Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Toxicity and Mutagenicity of Gulf of Mexico Waters During and After the Deepwater Horizon Oil Spill” n the comment on our paper entitled “Toxicity and Mutagenicity of Gulf of Mexico Waters During and After the Deepwater Horizon Oil Spill”, the Exxon scientists state that Paul et al. “makes extraordinary claims of environmental harm that are unsubstantiated, incorrect, or misleading”. I do not think we concluded anything regarding environmental harm; rather, we just report the time and space coordinates of the appearance of microbial toxicity and mutagenicity in the waters and on occasion in the sediment porewater near the Deepwater Horizon drill site, the NEGOM, and W. Florida Shelf. Further on the oil industry scientists state that our initial studies with oil and dispersant were flawed because “no analytical confirmation of exposure concentrations was provided, rendering test results unreliable for hazard assessment”. What we used was the water accommodating fraction (WAF) of the initial concentrations we employed, a common practice in such studies.1,2 We did not measure the concentration of oil or dispersant that was in the water accommodating fraction. Instead, we reported our results as a function of the target oil treatment added. There are several reasons for this. On an equal TPH concentration basis, the toxicity of fresh, unweathered crude oil is quite different from that of weathered crude found in the environment. “Fresh” oil is mass-dominated by short alkanes of lesser toxicity and proportionately less PAHs, the principle mutagenic fraction of crude oils. Weathered crude is proportionately higher in the genotoxic PAHs. Thus trying to do hazard assessment based on LCs or LDs derived from fresh crude and dispersant concentrations and trying to relate these to weathered oil in situ is a fruitless undertaking (sort of apples and oranges problem). Second, we thought using the mass added approach would be of more use to those doing similar oil and oil + dispersant experiments than trying to target an actual soluble oil concentration. Thus the findings reported in Figure 2 can be viewed as conservative estimate of toxicity of these compounds. The true concentration of oil soluble in the water would be in actuality much lower than the concentration added at the outset. Thus our estimates of toxic response per amount oil would actually be at lower dissolved concentrations than initial concentration targets. Therefore we have only underestimated the toxic response by reporting that which occurred at concentrations added and not the final analytically determined concentration. The Exxon scientists further discredit our work by the way we performed our Microtox and QwikLite assays because we did not correct for absorbance of the blue light emitted by these assays, the absorbance caused by brown colored particulates or scatter by turbidity in solution. We never observed turbidity or color in samples. Nonetheless we measured the absorbance in accordance of the method of Ashworth et al.3 of half of our Microtox positive samples. The OD was between 0.001 and 0.012 in comparison with the control (0.2 μm filtered offshore Gulf of Mexico water). Thus, there was no need to filter the
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water column samples. Sediment porewater samples were clarified by centrifugation and filtering. The rebuttle further says little detail had been given concerning statistical methods. We used Statgraphics Plus for Windows, version 1.11(Statistics Graphic Corporation). The rebuttal points out that the July cruise had four of twelve samples positive for QwikLite toxicity even though at that time, there were no hydrocarbons detected at any of the WFS stations. We concede that the QwikLite assay maybe responsive to other compounds, and in lab studies, it was found to have greater sensitivity (lower LC or LD50) than Microtox. QwikLite can be viewed as a “noisier” assay. “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”. It is also important to point out that these assays measure different things, QwikLite (general toxicity) and Microscreen (DNA damage), and are not comparable. The reviewers point out that after the July and August of 2010, we did not have a hydrocarbon chemist on the team. “Authors provide no evidence that the samples contain oil or dispersant or have any relevance to the blowout”. This is true and is, as such, reflected in the title of the paper. However, other papers in press show that upwelling on the WFS and inactivity of the Loop Current should have transported oil on the WFS. Another paper in review shows DWH oil in livers of demersal fish of the WFS. Our findings in this paper are consistent with other work in the publication pipeline. These researchers point out that concentrations needed to elicit a positive response in the field were much lower than those I found in the lab. Our concentrations of oil and dispersant used in our lab studies reflect the amount added at the outset of the experiments, not the actual soluble or detectable in the water. As mentioned above, this was to ensure consistency of experimental setup. The actual detectable concentrations were no doubt lower, as the actual LD50 and LC50 values probably are. Second, although most concentrations are expressed in units of TPH, the ratio of PAHs to TPH can be variable, and weathered samples would be expected to contain higher amounts of the genotoxic PAHs. Thus “fresh” Macondo oil would have the lion’s share of its hydrocarbon mass comprised of low mw HCs which would be less toxic and a lot less mutagenic than weathered oil that had shorter HC chains digested and composed of recalcitrant high mw PAHs. Stimulation of light output by samples: This happens a lot with phytoplankton when there may be nutrients in the water Published: March 3, 2014 3593
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Environmental Science & Technology
Correspondence/Rebuttal
samples. The whole NE GOM is influenced by nutrient runoff from Apalachicola Bay and the Mississippi River plume. The scientists find Figure S3 surprising. I find it startling, yet nothing surprises me in the post Deepwater Horizon era. I believe the general SE flow of mutagenesis onto the WFS to be consistent with the migration of recalcitrant PAHs from the DWH oil spill. However, other explanations exist. There was nothing erroneously said about dispersants; many have been shown to breakdown to products that have endocrine disruption activity. They claim we have made erroneous statements regarding the toxicity of Corexit 9500 to hydrocarbon bacteria based on a paper4 that cannot be extrapolated to the field. According to the cited literature, from the very first page, they say “These data suggest that hydro-carbon-degrading bacteria are inhibited by chemical dispersants, and that the use of dispersants has the potential to diminish the capability of the environment to bioremediate spills”. They also mention natural seeps of hydrocarbons, and the importance of microbes “Not much is known about the organisms metabolic activity that degrade oil in the water column and on coastal beaches”. It is not clear how this cannot be extrapolated to the field. Furthermore, Prince and Parkerton state that “there is no evidence that field exposures of dispersant inhibit microbial activity in the sea”, whereas Kuhl et al.1 state that “Both dispersant and dispersant oil mixtures remained toxic for at least 4 wk at the lowest salinity tested, suggesting increased sensitivity or reduced biodegradation of toxic components in low-saline environments.” However, I guess low salinity environments (like those that typically occur in estuaries) are technically not the sea, so Prince and Parkerton’s statement is correct. In conclusion, our claims are neither extraordinary nor misleading, and we point out that other explanations are indeed possible.
John H. Paul
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College of Marine Science, University of South Florida, St. Petersburg, Florida 33701, United States
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
REFERENCES
(1) Kuhl, A. J.; Nyman, A.; Kaller, M. D.; Green, C. C. Dispersant and salinity effects on weathering and acute toxicity of South Louisiana crude oil. Environ. Toxicol. Chem. 2013, 32, 2611−2620. (2) Lee, K.-W.; Shim, Won Joon; Yim, W. J.; Kang, U. H. J.H. Acute and chronic toxicity study of the water accommodated fraction (WAF), chemically enhanced WAF (CEWAF) of crude oil and dispersant in the rock pool copepod Tigriopus japonicas. Chemosphere 2013, 92, 1161−1168. (3) Ashworth, J.; Nijenhuis, E.; Glowacka, B.; Tran, L.; Schenk-Watt, L. Turbidity and color correction in the Microtox bioassay. Open Environ. Pollut. Toxicol. J. 2010, 2, 1−7. (4) 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.
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