Effects of Tetracycline on Growth and Nutrient Removal by Lemna aoukikusa and Spirodela polyrhiza Under Short-Term Cultivation

This study assessed the effects of tetracycline (TC) on growth of Lemna aoukikusa and Spirodela polyrhiza under batch conditions. The duckweeds were exposed to a range of 0.0–5.0 mg L−1 of TC for 7 days in a medium containing 10 mg L−1 total nitrogen (TN) and 1 mg L−1 total phosphorus (TP). The relative growth rate (RGR) of each species was determined from the frond area measurement using image analysis. The EC50 values as the TC concentrations causing a 50% reduction in RGR, were 4.4 mg L−1 for L. aoukikusa and 0.65 mg L−1 for S. polyrhiza. At 5.0 mg L−1 TC, TP removal decreased to 60% in the L. aoukikusa culture and 77% in the S. polyrhiza culture, compared to 85–91% and 96%, respectively, under lower TC exposure. Nevertheless, TN and TP removals were not significantly impaired at TC concentrations found in swine wastewater. The TC removals were 76–94% for the L. aoukikusa culture and 68–91% for the S. polyrhiza culture, which were attributed to adsorption and plant uptake. These findings highlight the feasibility of duckweed-based stabilization ponds for simultaneous antibiotic attenuation and nutrient removal.