Kinetic responses of enriched nitrifying systems to acute erythromycin and tetracycline exposure

Abstract

ABSTRACT This study investigated the acute inhibitory impacts of two widely used antibiotics, erythromycin (ERY) and tetracycline (TET), on ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) within an enriched nitrifying culture. For this purpose, a lab-scale wastewater treatment system sustaining enriched nitrifiers was exposed to antibiotic concentrations ranging from 25 to 100 mg L-1. Model evaluations of oxygen uptake rate (OUR) evolution, together with ammonia and nitrite oxidation profiles, revealed distinct metabolic disruptions. ERY severely impaired the system, dropping overall nitrification efficiency to as low as 25% and halving the growth rates for both AOB and NOB. In contrast, TET exposure largely maintained overall nitrification performance with slightly reduced NOB activity. Under TET stress, the half-saturation constants for ammonia (KNH) and nitrite (KNO) increased to 32 and 2 mg N L−1, respectively, indicating competitive inhibition and decreased substrate affinity. Both antibiotics reduced heterotrophic activity – more severely with ERY-triggering the release and utilisation of hydrolyzable biomass fractions. The results demonstrate that acute antibiotic toxicity and inhibitory impacts are markedly less pronounced in enriched nitrifying systems compared to conventional mixed activated sludge systems. GRAPHICAL ABSTRACTTwo schematic diagrams with two line graphs summarizing how erythromycin and tetracycline doses affect nitrifying culture kinetics.The figure shows a conceptual schematic of enriched nitrifying culture experiments with antibiotics and associated respirometry and kinetic diagrams. On the left, a cylindrical bioreactor with mixing equipment represents the enriched nitrifying culture. Below it, three cartoon bacteria are labeled AOB, NOB, and HET. Above the reactor, text labels identify the added antibiotics as Erythromycin and Tetracycline. Arrows from the reactor lead to two sets of flasks in the center, each linked to a line graph. The upper set has three Erlenmeyer flasks labeled 25 ERY, 50 ERY, and 100 ERY. Directly below these, a line graph shows oxygen uptake rate on the vertical axis, labeled OUR measured in milligrams oxygen per liter per hour, from 0 to 20 with tick marks every 2 units, versus time in hours from minus 1 to 15 with tick marks every 1 hour. Several colored model curves overlay scattered data points, with total oxygen uptake rate starting high then declining asymptotically. The lower set has two flasks labeled 25 TET and 100 TET. The corresponding line graph has oxygen uptake rate from 0 to 40 with tick marks every 5 units, again versus time from minus 1 to 15 hours, with a sharp peak followed by a gradual decline in all curves. On the right, kinetic schematics correspond to the erythromycin and tetracycline conditions. Each schematic includes AOB, NOB, and HET bacteria. There are parameters next to bacteria such as maximum growth rate for AOB and NOB, KNH, KNO, bAOB, bNOB, bH, and XH with arrows indicating their change. Both kinetic sections include the phrase Organic matter release. A long horizontal arrow across the top right suggests a progression of kinetic effects from the experiments.