Role of natural nanoparticles and colloids for the mobility and bioavailability of antibiotics in soil

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Natural colloids, including nanoparticles are ubiquitous in the environment and often important carriers for pharmaceuticals like antibiotics. However, almost nothing is known about how wastewater treatment and soil type modulate the prevalence of colloids and thus of colloid-associated antibiotics. Further, the effects of colloids on bioavailability of antibiotics in soil remain unclear. We hypothesize that i) large parts of antibiotics in wastewater, soil and soil effluent are bound to colloids, and that ii) a change in wastewater quality as well as iii) different soil types alter the composition of colloids as well as the portion of antibiotics bound to them. We further assume that iv) binding of antibiotics to colloids reduces their bioavailability and, accordingly, the selection of antibiotic resistance genes, while v) plant uptake is likely not influenced by wastewater treatment, as antibiotic concentrations in treated wastewater are likely lower but more available than in untreated water. To test these hypotheses, we will i) analyze antibiotics in the colloidal and truly dissolved phase of untreated wastewater, soil and soil effluent of the column and field experiment of the Phaeozem. We will ii) assess changes in the association of antibiotics with colloids as induced by changes in wastewater quality by analyzing antibiotics in the truly dissolved and in the colloidal phase of soils in the column and field experiment irrigated with untreated or treated wastewater, and iii) extend the analyses to to Leptosols and Vertisols (column experiment). A satellite experiment is performed to assess iv) effects of model soil colloids on the selection of antibiotic resistance genes together with SP 3, where we will determine minimum inhibitory concentrations and growth curves for bacteria in broths containing different antibiotic concentrations in the absence and presence of these colloids. To include real-soils with different colloid compositions from different soil types, we will also determine antibiotic concentrations as well as with SP 3 the minimal selective concentrations using isogenic resistant and susceptible strains in the truly dissolved and colloidal fraction of the central incubation experiment. v) Plant uptake is finally monitored in both central column and field experiments. To obtain information about the role of single colloidal constituents for antibiotic binding we will determine colloidal size ranges and composition using asymmetric Field Flow Fractionation for all wastewater and soil samples. Linking this information of colloid-antibiotics interactions to bulk soil concentrations (SP 2) and microbiological parameters from the other projects provides a unique chance to understand for the very first time the role of natural colloids for antibiotic mobility and bioavailability in soil under wastewater irrigation.