Characterization of antibiotic effects on ecosystems has largely been limited to standardized single-species ecotox-icology studies with survival, growth, or reproduction as the primary endpoints. Among the most sensitive aquatic primary producers to antibiotics are cyanobacteria (blue-green algae), which have been identified as more sensitive than green algae for a number of compounds. For example, in a series of toxicity tests with seven fluoroquinolones, EC50 for growth rates ranged from 7.9 to 1960mgl 1 for the cyanobacterium, Microcystis aeruginosa, with levofloxa-cin and flumequine the most and least toxic, respectively. Between two different laboratories, ciprofloxacin EC50 for M. aeruginosa ranged from 5 to 17 mgl . For the green algae, Pseudokirchneriella subcapitata, growth rate EC50s were at least 10 times greater, ranging from 1.1 to 23.0 mgl-1 with clinafloxacin and lomefloxacin the most and least toxic, respectively. In a separate study, the ciprofloxacin EC50 for P. subcapitata was reported to be 3mgl~. The marine cryptophycean, Rhodomonas salina, was less sensitive with growth rate EC50 of 10, 18, and 24mgl~1 for oxolinic acid, flumequine, and sarafloxacin, respectively. However, P. subcapitata may be more sensitive than R. salina to trimethoprim with EC50 values reported at 16 and 80.3 mgl~ , respectively.
Phytotoxicity of antibiotics warrants study because of the prokaryotic-like nature of the plant cell chloroplast. Most macrophyte toxicity data have focused on tetracyclines and fluoroquinolones with standardized toxicity tests using Lemna gibba or L. minor, in which the more sensitive response variables appear related to growth (e.g., frond number, wet weight). For L. minor, EC50 for reproduction (i.e., new frond growth) ranged from 51 mgl 1 for levofloxacin to 203 mgl 1 for ciprofloxacin among six fluroquinolones examined. Effects on other responses, such as wet weight, frond number, chlorophyll a, chlorophyll b, and carotenoids, have been studied in L. gibba. Of these, the most sensitive endpoints of exposure to five fluroquinolones were wet weight and frond number, and ranking of toxicity among compounds was generally the same for both endpoints. Wet weight EC50 ranged from 97 to 913 p.gl~ and EC50 for frond number ranged from 116 to 1146 p.gl~ for lomefloxacin, levofloxa-cin, ciprofloxacin, ofloxacin, and norfloxacin (in order from most to least toxic). Similar EC50 values have been observed for tetracyclines. Specifically, chlortetracycline, doxycycline, tetracycline, and oxytetracycline EC50s for wet weight and frond number were reported as 219, 316, 723, 1010 and 318, 473, 1114, and 1401 p.gl~ , respectively. A recent review paper on antibiotic effects on plants identified that triclosan presented the greatest risk to primary producers iftraditional endpoints (e.g., growth) and existing regulatory exposure models are used.
Tetracyclines have been shown to affect environmentally relevant microorganisms such as those within activated sludge from WWTPs. For aerobic sludge, growth inhibition EC50s were 0.08,0.03, and 0.08 mgl-1 for tetracycline, chlor-tetracycline, and oxytetracycline, respectively. One of the tetracycline degradation products, although not one of the primary degradation products, was 2.7 times more potent than the parent compound. It has also been shown that bacterial genera associated with stream water and several bacterial genera (i.e., Flavobacterium spp. and Pseudomonas spp.) isolated from the gut of detritivorous invertebrates are all within the spectrum of antimicrobial activity of at least one fluoroquinolone, ciprofloxacin.
Antibiotics generally appear to be far less toxic to invertebrates and vertebrates than to aquatic microorganisms or plants. Cladocerans may be most sensitive to tetracyclines. For the cladoceran Daphnia magna, the no observed effect concentration (NOEC) for survival during a 48 h tetracycline exposure period was 340 mgl 1 while the EC50 for reproduction was 44.8 p.gl~ . Cladocerans appear to be much less sensitive to erythromycin with reported acute EC50s for immobilization or mortality ranging from 200 to 400 mgl-1. In addition, a series of toxicity tests involving ciprofloxacin, levofloxacin, lomefloxacin, ofloxacin, enro-floxacin, flumequine, and clinafloxacin, all indicated very limited mortality for the cladoceran D. magna and the fathead minnow, Pimephales promelas, at concentrations as high as 10mgl~\ In another study, the reported ciprofloxacin NOECs for D. magna and the zebrafish, Brachydanio rerio, were 60 and 100 mgl-1, respectively. For many aquatic vertebrates, such as fish, mgl~ exposure concentrations can act as growth promoters and are used readily for this purpose in many aquaculture practices. However, potential ecological consequences (e.g., fitness) associated with accelerated growth promotion have not been explored.
Although antibiotics can be introduced to the terrestrial environment via dispersion of sewage treatment sludge on agricultural fields, to date very little research with limited number of compounds (e.g., tetracyclines and sulfona-mides) has been conducted on terrestrial impacts. It is also feasible that terrestrial systems that are closely linked to aquatic systems receiving antibiotic inputs could be indirectly affected by impacts to aquatic system structure and function. In agricultural soil, MIC50s (minimum inhibitory concentrations) for sensitive Pseudomonas spp. were 2.0, 0.5, and 1.0 mgl-1 for tetracycline, chlortetracycline, and oxytetracycline, respectively, and 0.25 mgl~ for other sensitive strains of bacteria for each of these compounds. It is important to note that the presence of divalent metals may reduce the antibacterial effects of tetracyclines via chelation reactions. Such bioavailability and effects relationships need further investigation.
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