In general, biodegradation of oil compounds is expected to follow the order n-alkanes > branched alkanes > low molecular weight aromatics > cyclic alkanes (Perry 1984). In cold seawater the same order is expected, although degradation will be highly influenced by the physico-chemical characteristics of the oil. The low temperature affects both dissolution from the non-aqueous (crude oil) to the aqueous phase (Schluep et al. 2001), and evaporation of volatile compounds, as described above.  

At temperatures above the freezing point of seawater (approximately -1.8°C) biodegradation of crude oil hydrocarbons is well documented. This is exemplified in Figure 5-2, showing the mineralization curves of ¹⁴C-labelled naphthalene, phenanthrene and hexadecane in seawater at 0°C when the compounds were spiked into crude paraffinic oil. Degradation of the n-alkane (hexadecane) was faster than for the aromatic compounds, and a smaller aromatic (naphthalene; 2-ring) degraded faster than larger aromatics (phenanthrene; 3-ring). This pattern followed the generally accepted order of crude oil compound biodegradation described above.

Figure 5-2. Mineralization in seawater at 0°C of 14C-labelled hydrocarbons spiked into crude oil (from Brakstad and Bonaunet 2006). No mineralization was measured in sterile controls (not shown).

One of the first attempts to study oil biodegradation in Arctic seawater at low temperatures (2-11°C) showed that shifts in microbial populations towards more oil-degrading bacteria, that abiotic oil losses were lower than expected, and that various classes of hydrocarbons (saturates, mono-, di- and polyaromatics) were subject to biodegradation (Horowitz and Atlas 1977). Several studies have compared oil biodegradation in seawater or with bacterial cultures at different temperatures, and results from some of these including temperatures relevant for the Arctic are summarized in Table 5-1.

In summary, these and most other relevant studies (e.g. MacNaughton et al. 2003) show slower biodegradation by lowering of the temperature, but the results also show that biodegradation at low seawater temperature is considerable. In a recent study with low concentrations (2.5 mg/L) of Alaska North Slope oil with Atlantic seawater, 80% was biodegraded (saturates, 2- to 4-ring aromatics) after 60 days at 8°C (Prince et al. 2012). While laboratory studies indicate that biodegradation in Arctic seawater may be slower than in temperate seawater, these results have not been confirmed by field studies. Seasonal biodegradation data and comparison of oil biodegradation from different geographic areas with the same oils and analytical procedures may be necessary to test these assumptions. Oil characteristics should also be addressed in more detail, for instance, by comparison of dispersed oil biodegradation of different oil types and weathering degrees at several seawater temperatures.  The physical properties of oil may decrease bioavailability of oil (e.g. larger droplets at lower temperatures would increase the surface area-to-volume ratio).

Table 5-1. Summary of selected biodegradation studies performed at different seawater temperatures 

^A) k1 is first-order rate coefficient; t_½ is half-life (0.69/k1) ^B) Reduction determined by comparison to sterile controls

Several biodegradation studies of oil in Arctic sediments have been conducted, most of these to investigate the potential for bioremediation of stranded oil in the Arctic (see later chapter). Studies on oil pollution of Arctic and Antarctic beaches has demonstrated the presence of indigenous hydrocarbon-degrading bacteria in these pristine environments (e.g. Grossman et al. 2000; Delille and Delille 2000; Powell et al. 2005). Oil removal from beach sediments may be attributed to several processes, including physical removal, photooxidation and biodegradation. For instance, significant depletion of total hydrocarbon concentrations and mineralization of radiolabelled hexadecane have been measured in Canadian Arctic soils at 4°C (Greer 2008). Anaerobic biodegradation has also been measured in the Arctic. Low-temperature degradation of PAH-compounds was reported from Arctic soils under anoxic and nitrate-reducing conditions at 7°C (Eriksson et al. 2003). In the Arctic winter the upper parts of marine sediments become frozen.  Whether biodegradation stops or continues at very slow rates under these conditions is not known, although microbial activity at subzero temperatures has been demonstrated (Doyle et al. 2012).  Several studies with oil-contaminated freeze-thaw cycled soil or permafrost have shown that microbial respiration takes place even at subzero temperatures and hydrocarbon degradation was observed (Rike et al. 2003; Børresen et al. 2007; Chang et al. 2011).  These studies therefore demonstrate that the lower limit for biodegradation can be below the freezing point.

If oil spills reach the marginal ice zone, the ice may become oil-infested. Once trapped within the ice, ocean currents can transport the oil over large distances.  A secondary discharge situation occurs during the spring melt season and, if the ice has been transported from the original spill site, this can result in contamination of new locations. In the spring and summer seasons, chemical alteration of the crude oil through photooxidation may also become an important process (Refer to Section 3).  Although the immediate impact of oil spills in ice has been studied (e.g. Fingas and Hollebone 2003) and is fairly well understood, little is known about the long-term fate and effects of such pollutants on ecosystems in polar environments.  To date, few studies have attempted to determine the transport and fate of individual water-soluble oil components in sea ice.  However, data from some recent studies have shown that the more water-soluble compounds (mainly naphthalenes, phenanthrenes and dibenzothiophenes) migrate through the brine channels in the ice (Figure 5-3).  As a result, such compounds come into contact with sea ice microbes in the brine and the underlying water (Faksness and Brandvik 2008a; Faksness and Brandvik 2008b).

In line with the results from studies with Arctic soils one should expect that biodegradation may also take place in marine ice at subzero temperatures. As described earlier, microbial metabolism and motility have been measured in the brine channels of marine ice (Breezee et al. 2004; June et al. 2002; Junge et al. 2003; Junge et al. 2004; Junge et al. 2006; Mykytzuk et al. 2013).  However, biodegradation of oil in marine ice has not yet been fully investigated. In a winter field study (February to June) performed on Svalbard with crude oil frozen into fjord ice, a slow reduction in the ratio between naphthalene and phenanthrene was measured in the parts of the ice with downward migration of soluble compounds, while no significant change in n-C₁₇/Pristane was measured, as shown in Figure 5-3 (Brakstad et al. 2008). However, the bulk oil stimulated bacterial biomass, including a few bacterial genera expected to be oil-degraders (Brakstad et al. 2008). The results from another field study performed at Svalbard showed that no significant degradation of oil hydrocarbons occurred in the ice at subzero temperatures, but at 0°C melt pool oil samples fertilized with inorganic nutrients showed a significant change in bacterial diversity (Gerdes and Dieckmann 2006). Marine ice represents an extreme environment for life.  The combination of low temperature and high salt content in the brine channels require that microbes be both halo- and psychro-tolerant.  Extremely halophilic or halotolerant microbes able to degrade oil have been reported (e.g. Diaz et al. 2002, Al-Mailem et al. 2010), but not so far in cold environments.  However, as described above, it has been demonstrated that oil pollution in marine ice may stimulate the growth of a few specific bacteria (Brakstad et al. 2008), but the ability to degrade oil compounds needs to be clarified. In addition, most oils will also be solidified under these conditions, but the migrating water-soluble compounds may be relevant target compounds for oil-degrading bacteria in this environment. If this is true, bacteria able to degrade small aromatics may be more relevant than alkane-degrading bacteria. 

Figure 5-3. Oil migration and degradation in marine ice. [The middle core shows the migration of different oil components through an ice core with the chromatograms of the components in the left panel and the relative ratios of specific components in the right panel (from Brakstad et al. 2008)].