitigating the environmental impacts of marine oil pollution, the evaluation of chemical dispersant effectiveness in enhancing biodegradation under increasing oil concentrations was the main focus of Chapter 2. As oil concentrations increased the percentage of hydrocarbons degraded over time significantly declined, however, the rate of degradation only declined at oil concentrations of 1000 mg L-1. With the addition of dispersant use degradation rate across all concentrations was enhanced. Bacterial communities were also shown to be impacted, notably with declines in Alteromonadaceae and Cellulophaga as oil concentrations increased above 100 mg L-1 compared to lower concentration of oil (1 – 10 mg L-1) . Another point of debate surrounding dispersant use was marine oil snow (MOS) formation, thus chapters three and four focused on MOS formation and their associated microbial communities under surface spill conditions. Using the ecological index of hydrocarbon exposure floc formations were shown to be sites of high hydrocarbon degradation potential, with upwards of 50% of the community being associated with the potential to utilise hydrocarbons as a viable carbon source. Observations of the bacterial succession of flocs further highlighted their hydrocarbon degrading potential with a clear succession over time from Oleispira dominated communities to a highly diverse community of obligate hydrocarbonoclastic bacteria and other generalistic potential degraders. This study also indicated microbial eukaryotes (algae, protists) as excellent biomarkers for hydrocarbon exposure and that they should also be considered alongside bacterial biomarkers for future spill monitoring models. Chapter four provided the opportunity to explore the impacts of the in-situ Poole Harbour 2023 spill on microbial communities and their functionality. Impacted communities within the Spill Site indicated potential biomarkers for hydrocarbon contamination such as Thiobacillus and Dechloromonas. Further analysis showcased the creeks capacity for hydrocarbon degradation through the presence of 24 hydrocarbon degrading genes targeting both alkane and aromatics and indicated an uplift in the presence of anaerobic hydrocarbon degradation genes at the spill-site compared to the rest of the creek and control site. This thesis highlights the effectiveness of remediation in mitigating the effects of marine oil pollution, whilst emphasising the environmental considerations that must be made when performing laboratory experiments. This thesis progresses our understanding of microbial and hydrocarbon interactions to improve biomonitoring techniques and decision-making via the Net Environment Benefit Analysis (NEBA).