Effects of selected pharmaceuticals on arbuscular mycorrhizal fungi

Pharmaceuticals enter the terrestrial environment through the amendment of agricultural soils with manure or biosolids and the irrigation of crops with reclaimed wastewater treatment plant effluent. The presence of pharmaceuticals in soils could impact plant health directly through phytotoxic mechanisms or indirectly through effects on plant interactions with beneficial microbes, such as arbuscular mycorrhizal fungi (AMF). The objectives of this thesis were to develop standardized bioassays and methods to evaluate the impacts of pharmaceuticals on AMF and to apply a tiered framework to address the current gap in knowledge regarding the effects of pharmaceuticals on terrestrial plants, AMF and the symbiotic relationship between these organisms.;Early stages of plant growth were insensitive to pharmaceuticals at environmentally relevant concentrations; however, Daucus carota consistently produced the lowest effective concentrations among the three plant species tested. The effect of pharmaceuticals on AMF, host root organ response and plant-AMF interactions were evaluated using a root organ culture method. Three pharmaceuticals showed selective mycotoxicity to carbamazepine, 17-alpha-ethynylestradiol and doxycycline. In higher tier soil microcosm tests, a similar degree of toxicity to AMF compared to root organ culture was observed for doxycycline, but not carbamazepine. Signalling experiments with carbamazepine and doxycycline evaluated whether the selective inhibition of mycorrhizal growth was due to interference of molecular signalling or direct toxic action on the AMF. Impaired recognition of the plant host was the cause of mycorrhizal inhibition for carbamazepine. Doxycycline inhibited the growth of AMF but did not interfere with plant host recognition. Doxycycline was selected to evaluate inter- and intra-species differences in relative sensitivity in host plants and AMF symbionts to a selectively mycotoxic chemical. This study showed little change in toxicity to either the plant host or AMF regardless of the species-genotype combinations. The collective effects data were used to evaluate the risk of pharmaceuticals to terrestrial plants and AMF in measured environmental soil concentrations using both deterministic and probabilistic methods. Pharmaceuticals were found to present a measureable but small risk to terrestrial plants and AMF.