99
pendant carbons on phenanthrene likely lowers the density below 1, while six substituents likely do the same for chrysene. Although GC/MS only usually detects
alkyl aromatics with up to four alkyl carbon substituents, the typical phenanthrene,
for example, is said to have a total of 18 alkyl carbons; presumably with such diversity that they are not individually resolved by gas chromatography (Cho et al. 2017).
The initial Macondo oil had a density of 0.8, but as the alkanes were degraded,
the average density increased. This was partially offset by the preferential degradation of parent PAH before alkylated forms, but the overall effect of biodegradation
was to preferentially remove the least dense hydrocarbons, leaving a denser residue
that would be neutrally buoyant or possibly sink. This is not a simple phenomenon.
While the buoyant density of large drops of oil led them to rise to the surface and
form a slick, the similar intrinsic buoyancy of smaller droplets was clearly overwhelmed by viscosity and turbulence to entrain them in the deep sea. Similarly,
even though intrinsically denser after the biodegradation of the alkanes and alkyl
aromatics, such tiny droplets may well have taken time to sink unless they became
entrained with dense material such as suspended sediment particles (Sørensen et al.
2014) or marine snow (Brakstad et al. 2018a; Gutierrez et al. 2018; Passow 2016).
As expected, oil residues in the sediment of the GoM within 5–8 km of the wellsite
are very biodegraded (Stout and Payne 2016; Stout et al. 2016; Bagby et al. 2017).
Quite likely some of the later biodegradation occurred after the partially degraded
droplets were incorporated in the sediment, perhaps even anaerobically (Kimes
et al. 2014), but the amount of oil residue was quite limited: if the residual oil is 80%
biodegraded as suggested in Stout et al. (2016), their subsequent quantitation of the
amount of oil residue in the sediment extrapolates to 4.4 g/m
2
(Stout et al. 2017).
Oil that promptly arrived at the surface formed large slicks, and biodegradation
was likely minimal while photooxidation began (Aeppli et al. 2012). The latter was
an important process for undispersed oil (Ward et al. 2018), but it is important to
remember that a major goal of the response was to encourage dispersion and hence
biodegradation—and most of the oil was dispersed as evidenced by its prompt disappearance from the surface after oil flow from the damaged well was stopped
(Houma ICP Aerial Dispersant Group 2010). It is not yet clear what effect photooxidation has on the biodegradability of the oil; the polymerization of the aromatics
(Prince et al. 2003) likely slows the process, but the unexpected generation of photooxidized saturates (Aeppli et al. 2012) may make them more bioavailable and
speed biodegradation. The chemical signature related to the photodegradation of
specific oil components is also helpful in tracing oil fate; for example, it can be
identified in areas such as sediments, helping to define the pathway that the oil took
before sedimentation.
An important ecological goal is to understand the food chain that develops after
the hydrocarbons are initially degraded. The bloom of oil-degrading microbes is
reasonably well characterized (Redmond and Valentine 2011; Dubinsky et al. 2013;
Yang et al. 2016; Hu et al. 2017), but we do not know what fraction of the hydrocarbon was mineralized to water and CO 2 and what fraction was assimilated to microbial biomass. Under nutrient-limited conditions, methane can be converted to
polyhydroxybutyrate with a yield of 45% on a weight basis (Helm et al. 2008), so a
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…
pendant carbons on phenanthrene likely lowers the density below 1, while six substituents likely do the same for chrysene. Although GC/MS only usually detects
alkyl aromatics with up to four alkyl carbon substituents, the typical phenanthrene,
for example, is said to have a total of 18 alkyl carbons; presumably with such diversity that they are not individually resolved by gas chromatography (Cho et al. 2017).
The initial Macondo oil had a density of 0.8, but as the alkanes were degraded,
the average density increased. This was partially offset by the preferential degradation of parent PAH before alkylated forms, but the overall effect of biodegradation
was to preferentially remove the least dense hydrocarbons, leaving a denser residue
that would be neutrally buoyant or possibly sink. This is not a simple phenomenon.
While the buoyant density of large drops of oil led them to rise to the surface and
form a slick, the similar intrinsic buoyancy of smaller droplets was clearly overwhelmed by viscosity and turbulence to entrain them in the deep sea. Similarly,
even though intrinsically denser after the biodegradation of the alkanes and alkyl
aromatics, such tiny droplets may well have taken time to sink unless they became
entrained with dense material such as suspended sediment particles (Sørensen et al.
2014) or marine snow (Brakstad et al. 2018a; Gutierrez et al. 2018; Passow 2016).
As expected, oil residues in the sediment of the GoM within 5–8 km of the wellsite
are very biodegraded (Stout and Payne 2016; Stout et al. 2016; Bagby et al. 2017).
Quite likely some of the later biodegradation occurred after the partially degraded
droplets were incorporated in the sediment, perhaps even anaerobically (Kimes
et al. 2014), but the amount of oil residue was quite limited: if the residual oil is 80%
biodegraded as suggested in Stout et al. (2016), their subsequent quantitation of the
amount of oil residue in the sediment extrapolates to 4.4 g/m
2
(Stout et al. 2017).
Oil that promptly arrived at the surface formed large slicks, and biodegradation
was likely minimal while photooxidation began (Aeppli et al. 2012). The latter was
an important process for undispersed oil (Ward et al. 2018), but it is important to
remember that a major goal of the response was to encourage dispersion and hence
biodegradation—and most of the oil was dispersed as evidenced by its prompt disappearance from the surface after oil flow from the damaged well was stopped
(Houma ICP Aerial Dispersant Group 2010). It is not yet clear what effect photooxidation has on the biodegradability of the oil; the polymerization of the aromatics
(Prince et al. 2003) likely slows the process, but the unexpected generation of photooxidized saturates (Aeppli et al. 2012) may make them more bioavailable and
speed biodegradation. The chemical signature related to the photodegradation of
specific oil components is also helpful in tracing oil fate; for example, it can be
identified in areas such as sediments, helping to define the pathway that the oil took
before sedimentation.
An important ecological goal is to understand the food chain that develops after
the hydrocarbons are initially degraded. The bloom of oil-degrading microbes is
reasonably well characterized (Redmond and Valentine 2011; Dubinsky et al. 2013;
Yang et al. 2016; Hu et al. 2017), but we do not know what fraction of the hydrocarbon was mineralized to water and CO 2 and what fraction was assimilated to microbial biomass. Under nutrient-limited conditions, methane can be converted to
polyhydroxybutyrate with a yield of 45% on a weight basis (Helm et al. 2008), so a
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…