Aarhus University Seal / Aarhus Universitets segl

No. 271: Arctic marine potential of microbial oil degradation

Wegeberg, S., Johnsen, A., Aamand, J., Lassen, P., Gosewinckel, U., Fritt-Rasmussen, J., Rigét, F., Gustavson, K. & Mosbech, A. 2018. Arctic marine potential of microbial oil degradation. Aarhus University, DCE – Danish Centre for Environment and Energy, 54 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 271 http://dce2.au.dk/pub/SR271.pdf

Summary

This review aims to collect and analyse relevant existing knowledge about microbial degradation of oil in the seawater around Greenland to add to the knowledge base for developing contingency plans and perform Net Environmental Benefit Analyses (NEBAs) for Greenland in relation to oil spill from oil exploration activities and shipping in Arctic waters. 

The main sources of oil pollution to the sea come from the offshore exploitation and transport of oil, but operational spills of oil from ships can also have a significant impact. Runoff from land, wastewater discharge and atmospheric deposition contribute to the load at regional scale. Furthermore, natural oil seeps in the ocean may act as a potential source at some locations. However, tanker accidents and blowouts are the most important sources of high-volume oil spills. 

In the accident in 1989 in Price Williams Sound of the tanker Exxon Valdez, 42 million liters of crude oil were leaked into the environment. Due to the naturally low nutrient levels at the site, biodegradation was enhanced by adding nutrients to the spilled oil. However, despite of intensive oil combat and clean-up, a fraction of the spilled oil was buried in the shoreline sediment in between stones and pebbles. Slow seeping of non-weathered oil from these oil pockets is assessed to be the source of continued oil pollution in some of the impacted areas, now twenty-five years after the incident. 

The Deepwater Horizon incident in the Mexican Gulf in 2010 is one of the world’s largest offshore oil spill accidents; ca. 780,000 m3 light crude oil were released into the marine environment from the seabed. It has been found that the oil dispersed in the water column was degraded by naturally occurring microorganisms in deep water (1,000 m) at temperatures of 4-5°C, which are not far from Greenland sea temperatures. The microbial flora needed for oil degradation was already present in the Mexican Gulf due to the numerous natural oil seeps in the seabed. Whether natural oil seeps in Greenland support a microbial flora capable of oil degradation is not yet known. 

Existing studies on oil degradation have primarily focused on the total removal of oil within certain boiling point intervals. Some studies have been supplemented with analyses of a few oil components such as benzene, toluene, ethylbenzene and xylenes (BTEX) and EPA16 (Polycyclic Aromatic Hydrocarbons, PAHs). Of the oil components, alkanes have the lowest toxicity and a relatively high biodegradability. However, the heavy aromatic fraction is more toxic and is slowly degradable. Results based solely on total oil may underestimate the toxicity of oil and overestimate the degradation of potentially toxic oil components in the environment. 

Oil consists of a very complex mixture of hydrocarbons, ranging from light gases to heavy residues. There can be millions of different hydrocarbons in oil. The hydrocarbon part of the crude oil mainly consists of straight and branched alkanes, cycloalkanes and aromatics, accounting for approximately 60-75%. These compounds are generally volatile and often easily degradable hydrocarbons. 

For biodegradation of oil in the marine environment, a combination of factors is important: 

  • Presence of a microbial organism capable of degrading the oil 
  • Sufficient nutrients for the degradation process to occur 
  • Possibly temperature. A microbial flora may be adapted to low temperatures but the general expectation is that low temperatures will lead to lower degradation rates. 

The presence of oil-degrading bacteria is a prerequisite for oil degradation to occur. The total number of bacteria in seawater is low compared with other environments such as soil or sediments. The number of oil-degrading bacteria constitutes only a fraction of the total number of bacteria in seawater. 

Although the number of oil-degrading bacteria is low in pristine water, the number may increase following an oil spill. The absence of specific oil-degrading bacteria may, however, limit oil degradation as was seen in experiments with seawater from Disko Bay where, for instance, only simple PAH degraders were found and in very low densities. It is highly likely that also more complex PAH degraders occur naturally in Arctic seawater, but in extremely low densities. 

The low temperatures along with low amounts of nutrients are limiting for oil degradation in the Arctic. However, in a recent laboratory study (5°C) with subsurface seawater from 150 m depth in Disko Bay, Greenland, rapid degradation of the alkane fraction of a crude oil was seen. The volatile alkanes were removed by both microbial degradation and abiotic processes, while the semi volatile and non-volatile alkanes were removed mainly by microbial degradation. In contrast, no degradation of either PAHs, dibenzothiophenes or alkyl-substituted homologue was observed during the 71-dayincubation period, which was explained by the pristine environment limited to bacteria adapted to degrade these structurally more complex molecules. However, slow degradation of the simpler PAHs naphthalene and 1-methyl naphthalene was recorded.  

Rapid degradation of crude oil was seen in Arctic seawater from the Chukchi Sea, Alaska, at an even lower water temperature of -1°C. About 60% of the total hydrocarbons were degraded within two months, and also PAHs and substituted PAHs were degraded, although slower than n-alkanes and branched alkanes. It was not possible, though, to distinguish biological oil degradation from physical removal processes like evaporation as non-biological controls were not included in the study. More rapid degradation of straight chain alkanes followed by branched alkanes and larger and alkylated aromatics was also observed in in Arctic sediment from Spitzbergen, Svalbard, and cold unpolluted seawater (~6.5°C) from the Trondheim Fjord, Norway.  

In seawater, one or more inorganic nutrients (e.g., nitrogen, phosphorus or iron) can become severely limiting to degradation processes, especially in the photic zone at times when the photosynthesising members of the trophic web sequester the limited nutrient supply. This may, however, to a certain extent be counterbalanced by grazing of the photosynthesizing organisms by other organisms by which the nutrients are recycled. Moreover, in general, mineralisation will take place and higher nutrient concentrations will exist below the photic zone. In some areas, nutrients can be brought to the photic zone by wind energy mixing and by upwelling. 

The knowledge presently available about natural degradation of oil under Arctic conditions shows a complex picture depending on, for example, oil type/components and environmental conditions. The marine environments of Greenland are also complex and consist of different water bodies characterised by different temperatures and nutrient levels. For instance, the East Greenland Current is cold and poor in nutrients compared with the West Greenland Current.   

In general, the potential for oil degradation in seawater depends on previous exposure of the environment to oil components. Following an oil spill, specific oil-degrading bacteria will proliferate until other factors such as nitrogen or phosphorus become limiting. Little is known about the time needed for growth of small degrader populations to the densities necessary for significant degradation to occur. This is especially the case for pristine environments such as the Arctic waters. A degradation potential may develop naturally due to exposure of oil components to hydrocarbons penetrating from the underground, the so-called natural oil seeps.  

Natural oil seeps are found in Greenland, especially in central West Greenland. It is thought that the microbial community in areas with seeps may have an increased hydrocarbon-oxidizing potential and that areas with a relatively high potential for microbial degradation of oil are also areas with relatively high temperatures and nutrient supplies. However, this warrants further study. 

The existing knowledge about biodegradation of oil in seawater in Greenland waters is presently limited to one study, which has direct focus on oil degradation in seawater in Disko Bay. 

Hence, to enhance our knowledge about the marine oil biodegradation potential in Greenland, it is recommended that future research should focus on: 

  1. getting a more generalised picture of the oil degradation potential in the seawater around Greenland. In the seawater from Disko Bay, limited degradation of aromatics compared with aliphatic hydrocarbons was seen, but it is unknown whether this is also the case for other type of localities around Greenland.
    The sea around Greenland is pristine and in general characterised by low temperatures and low levels of nutrients. There is a gap in our knowledge about bacterial adaptation to oil degradation under such conditions as well as on the timeframe of the adaptation processes. 

     2.  identifying Greenlandic localities showing similar oil-degrading characteristics.

The broad picture of water masses around Greenland shows distinct masses of different origin with different temperature and salinity characteristics as well as different nutrient richness. The biodegradability of different oil components in different water bodies around Greenland needs further study. 
It is unknown whether there is a priming effect on the microbial degrader community in the sea around Greenland in areas with natural oil seeps. 

3.   linking the degradation potential with environmental parameters and toxic effects.

We lack the mathematical models combining dispersal of oil with knowledge about degradation processes necessary for predicting the fate of oil in the Greenlandic marine environment.
An experiment has demonstrated that dispersants can exert a negative effect on microbial hydrocarbon degradation. There is a gap in our knowledge about the negative effects of dispersants on the degradation of oil in Arctic waters. 
Finally, increased toxicity of oil exposed to photochemical reactions is area topic deserving more research as such photochemical processes may be much more critical in Greenland during summer with all-day sunlight.