World Aquaculture September 2018

54 SEP TEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG Bacterial Antibiotic Resistance Bacterial antibiotic resistance is common in the environment, and is especially prevalent in water that is affected by pollutants resulting from human activities (Di Cesare et al . 2016). For instance, using antimicrobials for controlling disease in fish has long been a common activity in some countries; this practice is often used during international transportation of fish to inhibit growth of potential pathogens (Verner-Jeffreys et al . 2009). In recent decades, reports from The World Health Organization (WHO), National Institutes of Health (NIH), Food and Drug Administration (FDA), and Centers for Disease Control and Prevention (CDC) state that drug-resistant bacteria are posing a serious threat to human health. The rapid spread of antibioitc- resistant bacteria has resulted in widespread difficulty in treating common infections in humans and animals (Levy and Marshall 2004, Bax and Grif n 2012, Di Cesare et al . 2016). Contaminated water and food supplies appear to be helping to move antibiotic resistance into different environments (Suzuki and Hoa 2012). The factors that are driving selection for bacterial resistance are still largely unclear (Di Cesare et al. 2016) but heavy metals have the ability to play a potential role in enhancing co-selection for antibiotic resistance. In short, heavy metal pollutants increase bacterial tolerance to antibiotics related to co-regulation of resistance genes (Baker-Austin et al . 2006). The Experiment A 12-wk experiment was conducted to assess the partitioning and potential bioaccumulation of heavy metals in water, fish and plant tissue of an aquaponics system. The growth of aquaponics worldwide and the widespread presence of heavy metals in water sources indicates that there is a need to identify any potential health risks to consumers. Co-selection of antibiotic resistance in bacteria within aquaponics systems may also present a potential hazard to human health and the environment. To examine the distribution of heavy metals and the potential for antibiotic resistance development, an experiment was carried out in a greenhouse at the Controlled Environment Agriculture Center of the University of Arizona. A small-scale aquaponics systemwas developed with Nutrient Film Technique as the hydroponic method (Figs. 1 and 2). Systems were stocked with eight Nile tilapia Oreochromis niloticus fingerlings and ten Butterhead variety lettuce Lactuca sativa . Fish were fed at 2.5 percent body weight daily. Systems were operated for six weeks before initating the experiment. Three systems did not receive metal supplementation (control). As the background levels of heavy metals in potable water and fish feed are low, metals were added to three systems (treatment). Based on the MCL for potable water standards of the US Environment Protection Agency (EPA 2016), metals were added to reach a target of 60 percent of the MCL of four heavy metals (Cd, Pb, Hg and As). Heavy metal concentrations were determined in water samples collected every week. Samples of fish and plants were collected on the first and final days of the experiment. In addition, weekly water samples were collected for culturing bacteria to evaluate antibiotic resistance. Inductively coupled plasma mass spectrometry (ICP-MS) was performed to determine concentrations of heavy metals in samples of water, fish, and plants. Agar plating was used to evaluate bacterial antibiotic resistance. Heavy Metals inWater Concentrations of the four metals in the aquaponic systemwater varied over time. In general, concentrations of As were greater than those of other metals (Cd, Hg and Pb) due to high background levels and increased with time. Also, the As concentration of fish feed (3.03 μg/g feed) was approximately an order of magnitude greater than that of Cd (0.37 μg/g feed), Hg (0.80 μg/g feed) and Pb (0.30 μg/g feed), resulting in an increase in water As concentration over time (Fig. 3). Mercury levels decreased over time in both the treated and the control systems (Fig. 4). Cadmium and lead concentrations fluctuated over time, but concentrations were maintained within the MCL (5 μg/L). Lead concentrations were low in water of both treatment and control systems. At the end of the experiment, Pb concentrations were less than immediately after heavy metal addition to the system. FIGURE 1. Aquaponics system design. Three systems were dosed with heavy metals (treatment) and three were not (control). Water can be lost by evapotranspiration, but heavy metals might be concentrated with time. FIGURE 2. The main components of the systems for each replicate. From left to right: sump, lettuce growing hydroponically using nutrient film technique, biofilter, and fish tank. Water is pumped from the sump to the fish tank and then flows by gravity to the biofilter, lettuce tray and sump. ( C O N T I N U E D O N P A G E 5 6 )