Evaluation of Heavy Metals and Metalloid Accumulation in a Small-Scale Aquaponic System and an Effect on Bacterial Antibiotic Resistance

Abstract

Aquaponics is an environmentally friendly and sustainable technology that holds promise to promote global food production. However, in aquaponics, routine fish feeding, along with the natural phenomena of evaporation and transpiration from the water and plants, could lead to deterioration of water quality of the system and may concentrate organic and inorganic pollutants such as heavy metals (HMs). Aquaponics might present food safety hazards to consumers if they consume food that exceeds maximum allowable limits (MAL) of contaminants and toxins in food and feed set by the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO). This work evaluated the presence, diversity, distribution, and accumulation of artificially elevated toxic metalloids and heavy metals (HMs) (arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb)) after inoculating them into aquaponics water at levels of 20%, 15%, 1.0%, and 1.0%, respectively, of the U.S Environmental Protection Agency (U.S EPA) standards that dictate the maximum contaminant level (MCL) for drinking water. Work determined the concentrations, distribution, and the fractions of the applied HMs in the water, fish, plant, and sediment. The potential effect of the HMs accumulation on co-selection for bacterial antibiotic resistance (BAR) to ampicillin and tetracycline was also evaluated, and the effect of the HMs on the growth of lettuce, evaluated by root and shoot (edible leaves) elongation was also investigated. The HMs concentration in the aquaponics water over time were not consistent. In general, only As accumulated in the water (P<0.05), but none of the applied metals exceeded the MCL. The accumulation of As in the aquaponics water may have been due to concentrations of As in fish feed. By the end of the study period (35 days), Hg and Pb tended to bioaccumulate significantly (P<0.05) in the fish tissue (wet weight), though both were below the MAL. The root data showed significant (P>0.05) accumulation of As, Hg, and Pb, but no HMs bioaccumulated in the shoot of the plant. Unexpectedly, the concentrations of all HMs decreased in the sediment during the final sampling period (the last week), and only Pb was significantly decreased by the end of the trial. The resistance of bacteria within the aquaponics water to ampicillin and tetracycline showed no consistent patterns. Ampicillin-resistant bacteria decreased in the water over time, while tetracycline-resistant bacteria increased until the third week of the treatment, then decreased gradually until no resistant bacteria were found in samples collected during the final week in the treatment as well as in the control system. The concentrations of the HMs used in the present work did not have an effect on the root elongation and the shoot length. Also, the plant mass (dry weight) results showed no difference in the treatment compared to the control. This study can provide a general view of the behavior of HMs in aquaponics, and it can aid the sustainability of the aquaponics industry by ensuring the safety of the products for consumers. Further studies will be needed to shed light on the long-term effects of the HMs (longer period with higher levels).