Chemical dispersants: Oil biodegradation friend or foe? Shokouh Rahsepar a, ⁎, Martijn P.J. Smit a,1 , Albertinka J. Murk b , Huub H.M. Rijnaarts a , Alette A.M. Langenhoff a a Sub-department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, Wageningen, The Netherlands b Marine Animal Ecology Group, Wageningen University, P.O. Box 338, 6700 AH Wageningen, The Netherlands abstract article info Article history: Received 21 December 2015 Received in revised form 12 April 2016 Accepted 19 April 2016 Available online xxxx Chemical dispersants were used in response to the Deepwater Horizon oil spill in the Gulf of Mexico, both at the sea surface and the wellhead. Their effect on oil biodegradation is unclear, as studies showed both inhibition and enhancement. This study addresses the effect of Corexit on oil biodegradation by alkane and/or aromatic degrading bacterial culture in artificial seawater at different dispersant to oil ratios (DORs). Our results show that dispersant addition did not enhance oil biodegradation. At DOR 1:20, biodegradation was inhibited, espe- cially when only the alkane degrading culture was present. With a combination of cultures, this inhibition was overcome after 10 days. This indicates that initial inhibition of oil biodegradation can be overcome when different bacteria are present in the environment. We conclude that the observed inhibition is related to the enhanced dis- solution of aromatic compounds into the water, inhibiting the alkane degrading bacteria. © 2016 Elsevier Ltd. All rights reserved. Keywords: Biodegradation Dispersant Oil spill Enhanced dissolution 1. Introduction Large oil spills in the marine environment have been occurring since the early 1900s when oil and gas industries started extracting oil off- shore and using oil tankers for transportation (Burger, 1997). From 1970 to 2012, approximately 5.75 million tons of oil were released to the oceans as a result of tanker incidents (Oil Tanker Spill Statistics, 2015). Release of oil into the marine environment is the main cause of marine pollution (Holliger et al., 1997). The largest accidental marine oil spill in the history of the petroleum industry is the Deepwater Hori- zon oil spill, in April 2010 in the Gulf of Mexico (McNutt et al., 2012). Once oil is discharged into the marine environment, the properties of the spilled oil change due to a variety of physical, chemical and bio- logical processes. These processes, collectively known as weathering (Boehm et al., 2008; Wardlaw et al., 2008) change the oil's composition, its physical/chemical behaviour and its toxicity. An important weathering process is evaporation which transfers light-weight and more volatile compounds to the atmosphere (Mansuy et al., 1997). Gen- erally, this happens at the sea surface during the first few hours after a spill (Mansuy et al., 1997). Another important weathering process is biodegradation by which bacteria partially or completely transform oil to compounds that can be further degraded and become more soluble in water (Lepo et al., 2003; Pontes et al., 2013). The rate of biodegradation depends on many parameters, such as temperature, presence of electron acceptors and nutrients, composition of the oil, and the active microbial popula- tion. Moreover, the presence of other compounds influences the biodeg- radation rate by either enhancing or inhibiting the microbial conversion or by changing the bioavailability of oil and its toxicity to bacteria. Therefore, weathering processes iteratively affect the ongoing degrada- tion of the oil. Traditionally, oil spill management often includes the application of chemical dispersants on oil slicks to remove these from the water sur- face. Dispersants reduce the interfacial tension between the oil and sea- water, and stabilize the smaller oil droplets that are formed. As a result, the bioavailability of the oil increases, which can enhance oil biodegra- dation. At oil spills like the Deepwater Horizon spill, dispersants were injected under water to the crude oil (Kujawinski et al., 2011). In this case, the application of dispersants creates oil micro-emulsions, and benzene, toluene, ethylbenzene and xylene (BTEX) and polycyclic aro- matic hydrocarbons (PAHs) compounds dissolve faster. Since micro- emulsions cannot be separated easily from the water phase, and this often leads to a higher apparent water solubility of these compounds (Zheng & Obbard, 2002). Whether the addition of dispersant enhances or decreases oil degra- dation is not yet clear as in literature contradicting results were pub- lished (Brakstad et al., 2015; Lindstrom and Braddock, 2002). Previous studies showed the positive effect of Corexit on the oil biodegradation by mixed bacterial communities (Hazen et al., 2010; Valentine et al., 2012). However, some other studies have reported negative effect of Corexit on the oil biodegradation (Hamdan and Fulmer, 2011). Clearly, the scientific and technical understanding of the physicochemical inter- actions taking place and how they affect subsequently biological activi- ties not (yet) complete. Marine Pollution Bulletin xxx (2016) xxx–xxx ⁎ Corresponding author. E-mail address: shokouh.rahsepar@wur.nl (S. Rahsepar). 1 Current affiliation: Eurofins Analytico, P.O. Box 459, 3770 AL Barneveld, The Netherlands. MPB-07657; No of Pages 7 http://dx.doi.org/10.1016/j.marpolbul.2016.04.044 0025-326X/© 2016 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul Please cite this article as: Rahsepar, S., et al., Chemical dispersants: Oil biodegradation friend or foe?, Marine Pollution Bulletin (2016), http://dx. doi.org/10.1016/j.marpolbul.2016.04.044