Microcosm experiments of oil degradation by microbial mats Tirso Garcı ´a de Oteyza a , Joan O. Grimalt a, * , Marc Lliro ´s b , Isabel Esteve b a Department of Environmental Chemistry (IIQAB-CSIC), Jordi Girona, 18, 08034-Barcelona, Catalonia, Spain b Department of Genetics and Microbiology, Faculty of Sciences, Autonomous University of Barcelona, 08193 Bellaterra, Catalonia, Spain Received 28 July 2004; accepted 27 April 2005 Available online 1 June 2005 Abstract Several microcosm experiments were run in parallel to evaluate the efficiency of microbial mats for crude oil degradation as compared with physico-chemical weathering. The oils used in the experiments constituted representative examples of those currently used for commercial purposes. One was aliphatic and of low viscosity (33.4 American Petroleum Institute degrees, 8 API) and the other was predominantly aromatic, with high sulphur content (ca. 2.7%) and viscosity (16.68 API). After crude oil introduction, the microcosms were kept under cyclic changes in water level to mimic coastal tidal movements. The transformations observed showed that water weathering leads to more effective and rapid elimination of low molecular weight hydrocarbons than microbial mat metabolism, e.g. n -alkanes with chain length shorter than n -pentadecane or n - heptadecane, regular isoprenoid hydrocarbons with chain length lower than C 16 or C 18 or lower molecular weight naphthalenes. Microbial mats preserved these hydrocarbons from volatilization and water washing. However, hydrocarbons of lower volatility such as the C 24 –C 30 n -alkanes or containing nitrogen atoms, e.g. carbazoles, were eliminated in higher proportion by microbial mats than by water weathering. The strong differences in composition between the two oils used for the experiments were also reflected in significant differences between water weathering and microbial mat biodegradation. Higher oil viscosity seemed to hinder the former but not the later. D 2005 Elsevier B.V. All rights reserved. 1. Introduction Soon after the massive oil spills of the 1991 Gulf War (ca. 10.8 million barrels contaminating ca. 640 km of the Saudi Gulf coast) intensive growth of cyanobacterial mats was observed on top of the oil layers from the polluted beaches. The presence of these cyanobacteria was considered to represent a first step towards natural bioremediation (Sorkhoh et al., 1992) and was consistent with the observation of higher biodegradation potential in the coast than in open waters (Radwan et al., 1999). In other studies, partial elimination of the spilled oil was attributed to the combined effects of physico-chemical weathering and microbial degradation (Sauer et al., 1998). In 0048-9697/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2005.04.039 * Corresponding author. Tel.: +34 934006122; fax: +34 932045904. E-mail address: jgoqam@cid.csic.es (J.O. Grimalt). Science of the Total Environment 357 (2006) 12– 24 www.elsevier.com/locate/scitotenv