The effects of temperature and ice on biodegradation of crude oil
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Summary of thesis: This thesis investigates the effect of seawater temperature and presence of ice on biodegradation of oil. The work was done as a part of the project “Fate, behaviour and response to oil drifting into scattered ice and ice edge in the marginal ice zone”. As ice extent in the Arctic is declining, development and ship traffic will increase, with higher risk of oil spills to the marine environment. Oil drifting into ice may become trapped, or freeze into, the ice together with sea bacteria surriving in the brine channels of the ice matrix. With this scenario in mind, the main goal of this thesis has been to investigate and determine to what extent crude oil is degraded by bacteria at low temperature, and in the presence of different types of ice. The work has focused on both the microbial community dynamics during the degradation period, and on the chemical loss of oil components due to biodegradation. To achieve the goals of this thesis, four laboratory experiments were performed. In all the experiments biodegradation was determined as biotransformation by chemical analysis (GC-FID and GC-MS), whereas changes in the microbial communities were investigated by 16S rDNA amplicon sequencing, to examine the dynamics of the biodegradation process. First, the effect of decreasing seawater temperature, from 20 to 0°C, on biodegradation of oil was investigated, by examining the loss of immobilized oil, and the dynamics of the microbial community attached to the oil-water interface. The results of this study showed that half-lives of n-alkenes increased with increasing chain length, especially at 0°C. Onset of degradation was delayed by reduced seawater temperature, and temperature selected for different key n-alkane degraders, becoming dominant at different incubation temperatures. Next, the effect of newly formed frazil ice on biodegradation of oil was studied at a temperature of -2°C. Frazil ice was mixed with seawater and dispersed weathered oil, to assess the effect of ice on the degradation of oil and microbial community composition. At an incubation temperature of -2°C, the total amount of oil was not degraded to any significant degree, neither in the presence of frazil ice nor in sea water without ice. However, target n-alkanes and PAHs were biotransformed. While low bioavailability may be the major cause of slow degradation, the difference of half-lives of target compounds between treatments could be a result of differences between the microbial communities, as presence or absence of frazil ice selected for different microbes. In the third experiment, degradation of oil frozen into ice was studied at -10°C, aiming at determining what will happen if oil freeze into solid ice. The most water-soluble oil components, mainly naphthalene, were distributed into the brine fraction of the solid ice. No oil was degraded during the experimental period of 181 days, and the microbial community in sea ice with oil were not different from sea ice without oil. In the last experiment, dispersed weathered oil frozen into solid ice was melted (after 200 days of incubation as at -10°C) and the degradation potential of the microbial community released with the melting ice, together with oil, was studied at 4°C. Both total amount of oil and target components were degraded by the microbial community from the melted sea ice. However, oil released from melted sea ice was degraded at a slower rate compared to oil incubated in fresh seawater at the same temperature (4°C), by a different microbial community. Overall, the results from these experiments showed that half-lives of oil decreased with decreasing seawater temperature, and in the presence of ice. Based on the results from this PhD study, it can be expected that the total amount of dispersed oil frozen into ice would persist throughout the icy season, as no oil degradation was detected in solid ice. Therefore, from a biodegradation perspective, oil freezing into ice is not beneficial. Oil will be degraded when released when the sea ice melts, but the biodegradation rate is higher if oil is not in frozen into ice in the first place.