A protocol for assessing the effectiveness of oil spill dispersants in stimulating the biodegradation of oil

Abstract

Dispersants are important tools in oil spill response. Taking advantage of the energy in even small waves, they disperse floating oil slicks into tiny droplets (<70 μm) that entrain in the water column and drift apart so that they do not re-agglomerate to re-form a floating slick. The dramatically increased surface area allows microbial access to much more of the oil, and diffusion and dilution lead to oil concentrations where natural background levels of biologically available oxygen, nitrogen, and phosphorus are sufficient for microbial growth and oil consumption. Dispersants are only used on substantial spills in relatively deep water (usually >10 m), conditions that are impossible to replicate in the laboratory. To date, laboratory experiments aimed at following the biodegradation of dispersed oil usually show only minimal stimulation of the rate of biodegradation, but principally because the oil in these experiments disperses fairly effectively without dispersant. What is needed is a test protocol that allows comparison between an untreated slick that remains on the water surface during the entire biodegradation study and dispersant-treated oil that remains in the water column as small dispersed oil droplets. We show here that when this is accomplished, the rate of biodegradation is dramatically stimulated by an effective dispersant, Corexit 9500®. Further development of this approach might result in a useful tool for comparing the full benefits of different dispersants.

Chemical dispersants enhance the activity of oil- and gas condensate-degrading marine bacteria

“…(1: 186, 281 v/v) for dispersant and oilwith-dispersant microcosms, respectively. Except for the 42-day incubation time point in Thebaud winter microcosms, for which dispersant seemed to have a negative effect on short-chain alkane degradation, our results are generally in agreement with others (Lunel et al, 1997;Baelum et al, 2012;Lee et al, 2013;Prince and Butler, 2014) regarding the use of dispersant to stimulate oil degradation in SW. However, our data also suggest that while dispersant favors the degradation of n-alkanes, it does not affect the degradation of aromatic compounds.…”

Hydrocarbon-Degrading Bacteria Exhibit a Species-Specific Response to Dispersed Oil while Moderating Ecotoxicity

“…Studies conducted under conditions nearly identical to in situ dispersed-oil conditions (2.5 ppm oil by volume) revealed rapid oil degradation, little effect of dispersant addition, and no dispersant inhibitory effect on biodegradation. However, these studies should be extended to include the response of the microbial populations and biodegradation capacity, along with ecotoxicity (56,57).…”

Chemical dispersants can suppress the activity of natural oil-degrading microorganisms

“…Chemical dispersants emulsify surface oil slicks, reduce oil delivery to shorelines (19), and increase dissolved oil concentrations, which should make oil more bioavailable (20) and stimulate biodegradation (21). The efficacy of dispersants in stimulating oil biodegradation is debated (22) and negative environmental effects have been documented (23).…”