Comment on “Toxicity and Mutagenicity of Gulf of Mexico Waters

Correspondence/Rebuttal pubs.acs.org/est

Comment on “Toxicity and Mutagenicity of Gulf of Mexico Waters During and After the Deepwater Horizon Oil Spill” inconsistent with the much higher exposures required to demonstrate such responses in the lab tests included in this study as well as previous lab and field investigations;7,8 (2) field samples exhibited stimulatory effects of the same magnitude as samples judged to be toxic, seriously questioning the precision and accuracy of the tests for delineating toxicity gradients; and (3) the GOM has many natural oil seeps that have long been influencing this ecosystem.9−11 Given this latter point, it is essential to use forensic chemistry12 to assign the identity of any oil found in this area. Paul et al. used excitation emission matrix spectroscopy to identify oil, and gas chromatography of the aliphatic oil fraction, but provide no proof that the oil they found originated from the blowout even in August 2010. While the surface oil sample depicted in Figure S3 in ref 1 appears to be an evaporated, unbiodegraded crude oil, the two deeper samples are unusual and are most akin to chromatograms expected of a bunker fuel. The apparent lack of biodegradation of the surface sample, collected almost a month after well capping, is surprising given ample evidence of extensive biodegradation of spilled oil.13−15 Paul et al.1 also make several confusing and erroneous statements about dispersants. They correctly assert that the purpose of dispersants is to facilitate biodegradation but ignore substantial evidence that dispersants were very effective at dispersing oil into the water column in this response.16 Instead they misrepresent the work of Judson et al.17 by making the misleading statement “Dispersants have been shown to break down to products with endocrine disruption activity”, when in fact this paper states “Note that Corexit 9500, the currently used product, does not contain nonylphenol ethoxylates and did not show any estrogen receptor activity.” They also draw conclusions about the toxicity of Corexit 9500 to oil-degrading microbes from a paper that cannot be extrapolated to the field.18 Dispersants are applied to an aspirational dispersant:oil ratio of 1:20, which translates to an operational goal of 5 gallons per acre for surface slicks, so that the oil and dispersant dilutes to less than a few ppm within hours.19 Hamdan and Fulmer 18 tested concentrations of dispersant and nutrients that were far higher than would ever be encountered in oil spill response, and there is no evidence that field exposures of dispersant inhibit microbial activity in the sea.20 In conclusion, the extraordinary claims made by Paul et al.1 are not supported by objective data. The reported toxicity and mutagenicity in field samples were limited in extent and magnitude and provide little evidence of toxicity gradients nor underlying causality. In contrast, overwhelming evidence supports the conclusion that oil dispersed into the water column was biodegraded within months of the spill,13−15 and that the addition of dispersants16 was an effective tool in

While the Deepwater Horizon spill was larger than any previous spill in the Gulf of Mexico (GOM) and a major environmental insult, the paper by Paul et al.1 makes extraordinary claims of environmental harm that are unsubstantiated, incorrect or misleading. They conducted lab toxicity tests to assess the relative sensitivity of three assays to oil, Corexit 9500 dispersant, and a 1:1 oil and dispersant mixture at nominal concentrations from 0.1 to 100 000 mg/L. Unfortunately, no analytical confirmation of exposure concentrations was provided, rendering test results unreliable for hazard assessment.2,3 Further, the Microtox and QwikLite assays rely on bioluminescent test organisms, where toxicity is equated to reduction in light intensity relative to controls in artificial seawater. However, the blue light emitted by these organisms will be absorbed by brown colored material or scattered by turbidity in solution. The additional procedures required to correct for these potentially confounding problems were not performed.4,5. In fact light attenuation in test solutions clouded by oil droplets and dispersant likely partially explains the apparent 100-fold greater sensitivity of these assays when compared to the mutagenicity assay that is not bioluminescence-based. The assays were used to assess unfiltered water samples in GOM field surveys comparing observed responses to control samples of autoclaved and 0.2 μm filtered offshore seawater. As above, no consideration was given to potential confounding effects on bioluminescence attenuation. Further, little detail was provided regarding the statistical methods used. Microtox inhibition ranged from −18% (higher bioluminescence than control) in July 2010 to 18% in August 2010. Three surface water samples from the latter survey that exhibited significant toxicity (inhibition ranging from 10.7% to 18%) were attributed to nonchemically dispersed surface oil from the blowout. QwikLite inhibition was observed in 4 of 12 samples in July 2010, ranging from 29% to 63% (Table S2 in ref 1), and 4 of 13 samples, ranging from 28% to 57%, in August 2010. While no oil was detected in the July cruise, the similar August results were nevertheless attributed to chemically dispersed subsurface oil from the blowout. Mutagenicity, expressed as the percentage increase in phage abundance was reported in 14 samples from the August 2010 cruise and ranged from −92% to 283% with six samples flagged as statistically significant. Thus, this assay was more sensitive than the other two assays, which is contradictory to the relative sensitivity reported in lab toxicity tests. Subsequent surveys after August 2010 demonstrated consistently low Microtox inhibition below 10%. QwikLite and mutagenicity tests showed variable but some “positive” stations. However, the authors provide no evidence that the samples contain oil or dispersant or have any relevance to the blowout. Paul et al.1 imply that the spill has a major lingering acute toxicity, but they neglect the following: (1) the magnitudes of field responses under documented field exposures6 are © 2014 American Chemical Society

Published: March 3, 2014 3591

dx.doi.org/10.1021/es404846b | Environ. Sci. Technol. 2014, 48, 3591−3592

Environmental Science & Technology

Correspondence/Rebuttal

D.; Deng, Y.; Zhou, J.; Mason, O. U. Deep-sea oil plume enriches indigenous oil-degrading bacteria. Science 2010, 330, 204−208. (14) Edwards, B. R.; Reddy, C. M.; Camilli, R.; Carmichael, C. A.; Longnecker, K.; Van Mooy, B. A. S. Rapid microbial respiration of oil from the Deepwater Horizon spill in offshore surface waters of the Gulf of Mexico. Environ. Res. Lett. 2011, 6, No. 035301. (15) Baelum, J.; Borglin, S.; Chakraborty, R.; Fortney, J. L.; Lamendella, R.; Mason, O. U.; Auer, M.; Zemla, M.; Bill, M.; Conrad, M. E.; Malfatti, S. A.; Tringe, S. G.; Holman, H.; Hazen, T. C.; Jansson, J. K. Deep-sea bacteria enriched by oil and dispersant from the Deepwater Horizon spill. Environ. Microbiol. 2012, 14, 2405−2416. (16) BenKinney, M.; Brown, J.; Mudge, S.; Russell, M. ; Nevin, A.; Huber, C. Monitoring effects of aerial dispersant application during the MC252 Deepwater Horizon Incident. Proceedings of the 2011 International Oil Spill Conference, paper 367. Available at http:// www.ioscproceedings.org/doi/abs/10.7901/2169-3358-2011-1-368. (17) Judson, R. S.; Martin, M. T.; Reif, D. M.; Houck, K. A.; Knudsen, T. B.; Rotroff, D. M.; Xia, M.; Sakamuru, S.; Huang, R.; Shinn, P.; Austin, C. P.; Kavlock, R. J.; Dix, D. J. Analysis of eight oil spill dispersants using rapid, in vitro tests for endocrine and other biological activity. Environ. Sci. Technol. 2010, 44, 5979−5985. (18) Hamdan, L. J.; Fulmer, P. A. Effects of Corexit EC9500A on bacteria from a beach oiled by the Deepwater Horizon spill. Aquat. Microb. Ecol. 2011, 63, 101−109. (19) Lee, K.; Nedwed, T.; Prince, R. C.; Palandro, D. Lab tests on the biodegradation of chemically dispersed oil should consider the rapid dilution that occurs at sea. Mar. Pollut. Bull. 2013, 73, 314−318. (20) Prince, R. C.; McFarlin, K. M.; Butler, J. D.; Febbo, E. J.; Wang, F. C. Y.; Nedwed, T. J. The primary biodegradation of dispersed crude oil in the sea. Chemosphere 2013, 90, 521−526.

helping the indigenous microbes of the GOM to degrade the spilled oil.

Roger C. Prince*,† Thomas F. Parkerton*,‡ †

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ExxonMobil Biomedical Sciences, Inc. Annandale, New Jersey 08801, United States ‡ ExxonMobil Biomedical Sciences, Inc., Houston, Texas 77002, United States

AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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