Effects of the 2023 Poole Harbour oil spill on sediment bacterial communities and ecosystem functioning

In March 2023, approximately 27 t of fluid from an oil and gas reservoir (containing approximately 85 % water and 15 % crude oil spilt from a fractured pipeline beneath Ower Bay creek, entering Poole Harbour (Dorset, UK). This event provided a unique opportunity to investigate the impacts of hydrocarbon contamination on microbial communities in-situ in a temperate coastal, shallow, fine sediment environment. Our aims were to quantify hydrocarbon concentrations (via gas chromatography mass spectrometry (GC-MS)) and effects on microbial community structure and functional potential (via metagenomic sequencing) to understand the capacity for microbial biodegradation across the impacted region. Hydrocarbon contamination was localised to the Spill Site (approximately an area of 1500 m²) at the head of the creek, with minimal impact at the Mid Point (164 m from the Spill Site) and End Point (387 m from the Spill Site) and with no indication of contamination at Brownsea Island located in the heart of the harbour. By October 2023, n-alkane and 4-5 ring PAH concentrations had declined to background levels, highlighting the combined effects of the remediation response and natural hydrocarbon biodegradation at the Spill Site. Clear changes in bacterial community structure were observed in the seven months following the spill, with notable hydrocarbon-degrading bacteria i.e. Anaerolinea, Thiobacillus and Dechloromonas favouring the Spill Site, suggesting a significant increase in anaerobic biodegradation occurred as a result of significant increase in assA (anaerobic alkylsuccinate synthase), abcA (anaerobic benzene carboxylase) and ahyA (anaerobic alkane hydroxylase) genes. Overall, 24 alkane and aromatic hydrocarbon degradation genes, from both aerobic and anaerobic degradation pathways, were identified from contigs throughout the study site, being present within 48 out of 221 Metagenome-Assembled Genomes (MAGs), highlighting the sites capacity for hydrocarbon biodegradation under both aerobic and anaerobic conditions.