Antibiotic exposure in agroecosystems: Evidence for altered microbial community structure and ecosystem function

Background/Question/Methods

The continued use of antibiotics in livestock production has raised concerns regarding the promotion of antibiotic resistant genes (ARGs) in the environment. While specific linkages between increased antibiotic resistance in the environment and antibiotic resistant infections in humans continue to be investigated, the effect on the environment itself is also of concern. Increased abundance of antibiotics in the environment likely 'stress' bacterial communities, causing turnover followed by increased metabolic maintenance demands.  Because of increased maintenance demands, there is the potential that microbialy-moderated ecosystem processes will be altered (i.e. increased losses of carbon and nitrogen). The purpose of our study was to examine the effect of increased antibiotic exposure in agricultural settings in order to investigate the consequences both in terms of ecosystem function (i.e. respiration, nutrient cycling) and community composition. Nationwide, 11 paired sites (high vs. low antibiotic inputs) were compared to determine if antibiotic resistance was related to change in microbial community composition and if it had the potential to alter ecosystem processes.

Results/Conclusions

Variation between high and low sites in microbial community composition was apparent, specifically the fungal community showed greater variability at the high sites compared to the low sites. This potentially indicates a variety of responses from the fungal community in response to a perturbation amongst bacteria, likely stemming from a range of responses to a reduction in competition for resources. We found that both microbial mass specific respiration and the abundance of ARGs (tetO and ampC) were greater when antibiotic inputs were high. Additionally, mass specific respiration and ARG abundance were positively related, suggesting that the maintenance of ARGs leads to a more stressed microbial community. Further investigation using an information theoretic approach showed that both available DOC and ARG abundance were strong predictors (stronger even than soil pH and moisture) of mass specific respiration. These results suggest that increased antibiotic presence in agricultural soils can alter community composition, as well as function. This alteration is apparent through much greater mass specific respiration under high antibiotic loads, indicative of a shift in microbial resource allocation from growth to maintenance, which can have overarching effects on nutrient immobilization and carbon sequestration - potentially, leading to decreased ecosystem function.