Biotransformation of low sulfur fuel oil HDME 50 in seawater at sub-arctic  temperatures
Current oil spill response technologies include mechanical recovery, in situ burning and the use of oil dispersant. The use of dispersant enhances the dissolution of oil into the water by breaking it into smaller droplets, decreasing the oil accumulation at the water surface and reducing the amount of oil which may be drifted to shore. Biodegradation of dispersed oil can be fast and extensive if the oil is present at the ppm levels following a successful application of dispersants. Some studies however, also found that biodegradation might be inhibited by dispersants. Since using dispersant can be an effective method to enhance removal of spilled oil even in cold marine condition, biodegradation of dispersed oil in seawater at low temperature has gained much focus internationally . Biodegradation of dispersed oils has been demonstrated at temperature from 5 to -6 °C, which usually completed within several months.  The sulphur emission control areas (SECA) have been introduced in Europe since January 2015, to reduce harmful sulphur oxide (SOx) emissions from ship leading to the limit of sulfur content in ship fuel oil to 0.1%. In addition, the global regulations (MARPOL2020) came into effect from January 2020, which set the sulfur limit for fuel oil to 0.5% for sea areas outside SECA. To meet those requirements, the ships can switch to higher quality fuel oil with lower sulfur content. A new generation of marine fuel oil so called ultra-low sulfur fuel oil (ULSFO) or hybrid fuel oil, have been developed and used increasingly within and outside SECA. The hybrid fuel oils (LSFO) which has less than 0.1% sulfur tend to be rich in waxes, resulting in high pour point (). High waxes content and high pour point affect the dispersibility of the oil at low temperature presenting challenge for their removing from marine environment at low temperatures. Since LSFO has been used only recently, very little is known about the biodegradation of LSFO in general and of the dispersed LSFO in particularly. Knowledge about the biodegradation potential of LSFO is important knowledge in oil spill preparedness. In this study, we used Hybrid Wide Range Gas Oil (HDME 50), one of the few LSFOs which are available on the market. HDME 50 has been described as poorly dispersed naturally and after treatment with dispersants at temperatures below 5 °C. The oil however showed dispersibility at 13 °C. We therefore investigate the biotransformation of the chemically dispersed HDME 50 at 13 °C and non-dispersed oil film at 5 and 13 °C, a typical Norwegian summer and winter temperature in seawater.