ACCUMULATION OF HEAVY METALS IN AN AQUAPONICS SYSTEM AND EFFECT ON ANTIBIOTIC RESISTANCE OF BACTERIA

Almotairy, M.H*., Fitzsimmons, K., and McLain, J.E.
 
The University of Arizona
Department of Soil, Water, and Environmental Science
1177 E. 4th Street | P.O. Box 210038. Tucson, Arizona 85721-0038
Halmotairy@email.arizona.edu
 

Aquaponics is the combined culture of fish (aquaculture) and plants (hydroponic) in a recirculating system. It is a promising aquaculture technology which can enhance global food production. Despite the optimistic view, potential hazards like heavy metal accumulation could be a complicating factor if they exceed the maximum contaminant level goal (MCLG) in the system water. In addition, several studies report that water with elevated levels of heavy metals (HMs) contributes to bacterial antibiotic resistance. An 81-day experiment was conducted in a controlled greenhouse environment for evaluation of the presence, diversity, distribution, and accumulation of heavy metals and antibiotic resistance of bacteria in the aquaponic system. Three heavy metals and one metalloid (Cd, Pb, Hg, and As) (HMs) were mixed and inoculated in the replicate treatments based on 60% of the maximum permissible standard value for HMs in potable water into a small-scale (NFT) aquaponic system. Three replicates and three-control treatments were each stocked with eight fingerling (weighing 25-50 g) Tilapia fish (Oreochromis niloticus) and 10 plants of Butterhead lettuce (Lactuca sativa). Weekly water samples were collected while fish and plant samples were collected on day (0), and on the last day of the experiment for determining the concentration level of HMs. Inductively coupled plasma mass spectrometry (ICP-MS) was performed for assessing accumulation of HMs in the samples, and cultural method (plating) conducted to evaluate the bacterial antibiotic resistance after the inoculation day. Water samples that were collected weekly were used also for evaluation of bacterial antibiotic resistance. At harvest, wet and dry weight of fish and plants conducted as well. Water sample results showed that As and Cd increased steadily with duration of the experiment, but neither exceeded the permissible levels at the last day of the experiment. Hg and Pd did not accumulate, and their concentration declined after the inoculation. Tissues samples revealed dissimilar results; fish samples showed no excess of HMs to the MCLG, but HMs were found in elevated concentrations. Comparing with the control treatment, As, Hg, and Pb in fish samples showed accumulation of HMs while the Cd did not. In plant samples, Hg and Cd concentrations were elevated. While As and Pb did not accumulate in the plant samples. Pd concentrations levels were less than the initial concentration. The results of bacterial antibiotic resistance were uncertain. However, bacteria from the system were able to grow in both antibiotics (Ampicillin and Tetracycline). While bacteria grew steadily in all weeks of ampicillin culture, only, last week of culture tests (week 5) revealed a resistance in the diluted tetracycline culture. In general, bacteria results showed more resistance to ampicillin than to tetracycline.