115
to mass transfer limitations of electron acceptors. Similar trends both of
structure- dependent biodegradation susceptibility and oil concentration-driven degradation rates have also been observed in biodegraded oil reservoirs, in which the
biodegradation most rapidly occurs at the oil-water transition zone which is most
abundant with auxiliary chemical species needed for petroleum biodegradation
(Head et al. 2014). Moving away from the active zone of oil contamination, biodegradation rate likely decreases, due to mass transfer limitations and concentration
gradients – a “burnout effect” of a kind. In particular, biodegradation would be
inhibited when a large amount of petroleum is deposited, relative to the available
reactants involved in various biodegradation pathways. In such cases, one could
assume that deposited oil will be preserved over longer timescales, Fig. 7.1. An
additional control is interfacial/phase phenomena, as suspended oil components are
subjected to more pronounced (faster) degradation, contrary to oil deposited to sediment, which was degraded at slower rates (Fig. 7.2).
7.3.2 Microbial Community Response in Deep Sea Sediments
A substantial challenge in determining the impacts of oil deposition into deep
ocean sediments is the nearly complete lack of studies on benthic microbial communities performed prior to the DWH discharge (Overholt 2018). Due to interest
in petroleum exploration, previous studies in the Gulf focused almost entirely on
deep- subsurface sediments with the shallowest samples collected at ~4 m below
the seafloor or on sediments impacted by natural hydrocarbon seeps (Biddle et al.
2011; Nunoura et al. 2009; Orcutt et al. 2010). Due to this limitation in the knowledge of surficial microbial communities, the majority of studies on deep ocean
sediments associated with the DWH event used samples that were collected outside of impacted areas as controls (Kimes et al. 2014; Mason et al. 2014; Yang
et al. 2016; Liu et al. 2017). Sediments were primarily impacted as deepwater oil
intrusion layers impinged on the seafloor and by a large sedimentation event of
marine oil-derived snow (Brooks et al. 2015; Passow et al. 2012; Valentine et al.
2014; Romero et al. 2017). While the datasets on benthic microbes are limited to
relatively few samples (Overholt 2018), results indicate that microbial communities responded quickly to oil perturbation, and shifts in community composition as
well as total metabolic potential were observed (Kimes et al. 2014; Mason et al.
2014; Yang et al. 2016).
In areas immediately surrounding the DWH wellhead that were heavily contaminated to above EPA limits, microbial communities shifted to a different state dominated by known hydrocarbon degraders of the Colwellia group as well as uncultured
Gammaproteobacteria that comprised up to 18% of the total community (Mason
et al. 2014; Overholt 2018). Total metabolic potential was enriched in both aliphatic
and PAH degradation pathways as well in anaerobic respiratory metabolism (Mason
et al. 2014). Yang et al. (2016) used a clone library approach to follow the succession in deep benthic communities, including a pulse in sulfate-reducing bacterial
7 Biodegradation of Petroleum Hydrocarbons in the Deep Sea
to mass transfer limitations of electron acceptors. Similar trends both of
structure- dependent biodegradation susceptibility and oil concentration-driven degradation rates have also been observed in biodegraded oil reservoirs, in which the
biodegradation most rapidly occurs at the oil-water transition zone which is most
abundant with auxiliary chemical species needed for petroleum biodegradation
(Head et al. 2014). Moving away from the active zone of oil contamination, biodegradation rate likely decreases, due to mass transfer limitations and concentration
gradients – a “burnout effect” of a kind. In particular, biodegradation would be
inhibited when a large amount of petroleum is deposited, relative to the available
reactants involved in various biodegradation pathways. In such cases, one could
assume that deposited oil will be preserved over longer timescales, Fig. 7.1. An
additional control is interfacial/phase phenomena, as suspended oil components are
subjected to more pronounced (faster) degradation, contrary to oil deposited to sediment, which was degraded at slower rates (Fig. 7.2).
7.3.2 Microbial Community Response in Deep Sea Sediments
A substantial challenge in determining the impacts of oil deposition into deep
ocean sediments is the nearly complete lack of studies on benthic microbial communities performed prior to the DWH discharge (Overholt 2018). Due to interest
in petroleum exploration, previous studies in the Gulf focused almost entirely on
deep- subsurface sediments with the shallowest samples collected at ~4 m below
the seafloor or on sediments impacted by natural hydrocarbon seeps (Biddle et al.
2011; Nunoura et al. 2009; Orcutt et al. 2010). Due to this limitation in the knowledge of surficial microbial communities, the majority of studies on deep ocean
sediments associated with the DWH event used samples that were collected outside of impacted areas as controls (Kimes et al. 2014; Mason et al. 2014; Yang
et al. 2016; Liu et al. 2017). Sediments were primarily impacted as deepwater oil
intrusion layers impinged on the seafloor and by a large sedimentation event of
marine oil-derived snow (Brooks et al. 2015; Passow et al. 2012; Valentine et al.
2014; Romero et al. 2017). While the datasets on benthic microbes are limited to
relatively few samples (Overholt 2018), results indicate that microbial communities responded quickly to oil perturbation, and shifts in community composition as
well as total metabolic potential were observed (Kimes et al. 2014; Mason et al.
2014; Yang et al. 2016).
In areas immediately surrounding the DWH wellhead that were heavily contaminated to above EPA limits, microbial communities shifted to a different state dominated by known hydrocarbon degraders of the Colwellia group as well as uncultured
Gammaproteobacteria that comprised up to 18% of the total community (Mason
et al. 2014; Overholt 2018). Total metabolic potential was enriched in both aliphatic
and PAH degradation pathways as well in anaerobic respiratory metabolism (Mason
et al. 2014). Yang et al. (2016) used a clone library approach to follow the succession in deep benthic communities, including a pulse in sulfate-reducing bacterial
7 Biodegradation of Petroleum Hydrocarbons in the Deep Sea