Temperature Effects on Biodegradation of Dispersed Crude Oil: Degradation Rates and Microbial Communities Associated with Degradation of Oil Droplets
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Depending on the weather- and sea conditions, oil spilled at sea may disperse into the water column. Dispersion of oil promotes biodegradation of oil hydrocarbons (HCs) due to an increased surface-to-volume ratio. In this study oil was immobilized on hydrophobic Fluortex adsorbents to simulate dispersed oil, by generating a thin oil film of the same thicknesses as the diameter of a small oil droplet. Adsorbents were used to investigate microbial communities associated with attachment and degradation of dispersed oil. Immobilized oil was incubated in seawater at four different temperatures (0, 5, 10 and 20oC) during a test period of 56 days. The seawater was collected from a depth of 90 meters, holding a constant temperature of ~5oC. Biodegradation of the saturated oil fraction immobilized on Fluortex adsorbents were quantified using gas chromatography, and biodegradation rates of the total amount of oil (C10- C36)(TEM), six selected pseudo-oil component groups and n-alkanes were determined. Microbial communities attached to the oil surfaces on the adsorbents during the degradation period were characterized by polymerase chain reaction (PCR) amplification of bacterial 16S rDNA gene fragments and denaturing gradient gel electrophoreses (DGGE) analysis. Chemical analysis of oil compounds showed that biodegradation was affected by temperature. Highest biodegradation rates were observed at 20oC, significantly lower rates were observed at 0oC. Degradation rates of TEM and the pseudo-oil components at 5oC were similar to rates at 10oC, whereas degradation rates of n-alkanes at 5oC were higher than rates at 10oC. Degradation analysis of the pseudo-oil component groups and n-alkanes showed that the higher molecular weight fractions of the oil were affected more by a decrease in temperature than the lower molecular weight fractions. The results indicated that temperature affects HC bioavailability, and that increased temperature does not necessarily increase biodegradation rates. Analysis of bacterial communities showed that oil degraders successfully attached to, and colonized, the oil-coated adsorbents. The community composition changed dynamically during the incubation period, at all incubation temperatures. Number of operational taxonomic units (OTUs) increased during the same period. An increase in bacterial diversity was observed after depletion of n-alkanes. OTUs dominant in the early phases of biodegradation are suspected to be n-alkane degraders, whereas bacteria appearing in the later part are suspected to be associated with degradation of the aromatic and heavier fractions of the oil.