113
Romero et al. 2017). Similar to planktonic environments, the biodegradation of
hydrocarbons in deep sea sediments is controlled by the abovementioned suite of
environmental parameters including temperature, pressure, oxygen and nutrient
availability, and physical or chemical form of the oil (Fig. 7.1) (Head et al. 2006).
Oxygen is consumed rapidly by microbial activity in sediments, and it is resupplied
slowly by diffusion or intermittently by bioturbation in the deep sea. Thus, sediments differ in that petroleum hydrocarbons (PHCs) become buried in an environment that is limited in the supply of oxygen and other electron acceptors. In addition,
rates of biodegradation were shown to inversely correlate with oil concentration,
and since sediments serve as a repository for oil deposition, oil concentrations are
elevated, and slower rates are to be expected (Prince et al. 2017).
Understanding of the metabolic pathways of hydrocarbon degradation under
anoxic conditions remains in its infancy (Meckenstock et al. 2016), and this represents a key knowledge gap for predicting the long-term fate of recalcitrant oil compounds in fine-grained sediments that cover much of the seafloor (Shin et al.
2019; Shin 2018). Activation of hydrocarbon molecules during degradation is
energetically demanding for microorganisms in the absence of oxygen, and thus
anaerobic hydrocarbon degradation is much slower in comparison to aerobic
metabolism. Since aerobic hydrocarbon-degrading bacteria preferentially utilize
short-chain n-alkanes or simple polycyclic aromatic hydrocarbons (PAHs) more
readily, PHCs buried in anoxic sediments tend to be enriched in more recalcitrant
compounds, further exacerbating degradation processes (Shin et al. 2019; Shin
2018). Once buried in the anoxic zone, microorganisms metabolize hydrocarbons
through respiration pathways using nitrate, iron, or sulfate as their terminal electron acceptor or by fermentation. Since sulfate is the most abundant electron acceptor present in marine sediments, anaerobic hydrocarbon degradation coupled to
sulfate reduction is presumably quantitatively more important than degradation
coupled to other electron acceptors.
The schematic in Fig. 7.2 illustrates how long-term oil degradation is controlled
by oxygen depletion in surficial sediments. The rate-limiting steps are likely mass
transport (principally diffusion), both of petroleum species within the oilcontaminated zone and also nutrients and electron acceptors within the more highly
water-saturated burnout zones above and below oil-saturated sediments. Thicker
oil-saturated zones will degrade much more slowly than thin ones, with degradation
timescales likely scaling as the square of oil-saturated zone thickness and scaling
inversely with diffusion coefficients for species transfer within the oil zone itself,
which will decrease with increasing net levels of biodegradation. Very high oil saturations in near-surface sediments are likely enhanced by a so-called Marine Oil
Snow Sedimentation and Flocculent Accumulation (MOSSFA), aggregation of oil
with mineral particles, extracellular polymers, and biomass, which caused sinking
of a significant amount of oil following major oil spills (Brooks et al. 2015; Daly
et al. 2016; Quigg et al. 2020).
7 Biodegradation of Petroleum Hydrocarbons in the Deep Sea
Romero et al. 2017). Similar to planktonic environments, the biodegradation of
hydrocarbons in deep sea sediments is controlled by the abovementioned suite of
environmental parameters including temperature, pressure, oxygen and nutrient
availability, and physical or chemical form of the oil (Fig. 7.1) (Head et al. 2006).
Oxygen is consumed rapidly by microbial activity in sediments, and it is resupplied
slowly by diffusion or intermittently by bioturbation in the deep sea. Thus, sediments differ in that petroleum hydrocarbons (PHCs) become buried in an environment that is limited in the supply of oxygen and other electron acceptors. In addition,
rates of biodegradation were shown to inversely correlate with oil concentration,
and since sediments serve as a repository for oil deposition, oil concentrations are
elevated, and slower rates are to be expected (Prince et al. 2017).
Understanding of the metabolic pathways of hydrocarbon degradation under
anoxic conditions remains in its infancy (Meckenstock et al. 2016), and this represents a key knowledge gap for predicting the long-term fate of recalcitrant oil compounds in fine-grained sediments that cover much of the seafloor (Shin et al.
2019; Shin 2018). Activation of hydrocarbon molecules during degradation is
energetically demanding for microorganisms in the absence of oxygen, and thus
anaerobic hydrocarbon degradation is much slower in comparison to aerobic
metabolism. Since aerobic hydrocarbon-degrading bacteria preferentially utilize
short-chain n-alkanes or simple polycyclic aromatic hydrocarbons (PAHs) more
readily, PHCs buried in anoxic sediments tend to be enriched in more recalcitrant
compounds, further exacerbating degradation processes (Shin et al. 2019; Shin
2018). Once buried in the anoxic zone, microorganisms metabolize hydrocarbons
through respiration pathways using nitrate, iron, or sulfate as their terminal electron acceptor or by fermentation. Since sulfate is the most abundant electron acceptor present in marine sediments, anaerobic hydrocarbon degradation coupled to
sulfate reduction is presumably quantitatively more important than degradation
coupled to other electron acceptors.
The schematic in Fig. 7.2 illustrates how long-term oil degradation is controlled
by oxygen depletion in surficial sediments. The rate-limiting steps are likely mass
transport (principally diffusion), both of petroleum species within the oilcontaminated zone and also nutrients and electron acceptors within the more highly
water-saturated burnout zones above and below oil-saturated sediments. Thicker
oil-saturated zones will degrade much more slowly than thin ones, with degradation
timescales likely scaling as the square of oil-saturated zone thickness and scaling
inversely with diffusion coefficients for species transfer within the oil zone itself,
which will decrease with increasing net levels of biodegradation. Very high oil saturations in near-surface sediments are likely enhanced by a so-called Marine Oil
Snow Sedimentation and Flocculent Accumulation (MOSSFA), aggregation of oil
with mineral particles, extracellular polymers, and biomass, which caused sinking
of a significant amount of oil following major oil spills (Brooks et al. 2015; Daly
et al. 2016; Quigg et al. 2020).
7 Biodegradation of Petroleum Hydrocarbons in the Deep Sea