303
Marine snow is generally defined as rapidly sinking composite particles (>0.5 mm)
that consist of many particles including bacteria, phytoplankton, and feces (Daly
et al. 2016; Passow et al. 2012, 2014). Microbes (e.g., bacteria, phytoplankton, and
bacterioplankton) secrete sticky extracellular polymeric substances (EPS) to absorb
nutrients, to create a protective microenvironment, and to act as a sink of excess
fixed carbon (Bullerjahn and Post 2014). The resulting formed marine snow can
also have an effect on the degradation of oil during an oil spill.
This chapter will discuss the effect of chemical dispersants and marine snow on
oil biodegradation, using both weathered and non-weathered oil.
18.2 Effect of Chemical Dispersants on Oil Biodegradation
To date, contradicting results are published in the literature regarding the effect of
chemical dispersants on oil biodegradation, as both stimulation and inhibition have
been described. Various mechanisms play a role and interfere with each other, such
as (1) oxygen competition between oil- and dispersant-degrading bacteria; (2)
enhanced bioavailability of oil due to the dispersion; and (3) decreased biodegradation of oil due to acute release of toxic oil components by chemical dispersants.
These three mechanisms are discussed in the next paragraphs.
18.2.1 Oxygen Competition Between Oil- and DispersantDegrading Bacteria
The added chemical dispersants themselves can be degraded by bacteria present in
the ocean, thus competing with oil-degrading bacteria for the available oxygen. The
two chemical dispersants that were used during the DWH oil spill (Corexit 9500A
and 9527) are composed of various solvents and nonpolar surfactants including
petroleum distillates, propylene glycol, 2-butoxyethanol, Tween 80, Tween 85,
Span 80, dioctyl sulfosuccinate (DOSS), and dipropylene glycol butyl ether (Glover
et al. 2014). DOSS is the most commonly studied chemical of this mixture, and a
combination of biodegradation and hydrolysis is responsible for its breakdown
(Campo et al. 2013).
When comparing the oxygen consumption for the degradation of dispersants and
oil, results show that oxygen depletion was much faster in mixtures of oil and seawater, compared to mixtures of dispersant and seawater. This shows that aerobic
degradation occurs much faster in oil compared to Corexit (Fig. 18.1).
Limited oxygen consumption (less than 1%) was observed during Corexit degradation, compared to the amount used for oil degradation after 7 days of incubation,
dropping from 20% to 8% in the headspace. These data suggest that biodegradation
of Corexit will not result in a significant competing effect with oil-degrading bacteria. Similar observations were made regarding the degradation of DOSS, one of the
18 Effect of Marine Snow on Microbial Oil Degradation
Marine snow is generally defined as rapidly sinking composite particles (>0.5 mm)
that consist of many particles including bacteria, phytoplankton, and feces (Daly
et al. 2016; Passow et al. 2012, 2014). Microbes (e.g., bacteria, phytoplankton, and
bacterioplankton) secrete sticky extracellular polymeric substances (EPS) to absorb
nutrients, to create a protective microenvironment, and to act as a sink of excess
fixed carbon (Bullerjahn and Post 2014). The resulting formed marine snow can
also have an effect on the degradation of oil during an oil spill.
This chapter will discuss the effect of chemical dispersants and marine snow on
oil biodegradation, using both weathered and non-weathered oil.
18.2 Effect of Chemical Dispersants on Oil Biodegradation
To date, contradicting results are published in the literature regarding the effect of
chemical dispersants on oil biodegradation, as both stimulation and inhibition have
been described. Various mechanisms play a role and interfere with each other, such
as (1) oxygen competition between oil- and dispersant-degrading bacteria; (2)
enhanced bioavailability of oil due to the dispersion; and (3) decreased biodegradation of oil due to acute release of toxic oil components by chemical dispersants.
These three mechanisms are discussed in the next paragraphs.
18.2.1 Oxygen Competition Between Oil- and DispersantDegrading Bacteria
The added chemical dispersants themselves can be degraded by bacteria present in
the ocean, thus competing with oil-degrading bacteria for the available oxygen. The
two chemical dispersants that were used during the DWH oil spill (Corexit 9500A
and 9527) are composed of various solvents and nonpolar surfactants including
petroleum distillates, propylene glycol, 2-butoxyethanol, Tween 80, Tween 85,
Span 80, dioctyl sulfosuccinate (DOSS), and dipropylene glycol butyl ether (Glover
et al. 2014). DOSS is the most commonly studied chemical of this mixture, and a
combination of biodegradation and hydrolysis is responsible for its breakdown
(Campo et al. 2013).
When comparing the oxygen consumption for the degradation of dispersants and
oil, results show that oxygen depletion was much faster in mixtures of oil and seawater, compared to mixtures of dispersant and seawater. This shows that aerobic
degradation occurs much faster in oil compared to Corexit (Fig. 18.1).
Limited oxygen consumption (less than 1%) was observed during Corexit degradation, compared to the amount used for oil degradation after 7 days of incubation,
dropping from 20% to 8% in the headspace. These data suggest that biodegradation
of Corexit will not result in a significant competing effect with oil-degrading bacteria. Similar observations were made regarding the degradation of DOSS, one of the
18 Effect of Marine Snow on Microbial Oil Degradation