Biotransformation and dissolution of petroleum hydrocarbons in natural flowing seawater at low temperature

The objective of this study was to establish methods for controlled studies of hydrocarbon depletion from thin oil films in cold natural seawater, and to determine biotransformation in relation to other essential depletion processes. Mineral oil was immobilized on the surface of hydrophobic Fluortex fabrics and used for studies of microbial biodegradation in an experimental seawater flow-through system at low temperatures (5.9-7.4 degrees C) during a test period of 42 days. The seawater was collected from a depth of 90 m, and microbial characterization by epifluorescence microscopy, fluorescence in situ hybridization, and most-probable number analysis showed relatively larger fractions of archaea and oil-degrading microbes than in the corresponding surface water. Chemical analysis of hydrocarbons attached to the fabrics during the test period showed that n-alkanes (C10-C36) were decreased by 98% after 21 days, while naphthalenes were depleted by 99-100% during the same period. At the end of the period 4-5 ring polyaromatic hydrocarbon (PAH) compounds were removed by 82% from the fabrics. Analysis of the recalcitrant pentacyclic triterpane C30 17alpha(H),21beta(H)-hopane showed that the oil remained adsorbed to the fabrics during the test period. Comparison of depletion analysis with calculation of hydrocarbon dissolution in a flow-through system indicated that naphthalenes and smaller PAH compounds (alkylated 2-ring and 3-ring compounds) were removed from the fabrics by dissolution. The data further implied that depletion of n-alkanes and 4-5 ring PAH hydrocarbons were the result of biotransformation processes. PCR amplification of bacterial 16S rRNA genes from microbes adhering on the immobilized oil surfaces showed the dominance of a few bands when analysed in denaturing gradient gel electrophoresis (DGGE). Sequence analysis of DGGE bands revealed phylogenetic affiliation to the alpha- and gamma-subdivisions of proteobacteria and to the Chloroflexus-Flavobacterium-Bacteroides group.