Abstract
Manufactured Zn oxide nanoparticle (ZnO-NP) are extensively used world-wide in personal care and industrial products and are important contaminants of aquatic environments. To understand the overall impact of ZnO-NP contamination on aquatic ecosystems, investigation of their toxicity on aquatic biofilms is of particular consequence, given biofilms are known sinks for NP contaminants. In order to assess alterations in the functional activity of river microbial biofilm communities as a result of environmentally-relevant ZnO-NP exposure, biofilms were exposed to ionic zinc salt or ZnOPs that were uncoated (hydrophilic), coated with silane (hydrophobic) or stearic acid (lipophilic), at a total concentration of 188 μg l–1 Zn. ICP-MS analyses of biofilms indicated ZnO-NP concentrated in the biofilms, with hydrophilic, hydrophobic, and lipophilic treatments reaching 0.310, 0.250, and 0.220 μg Zn cm–2 of biofilm, respectively, while scanning transmission X-ray microspectroscopy (STXM) analyses of biofilms confirmed that Zn was extensively- and differentially-sorbed to biofilm material. Microbial community composition, based on taxonomic affiliation of mRNA sequences and enumeration of protozoa and micrometazoa, was not affected by these treatments, and the total transcriptional response of biofilms to all experimental exposures was not indicative of a global toxic-response, as cellular processes involved in general cell maintenance and housekeeping were abundantly transcribed. Transcripts related to major biological processes, including photosynthesis, energy metabolism, nitrogen metabolism, lipid metabolism, membrane transport, antibiotic resistance and xenobiotic degradation, were differentially expressed in Zn-exposures relative to controls. Notably, transcripts involved in nitrogen fixation and photosynthesis were decreased in abundance in response to Zn-exposure, while transcripts related to lipid degradation and motility-chemotaxis were increased, suggesting a potential role of Zn in biofilm dissolution. ZnO-NP and ionic Zn exposures elicited generally overlapping transcriptional responses, however hydrophilic and hydrophobic ZnO-NPs induced a more distinct effect than that of lipophilic ZnO-NPs, which had an effect similar to that of low ionic Zn exposure. While the physical coating of ZnO-NP may not induce specific toxicity observable at a community level, alteration of ecologically important processes of photosynthesis and nitrogen cycling are an important potential consequence of exposure to ionic Zn and Zn oxides.
Highlights
Manufactured nanoparticle (NP) contamination of aquatic environments is of world-wide concern, as they are ubiquitously used in commonly-consumed products and present at increasingly-high concentrations in systems that include waste water treatment plant effluents, surface water, sewage sludge, and biosolids (Mueller and Nowack, 2008; Gottschalk et al, 2009; Nowack, 2009; Keller and Lazareva, 2014; Dumont et al, 2015)
Based on ICP-MS analyses, ZnO-NPs occurred in the biofilms with the hydrophilic, hydrophobic and lipophilic treatments, reaching 0.31, 0.25, and 0.22 μg Zn/cm2 of biofilm, respectively, while the high ionic exposure resulted in 0.184 μg Zn/cm−2, with background levels of 0.014 μg Zn/cm−2 of biofilm
These analyses indicated that the maximum Zn thickness for hydrophilic ZnO was 198 nm, hydrophobic ZnO was 168 nm, lipophilic ZnO was 243 nm, and high ionic was 328 nm, while average thicknesses of Zn throughout the biofilms were 9.0, 9.0, 13.0 and 14.1 nm of Zn, respectively
Summary
Manufactured nanoparticle (NP) contamination of aquatic environments is of world-wide concern, as they are ubiquitously used in commonly-consumed products and present at increasingly-high concentrations in systems that include waste water treatment plant effluents, surface water, sewage sludge, and biosolids (Mueller and Nowack, 2008; Gottschalk et al, 2009; Nowack, 2009; Keller and Lazareva, 2014; Dumont et al, 2015). Modeling suggests that ZnO-NPs may occur in wastewater treatment plant effluents at concentrations between 0.22 and 0.74 μg l−1 (Gottschalk et al, 2009). Other modeling-based estimates suggest that the levels of ZnO in surface water and sewage sludge may approach 0.01 and 0.5 μg l−1, respectively (Mueller and Nowack, 2008; Nowack, 2009). Other modeling-based estimates suggest that the levels of ZnO in surface water and sewage sludge may approach 0.01 and 0.5 μg l−1, respectively (Mueller and Nowack, 2008; Nowack, 2009). Dumont et al (2015) predicted that the concentration of ZnO-NPs in European surface waters was 0.1–0.5 μg l−1, while Keller and Lazareva (2014) predicted that biosolids could contain up to 80 mg kg−1 ZnO-NPs; in both studies there was a general trend to increasing concentrations
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