Long-term transformation of oxolinic acid in water from an eel pond Hong-Thih Lai a,1 , Yew-Hu Chien b, ⁎ , Juo-Shan Lin a a Department of Aquatic Biosciences, National Chiayi University, 300 University Rd., Chiayi, 60004 Taiwan, ROC b Department of Aquaculture, National Taiwan Ocean University, No.2 Beining Rd., 20224 Keelung, Taiwan, ROC Received 26 September 2007; received in revised form 15 January 2008; accepted 15 January 2008 Abstract Oxolinic acid (OA) is one of the widely used antibiotics in aquaculture. Its residue in the aquatic environment is often detected globally and has raised environmental and public health concerns. To understand its transformation in water, samples obtained from an eel pond was added at three OA concentrations (5, 10, 20 mg l - 1 ) and then incubated under a combination of two illumination regimes (light-5800 lux, dark) and two oxygen profiles (aerobic, anaerobic) for 691 d. OA was highly resistant to transformation and its residue at the end of the experiment still ranged from 1.1 to 6.6 mg l - 1 . Illumination regime played the key role among the three factors used in the study. OA decayed faster under light than in the dark, with a mean half-life of 298 d and 509 d, respectively. High added OA concentration resulted in faster transformation. No difference in OA transformation was found between aerobic and anaerobic incubation. Further research on transformation of OA under light of higher intensity and/ or ultraviolet wavelength is recommended to resolve the slow OA transformation problem. © 2008 Elsevier B.V. All rights reserved. Keywords: Oxolinic acid; Transformation; Light regime; Oxygen profile; Residue 1. Introduction The use of antibiotics to control disease outbreaks and enhance growth and feed efficiencies increased significantly with the development of culture systems in recent years (Sarmah et al., 2006). However, the overuse and misuse of certain antibiotics have led to adverse environmental effects or increased human health risk (GESAMP, 1997; Heberer, 2002; Sarmah et al., 2006). The presence of antibiotics, their metabolites and transformation products in the environment is becoming a matter of concern (Hirsch et al., 1999; Kümmerer, 2003; Debska et al., 2004). High concentrations of oxolinic acid (OA) residue have been found in the aquatic environment (Burka et al., 1997; Delepee et al., 2004; Le and Munekage, 2004). For example, antibiotics were often applied in the first month of each shrimp crop, which resulted in accumulation of 0.01 to 2.50 ppm (surface water) and 0.01 to 2.31 ppm (bottom water) of OA residues in ponds (Le and Munekage, 2004). This can be attributed to the frequent use of OA in shrimp ponds, its low bioavailability in aquatic animals, and possibly its poor transformation in environment. OA is one of the quinolone antibiotics commonly used in veterinary or aquaculture as prophylactic or disinfectant to prevent diseases, or as chemotherapy agent to control diseases (Gräslund and Bengtsson, 2001; Samuelsen, 2006). It is predicted that approximately 50 kg of OA was released into Mediterranean from fish farms in Greece per year (Rigos et al., 2004). Good clinical effects at a low dosage rate have made OA popular in both fresh and salt water aquaculture (Barker et al., 1990; GESAMP, 1997; Le and Munekage, 2004; Le et al., 2004; Samuelsen, 2006) in tropical and temperate countries (Björklund et al., 1991; Burka et al., 1997). OA is effective against all common bacterial infections (Sarmah et al., 2006) and convenient to use through either medicated feed or bath treatment (Barker et al., 1990; Hernández Serrano, 2005; Samuelsen, 2006). However, toxicity of OA for rainbow trout is reported to be as low as 1 mg l - 1 (Oncorhynchus mykiss) (Lundén and Bylund, 2000). Despite the popular use of OA, most farmers are unaware of its low bioavailability in aquatic animals. Samuelsen (2006) Available online at www.sciencedirect.com Aquaculture 275 (2008) 96 – 101 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Tel.: +886 2 24622192x5204; fax: +886 2 24625393. E-mail addresses: htlai@mail.ncyu.edu.tw (H.-T. Lai), yhchien@mail.ntou.edu.tw (Y.-H. Chien). 1 Tel.: +886 52717848; fax: +886 52717847. 0044-8486/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2008.01.017