Abstract
Enrofloxacin (EFX) is one of the most frequently used broad-spectrum veterinary drugs, and copper (Cu) is a heavy metal that could easily bind to certain antibiotic molecules. Hence EFX and Cu were chosen as representatives of antibiotics and heavy metals to explore the abundance and variation of soil microbial populations with a plate-counting technique, as well as the copy numbers of amoA gene in ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) by quantitative PCR methods in Argosols samples. Treatments of applied EFX (0.05, 0.20, 0.80 mmol/kg), Cu (1.60 mmol/kg) and combined EFX and Cu (at molar ratios of 1:32,1:8,1:2) exhibited various effects on different soil microorganisms. The toxicity of combined EFX and Cu was more strongly expressed on both soil microbial populations and amoA gene (AOA and AOB) compared to the EFX treatment alone, in most cases, time and dose effects were observed. With respect to the amoA gene, the AOA-amoA gene was more abundant than the AOB-amoA gene, and the ratio ranged from ~8 to ~11. Moreover, the interaction types of EFX and Cu were more likely to be antagonistic (64.29%) than synergistic (35.71%) on soil abundance and function, which may be related to the incubation time and the ratio of EFX to Cu in the soil.
Highlights
Since their first discovery, antibiotics have played incomparable roles in treatment of diseases and promotion of animal feed efficiency[1]
Antibiotics are deployed against a broad spectrum of bacteria, and high concentrations of heavy metals may be toxic to microorganisms
The toxicity of combined EFX and Copper sulfate (Cu) on soil microbial populations was more strongly expressed compared to the EFX treatment alone, and the highest concentration of EFX and Cu treatment (EFX0.80_Cu) exhibited the greatest impact on soil microorganisms
Summary
Antibiotics have played incomparable roles in treatment of diseases and promotion of animal feed efficiency[1]. Coexistence of antibiotics and heavy metals, including copper (Cu), zinc (Zn), cadmium (Cd), chromium (Cr), and lead (Pb) have been detected with high concentrations in wastewater[13], sewage treatment plants[14], manures and soils[15]. Antibiotics are deployed against a broad spectrum of bacteria, and high concentrations of heavy metals may be toxic to microorganisms. Ammonia oxidation process[21], but recent studies have shown that AOA can be detected with high abundance in soils[22,23], suggesting that both AOA and AOB play pivotal roles in N cycling[23]. In this study, we chose EFX and Cu as representatives of antibiotic and heavy metal contaminants to explore their effects alone, their combined effects and their interaction types on soil microorganisms in Argosols samples from Taian, China. Our objectives are to answer three questions: (1) Do EFX and Cu, either alone or in combination, affect the abundance of living bacteria, fungi, and actinomycetes in consistent patterns? (2) Do EFX or Cu affect the abundance of ammonia-oxidizing genes in soil? (3) Does the molar ratio of EFX to Cu affect the degree of toxicity for different microbial communities in similar ways?
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