305
abundance and expression of oil-degrading genes in near-surface seawater from the
vicinity of crude oil and natural gas production facilities off eastern Canada. This
again shows the stimulating effect of chemical dispersants on oil degradation.
A less pronounced effect was shown by microbial hydrocarbon enrichments
grown at 5 and 25 °C. Only slight increases in biodegradation were observed in the
presence of Corexit 9500A at both temperatures (Techtmann et al. 2017), with oil
biodegradation patterns consistent with those reported in the literature (i.e., aliphatics were degraded faster than aromatics).
18.2.3 Decreased Biodegradation of Oil Due to Dispersion
by Chemical Dispersants
The stimulating effect of chemical dispersants is often related to the enhanced dissolution of oil; a better bioavailability leads to a better biodegradation. However,
these smaller droplets also increase the dissolution of toxic oil compounds into the
water phase, such as BTEX.
Lindstrom and Braddock (2002) compared biodegradation of fresh, weathered,
or dispersed oil with and without the chemical dispersant Corexit 9500 and showed
that aromatic compounds and alkanes were most rapidly mineralized in fresh oil,
followed by weathered oil, and finally dispersed oil. However, when looking at
individual components of crude oil, variable effects of the presence of dispersed and
non-dispersed compounds were observed (Lindstrom and Braddock 2002).
However, especially with light, fresh oil, the acutely released toxic compounds can
in turn result in a toxic effect on oil-degrading bacteria, which might inhibit oildegrading bacteria.
This toxic effect was shown in a recent study by Rahsepar et al. (2016) using
pure bacterial cultures, fresh and weathered Macondo oil, and Corexit 9500A. When
the alkane degrader Rhodococcus qingshengii TUHH-12 was introduced to a
Corexit and fresh oil mixture, biodegradation inhibition was observed to occur for a
period of at least 50 days (the incubation time), whereas when added to Corexit the
weathered oil mixture, biodegradation was only inhibited for 10 days (Fig. 18.2).
The difference in inhibition may be attributed to the difference in chemical composition between crude and weathered oil. Chemical analyses showed that crude oil
had high concentrations of BTEX compounds (≈2000 μg/g), whereas weathered oil
contains negligible amounts of BTEX compounds (0.25 μg/g oil), and less light
aliphatic compounds (Rahsepar et al. 2016). The addition of Corexit 9500A to the
fresh oil results in higher concentrations of BTEX in the dispersed small oil droplets
and in the water phase (Prince 2015). Those higher concentrations of BTEX compounds in the water column inhibited the activity of the bacteria, thus explaining the
inhibitory effect observed for R. qingshengii TUHH-12. Since weathered oil is
devoid of toxic monoaromatic compounds, this inhibitory effect of high concentration of dissolved BTEX for R. qingshengii TUHH-12 did not occur in chemically
dispersed weathered oil. In follow-up experiments, both a BTEX-degrading culture
18 Effect of Marine Snow on Microbial Oil Degradation
abundance and expression of oil-degrading genes in near-surface seawater from the
vicinity of crude oil and natural gas production facilities off eastern Canada. This
again shows the stimulating effect of chemical dispersants on oil degradation.
A less pronounced effect was shown by microbial hydrocarbon enrichments
grown at 5 and 25 °C. Only slight increases in biodegradation were observed in the
presence of Corexit 9500A at both temperatures (Techtmann et al. 2017), with oil
biodegradation patterns consistent with those reported in the literature (i.e., aliphatics were degraded faster than aromatics).
18.2.3 Decreased Biodegradation of Oil Due to Dispersion
by Chemical Dispersants
The stimulating effect of chemical dispersants is often related to the enhanced dissolution of oil; a better bioavailability leads to a better biodegradation. However,
these smaller droplets also increase the dissolution of toxic oil compounds into the
water phase, such as BTEX.
Lindstrom and Braddock (2002) compared biodegradation of fresh, weathered,
or dispersed oil with and without the chemical dispersant Corexit 9500 and showed
that aromatic compounds and alkanes were most rapidly mineralized in fresh oil,
followed by weathered oil, and finally dispersed oil. However, when looking at
individual components of crude oil, variable effects of the presence of dispersed and
non-dispersed compounds were observed (Lindstrom and Braddock 2002).
However, especially with light, fresh oil, the acutely released toxic compounds can
in turn result in a toxic effect on oil-degrading bacteria, which might inhibit oildegrading bacteria.
This toxic effect was shown in a recent study by Rahsepar et al. (2016) using
pure bacterial cultures, fresh and weathered Macondo oil, and Corexit 9500A. When
the alkane degrader Rhodococcus qingshengii TUHH-12 was introduced to a
Corexit and fresh oil mixture, biodegradation inhibition was observed to occur for a
period of at least 50 days (the incubation time), whereas when added to Corexit the
weathered oil mixture, biodegradation was only inhibited for 10 days (Fig. 18.2).
The difference in inhibition may be attributed to the difference in chemical composition between crude and weathered oil. Chemical analyses showed that crude oil
had high concentrations of BTEX compounds (≈2000 μg/g), whereas weathered oil
contains negligible amounts of BTEX compounds (0.25 μg/g oil), and less light
aliphatic compounds (Rahsepar et al. 2016). The addition of Corexit 9500A to the
fresh oil results in higher concentrations of BTEX in the dispersed small oil droplets
and in the water phase (Prince 2015). Those higher concentrations of BTEX compounds in the water column inhibited the activity of the bacteria, thus explaining the
inhibitory effect observed for R. qingshengii TUHH-12. Since weathered oil is
devoid of toxic monoaromatic compounds, this inhibitory effect of high concentration of dissolved BTEX for R. qingshengii TUHH-12 did not occur in chemically
dispersed weathered oil. In follow-up experiments, both a BTEX-degrading culture
18 Effect of Marine Snow on Microbial Oil Degradation