Masters Thesis

The effects of selected antibiotics on nitrogen uptake by Spirodela punctata

The purpose of this study was to determine the effects of nitrogenous compound removal by the aquatic macrophyte, Spirodela punctata, when exposed to three selected antibiotics. Recent research has shown that certain antibiotics target the chloroplasts of aquatic species such as Lemna and Myriophyllum. Studies have demonstrated antibiotic toxicity to Lemna gibba at concentrations as low as 10 µg/L. Meanwhile, antibiotic concentrations in domestic wastewater lie in the nanogram to microgram range with an average of approximately 50 µg/L. In this study, Spirodela punctata was grown in a mineral salts medium containing the antibiotics chlortetracycline, lomefloxacin, and sulfamethoxazole in concentrations ranging from 10 µg/L to 300 µg/L. Fronds were allowed to grow in the medium for seven, fourteen, and twenty-one day periods. Following the growth periods, the medium was analyzed for nitrate and total nitrogen concentrations. Dry weights of fronds were taken and the dried plant material was analyzed for Total Kjeldahl Nitrogen (TKN) content. Effective concentrations (EC25 and EC50) that impacted total nitrogen and nitrate removal from the growth medium as well as dry weights and Total Kjeldahl Nitrogen of the plant tissue were calculated. Of the antibiotics tested, chlortetracycline had the most significant responses. The antibiotic reduced nitrate and total nitrogen removal from the medium and decreased plant biomass. For nitrate removal chlortetracycline had an EC25 of 32 µg/L after three weeks of exposure. Dry weight EC25 for chlortetracycline was 73 µg/L after seven days of exposure to the antibiotic. Control treatments containing the antibiotic and no plant material had similar results indicating that the compounds added nitrogen. Conversely, other compounds tested resulted in very few significant responses. Lomefloxacin only showed significance for nitrate removal during week two and total nitrogen for week one. Surprisingly, the antibiotic seemed to stimulate the ability of Spirodela punctata to remove total nitrogen during the first week of exposure. Sulfamethoxazole had only one significant response during the test period. The combination of the three antibiotics resulted in only a reduction in nitrate removal during the first week after which there was no significant effect. Overall, selected antibiotics had little direct effect on ability of Spirodela punctata to remove nitrogenous compounds. However, with respect to chlortetracycline, the reduction in the production of biomass resulted in the reduction in nitrate and total nitrogen removal. This reduction in biomass is perhaps the greatest impact on S. punctata's ability to remove nitrogenous compounds from wastewater.

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