117
(Schwarz et al. 1974). Sediment samples collected off the coast of Florida from a
depth of 4940 m were enriched in a mineral salts medium containing n-hexadecane,
and biodegradation was monitored at either atmospheric or elevated pressure
(51 MPa = 500 atm). Growth and degradation rates were comparable between the
two pressure treatments. However, the experimental conditions did not adequately
resemble the deep sea as initial enrichments were performed at room temperature.
Subsequently, Schwarz et al. conducted a similar enrichment culture experiment at
4 °C (Schwarz et al. 1975). Despite the lower cell density and the significantly lower
degradation rates (tenfold decrease) observed at high pressure, the observed total
amount of degraded n-hexadecane was similar between the two tested pressures
(Schwarz et al. 1975). Lowering the temperature amplified any effects of pressure
on the microbial metabolism and subsequently lowered the rates of hydrocarbon
degradation, indicating that low temperature and high pressure act synergistically to
slow the rates of hydrocarbon degradation. In these early studies, preincubation at
atmospheric pressure may have inhibited the growth and metabolism of microbial
populations that are adapted to high pressure.
Few studies of PHC biodegradation were conducted at pressures relevant to the
deep sea from the 1970s until the DWH spill. Since that time, accumulating evidence
points to a threshold in pressure effects, whereby hydrocarbon degradation is only
moderately affected at 10–15 MPa, and more dramatic effects are observed at
extremely high pressures approaching 50 MPa. Prince et al. (2016) reported that
hydrocarbon degradation rates diminished by approximately one-third at 15 MPa in
comparison to atmospheric pressure in incubations of seawater collected from a
depth of 8 m off the coast of Newfoundland (Canada). The Prince et al. (2016) study
is notable in that weathered crude oil (3 ppm) and dispersant (dispersant/oil ratio of
1:15) were amended at close to in situ concentrations expected during response
efforts. The majority of n-alkanes were degraded within the first 7 days of the incubation, while the aromatic fraction was only partially degraded by day 35 at 15 MPa
(>100 day half-life for aromatic fraction).
Results from seawater enrichments were corroborated by investigations of deep
sea sediments. Nguyen et al. (2018) incubated sediments collected from a range of
water depths (62–1520 m) in the Gulf of Mexico amended with crude oil at in situ
pressure (0.1–15 MPa) and temperature (4, 10, and 20 °C). The authors confirmed
an inhibitory effect of pressure on hydrocarbon degradation at a pressure of 15 MPa.
Total n-alkanes were degraded under all tested conditions following 18 days of
incubation; however the extent of biodegradation was inversely proportional to
pressure with greater degradation observed at shallower sites. For every 1 MPa of
pressure increase, a 4% decrease in the rate of alkane degradation was estimated
(Nguyen et al. 2018). Polycyclic aromatic hydrocarbons were also degraded but to
a smaller extent (3–60%) compared to the observed alkane degradation (90%).
Marietou et al. (2018) investigated the impacts of pressure on microbial communities along with hydrocarbon degradation in enrichments at in situ temperature (4 °C) and a range of pressures (0.1–30 MPa) in seawater samples collected
at 1070 m water depth from the Gulf of Mexico. Using the abundance of hydro7 Biodegradation of Petroleum Hydrocarbons in the Deep Sea
(Schwarz et al. 1974). Sediment samples collected off the coast of Florida from a
depth of 4940 m were enriched in a mineral salts medium containing n-hexadecane,
and biodegradation was monitored at either atmospheric or elevated pressure
(51 MPa = 500 atm). Growth and degradation rates were comparable between the
two pressure treatments. However, the experimental conditions did not adequately
resemble the deep sea as initial enrichments were performed at room temperature.
Subsequently, Schwarz et al. conducted a similar enrichment culture experiment at
4 °C (Schwarz et al. 1975). Despite the lower cell density and the significantly lower
degradation rates (tenfold decrease) observed at high pressure, the observed total
amount of degraded n-hexadecane was similar between the two tested pressures
(Schwarz et al. 1975). Lowering the temperature amplified any effects of pressure
on the microbial metabolism and subsequently lowered the rates of hydrocarbon
degradation, indicating that low temperature and high pressure act synergistically to
slow the rates of hydrocarbon degradation. In these early studies, preincubation at
atmospheric pressure may have inhibited the growth and metabolism of microbial
populations that are adapted to high pressure.
Few studies of PHC biodegradation were conducted at pressures relevant to the
deep sea from the 1970s until the DWH spill. Since that time, accumulating evidence
points to a threshold in pressure effects, whereby hydrocarbon degradation is only
moderately affected at 10–15 MPa, and more dramatic effects are observed at
extremely high pressures approaching 50 MPa. Prince et al. (2016) reported that
hydrocarbon degradation rates diminished by approximately one-third at 15 MPa in
comparison to atmospheric pressure in incubations of seawater collected from a
depth of 8 m off the coast of Newfoundland (Canada). The Prince et al. (2016) study
is notable in that weathered crude oil (3 ppm) and dispersant (dispersant/oil ratio of
1:15) were amended at close to in situ concentrations expected during response
efforts. The majority of n-alkanes were degraded within the first 7 days of the incubation, while the aromatic fraction was only partially degraded by day 35 at 15 MPa
(>100 day half-life for aromatic fraction).
Results from seawater enrichments were corroborated by investigations of deep
sea sediments. Nguyen et al. (2018) incubated sediments collected from a range of
water depths (62–1520 m) in the Gulf of Mexico amended with crude oil at in situ
pressure (0.1–15 MPa) and temperature (4, 10, and 20 °C). The authors confirmed
an inhibitory effect of pressure on hydrocarbon degradation at a pressure of 15 MPa.
Total n-alkanes were degraded under all tested conditions following 18 days of
incubation; however the extent of biodegradation was inversely proportional to
pressure with greater degradation observed at shallower sites. For every 1 MPa of
pressure increase, a 4% decrease in the rate of alkane degradation was estimated
(Nguyen et al. 2018). Polycyclic aromatic hydrocarbons were also degraded but to
a smaller extent (3–60%) compared to the observed alkane degradation (90%).
Marietou et al. (2018) investigated the impacts of pressure on microbial communities along with hydrocarbon degradation in enrichments at in situ temperature (4 °C) and a range of pressures (0.1–30 MPa) in seawater samples collected
at 1070 m water depth from the Gulf of Mexico. Using the abundance of hydro7 Biodegradation of Petroleum Hydrocarbons in the Deep Sea