While single-species toxicity tests provide valuable information, these short-term laboratory tests may not be representative of the most sensitive species and are not intended to assess structural or functional responses to chemical stressors. Few studies have examined the effects of antibiotics on aquatic microbial communities and populations. One study, using microcosms consisting of stream sediment, water, and leaf material, reported an effect of 12 day ciprofloxacin exposures on function of aerobic microbial communities, specifically communitylevel physiological profiles. Shifts in microbial community function were observed at concentrations of 10mgl1 and above, whereas at 1 mgl-1, function of communities in terms of carbon substrate utilization was similar to controls. Similarly, in ternary combination with two other pharmaceutical compounds, 10 mgl-1 ciprofloxacin exposures did not alter bacterial abundance in aquatic microcosms. Another freshwater lentic microcosms study assessed ecological responses to a mixture of tetracycline, oxytetracycline, dox-ycycline, and chlortetracycline. While both structural and functional effects were observed in this study, effect levels were determined at higher levels than those reported in aquatic systems.
Ecosystem-level responses such as leaf litter breakdown by detritivores and microbes as well as detritivore growth and condition indices (i.e., responses that can be directly related to bacterial populations) were not affected by cipro-floxacin exposure concentrations of 10, 1, and 0.1 mgl-1 in laboratory microcosm experiments. In the multiple pharmaceutical microcosm exposure involving ciprofloxacin (as described above), community-level effects on zooplankton and phytoplankton were only observed at the greatest ciprofloxacin exposure concentration (1000 mgl ). For both phytoplankton and zooplankton, overall abundance increased at this concentration while diversity declined, indicating disruption of community structure and suggesting possible release from competition or predation processes for some taxa.
While the potential for greatest effects of antibiotics may be observed in stream ecosystems, no studies have assessed antibiotic effects on structural or functional response variables in model stream systems. Lotic systems involve physical, chemical, and biotic characteristics that vary greatly from lentic systems and variation in these characteristics could significantly influence occurrence, fate, and effects of antibiotics. Thus, it is likely that a robust understanding of cause and effect relationships between environmentally realistic antibiotic exposures and stream ecosystem functional responses will only be possible if sophisticated lotic mesocosms are employed in future studies. In addition, it is likely that most cases of antibiotic contamination in streams will be accompanied by excessive nutrients and particulate matter due to co-contamination of either sewage or animal waste. Here again, appropriately designed stream mesocosm experiments coupled with other lines of evidence (e.g., biomarkers of exposure or effect) can be used to appropriately characterize antibiotic and other stressor effects on aquatic ecosystems.
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