Aerobic biodegradation of crude oil from the Deepwater Horizon oil spill

Much research has been done on biodegradation of crude oil, in particular in the marine environment (e.g. Atlas 1981, Colwell and Walker 1977, Head et al. 2006, Leahy and Colwell 1990, Yakimov et al. 2007). Since 2010, the number of studies, especially in context of the DHW incident, is rising (e.g. Bælum et al. 2012, Hazen et al. 2010, Kessler et al. 2011, Kleindienst et al. 2015a, Passow 2014).

In the DWH spill, oil-adapted indigenous microorganisms responded rapidly to the oil and thus played a significant role in reducing the environmental impact of the oil (Atlas and Hazen 2011). From the discharged hydrocarbons, probably 43 to 61% have been microbially oxidised (Joye 2015). A substantial proportion of hydrocarbons in the plumes was converted to biomass (about 0.8 to 2x10¹⁰ mol carbon) (Shiller and Joung 2012), resulting in bacterial blooms. These blooms, which indicate that indigenous oil-degrading bacteria were enriched by the high supply of released hydrocarbons in the oil plumes, were observed in the months following the DHW accident (Bælum et al. 2012, Hazen et al. 2010, Kessler et al. 2011, Redmond and Valentine 2012, Valentine et al. 2010 and 2012).

The biodegradation rates of crude oil and gaseous hydrocarbons in the plumes were debated (Daley et al. 2016). While Camilli et al. (2010) suggested very low biodegradation of the hydrocarbon plume (requiring many months), Hazen et al. (2010) suggested fast hydrocarbon biodegradation at 5°C (oil half-lives in order of days) and reported high cell densities in the plume compared to outside the plume. Hazen et al. (2010) gave several reasons for this: (1) The oil from the DWH blowout was light crude oil, which can be more readily biodegraded than heavy crude oil, (2) the particle size of the oil droplets dispersed in the deep plume was small and (3) an oil-adapted bacterial community was already stimulated by oil leaks from natural deep-sea seeps in the GoM. Similarly, Kimes et al. (2014) and King et al. (2015) concluded in their reviews that the overall response of the microbial community to the oil and gas was rapid and robust.

Corexit® was found to have differing effects on the biodegradation rates. For instance, Bælum et al. (2012) found no negative effects of Corexit® EC9500A on growth of indigenous bacteria and an improved oil degradation in enrichment experiments. Kleindienst et al.

(2015a) reported that crude oil biodegradation of a microbial community was either suppressed or not stimulated when dispersants were added. Overholt et al. (2016),

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however, found both dispersants-induced inhibition as well as dispersants-induced stimulation of oil degradation and growth of certain model oil degraders. Corexit® EC9500A is a mixture of hydrocarbons (50%), glycols (40%) and dioctylsulfosuccinate (DOSS) (10%) (Bælum et al. 2012). These components can be degraded as well(Bælum et al. 2012, Campo et al. 2013, Chakraborty et al. 2012, Kleindienst et al. 2015a, Lindstrom and Braddock 2002, Lindstrom et al. 1999, Overholt et al. 2016).

The oil plumes were found to be associated with a decrease in dissolved oxygen concentration (oxygen anomaly), which was supposed to be caused by microbial respiration during the hydrocarbon degradation (Hazen et al. 2010, Joye et al. 2011b). Kessler et al.

(2011) reported that within 120 days a bloom of methanotrophic bacteria in the deep sea metabolised almost all the released methane and that this event was accounting for the anomalous oxygen depression in the plume. However, this interpretation was subject of debate (Crespo-Medina et al. 2014, Joye et al. 2011a). Other gases, such as ethane and propane, were also degraded rapidly in the plume (King et al. 2015). Valentine et al. (2010) reported that rapid microbial respiration of propane and ethane, mainly by Colwellia (Redmond and Valentine 2012), was responsible for up to 70% of the oxygen depletion and that these hydrocarbon gases were the primary drivers of microbial respiration early in the spill.

1.2.4.1 Succession of the bacterial community composition

The bacterial community composition in the deep-sea plumes as well as other GoM locations changed over time and space in response to the varying oil composition and quantity (see Figure 1.8) (Atlas and Hazen 2011, Dubinsky et al. 2013, Kimes et al. 2014).

The communities were dominated by a few types of Gammaproteobacteria (Dubinsky et al.

2013, Hazen et al. 2010, Redmond and Valentine 2012, Valentine et al. 2010).

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Figure 1.8: Changes in dominant members of the GoM microbial communities in response to the DWH oil spill (figure from Kimes et al. 2014).

During unmitigated flow of crude oil from the wellhead early in the spill, from end of May to beginning of June 2010, which resulted in high concentrations of n-alkanes and cycloalkanes, the dominating taxa in the deep-sea plumes were Oceanospirillales and Pseudomonas, which are alkane degraders (Dubinsky et al. 2013, Hazen et al. 2010, Mason et al. 2012, Redmond and Valentine 2012). In early June 2010, hydrocarbons were partially captured at the wellhead, hydrocarbon concentrations decreased and the amount of BTEX relative to alkanes increased. During this time, there was a shift in the plume community to dominance of Colwellia, Cycloclasticus, Pseudoalteromonas and Thalossomonas, which are capable of degradation of hydrocarbon gases (propane and ethane) or degradation of aromatic hydrocarbons (Dubinsky et al. 2013, Redmond and Valentine 2012, Valentine et al.

2010). After the well shut-in in mid-July 2010, the community in the dissolved oxygen anomaly of the water column was dominated by methylotrophs of the taxa Methylococcaceae (methane oxidisers), Methylophaga and Methylophilaceae (both secondary consumers of C1 compounds) (Kessler et al. 2011, Kimes et al. 2014), as well as

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Flavobacteria, Rhodobacteraceae and Alteromonadaceae, which are degraders of high molecular weight hydrocarbons and of complex organic matter (Dubinsky et al. 2013, Kessler et al. 2011). They probably scavenged organic matter and cell biomass from the decaying bacterial bloom (King et al. 2015).

Similarly, the bacterial community in the oil-contaminated deep-sea sediments responded to the oil from the DWH blowout. Mason et al. (2014) found highly oil-contaminated surface sediments to be most abundant with an uncultivated Gammaproteobacterium and a Collwellia species. In contrast, Liu and Liu (2013) found mainly Gammaproteobacteria (Methylococcus, Vibiro and Pseudomonas), Alphaproteobacteria (Methylobacterium), Flavobacteria and Acidobacteria.

Oiled coastal sands were also dominated by members of the class Gammaproteobacteria, such as Alcanivorax, Marinobacter, Pseudomonas and Acinetobacter (Kostka et al. 2011). Oil-contaminated coastal salt marshes were found to be dominated by Proteobacteria, Bacteroides, Actinobacteria and Firmicutes (Beazley et al. 2012).

In oil-contaminated surface waters a dominance of Gammaproteobacteria (including Marinobacter, Alcanivorax, Pseudomonas and Alteromonas), Alphaproteobacteria and Cyanobacteria was reported (Liu and Liu 2013, Redmond and Valentine 2012). However, Yang et al. (2014) found Gammaproteobacterium Cycloclasticus to be dominant in surface-water samples.

In conclusion, oil acted as a strong selective force to stimulate particular, specialised, oil-degrading bacteria and reduced the community diversity (Head et al. 2006). The response of bacterial communities to the oil probably depended on the respective environmental conditions (Liu and Liu 2013).