Abstract
Subsurface dispersant injection (SSDI) is an operation tool to reduce oil surfacing during subsurface oil discharges. The dispersant treatment results in small-droplet oil dispersions and thereby enhances oil biodegradation in the marine water column. To study dispersed oil biodegradation and bacteria involved associated with degradation processes, we have developed a novel oil dispersion generator and carousel system. Studies in this system were performed with a crude paraffinic oil (premixed with dispersant) dispersed in natural seawater at 4-5°C. Oil dispersions were generated at defined concentrations (2-3 mg/L oil) and droplet size distribution (10-20 µm) in the oil dispersion generator, while incubation (4-5°C; 64 days) was performed in the carousel system to maintain the dispersions. Biotransformation was determined for saturates and aromatic hydrocarbons by GC-FID and GC-MS analyses, while microbial communities and metagenomes associated with oil compound biodegradation was investigated by 16S rRNA gene amplicon sequencing (Illumina MySeq). Flocculation was observed in the systems after 2-3 weeks of incubation. Typically, n-alkanes and BTEX were biotransformed during the first 2-4 weeks of incubation, while PAH compound transformation was transformed at slower rates. After 4 weeks of incubation the oil residues were predominated by unresolved complex mixture. Bacterial communities showed successions of Colwellia, Oleispira and Umboniibacter, which were associated with n-alkane degradation, followed by later abundances of Cycloclasticus and Marinospirillum during degradation of 2- to 4-ring aromatic hydrocarbon compounds. Closer examination of bacteria adhering to oil-seawater interfaces, using oil immobilized on hydrophobic adsorbents, showed that Oleispira was predominant on the oil surfaces during n-alkane biodegradation. Metagenome analyses resulted in successions of genes coding for alkane monooxygenase, followed by alcohol and aldehyde dehydrogenase encoding genes, associated with degradation of simple n-alkanes (nC25), aromatics and PAHs peaked in abundance. As the knowledge of microbial communities and genes involved in biodegradation of specific oil compounds is increasing, microbial community and metagenome/metatranscriptome analyses will become important tools to determine the state of biodegradation and the restitution of the environment after an oil spill, as well as the influence of oil spill response actions like SSDI.