Differential silver nanoparticle toxicity to microbes and macrophytes leads to carbon dioxide, nitrous oxide, and methane pulses

Background/Question/Methods:

Given their known toxicity to organisms across taxanomic boundaries, and their potential fate in aquatic ecosystems, we ask: what are the effects of AgNP in wetlands?  Here we report results from the first month of an ongoing wetland mesocosm experiment in which we dosed large (1.2x1.2x3.7m) mesocosms with two types of AgNPs (6nm gum arabic coated nanoparticles, GA-AgNPs, and 40nm polyvinylpyrrolidone coated particles, PVP-AgNPs; 3 reps each). Responses were compared relative to negative controls (deionized water addition; 4 reps), coating controls (assessing GA or PVP affects; 3 reps) and dissolved-silver controls (equal Ag mass as AgNO₃; 3 reps; all other treatments received equivalent NO₃^- as KNO₃).  In silver treatments, the watercolumn was dosed to achieve a final concentration of 2.5mg L^-1 Ag.

Results/Conclusions:

Silver concentrations dropped 1000-fold over the first week across treatments, with most silver ending up in sediment. Silver, added as either Ag⁺ or Ag⁰, was quickly transformed to Ag₂S or Ag-thiol, suggesting that sulfide and thiol groups were a strong sink for silver in this system. The abundant macrophytes, Egeria densa and Potamogeton diversifolius, were immediately affected by exposure to both AgNO₃ and GA- AgNPs.  Both forms of Ag addition led to plant dieback within 24 hours and concomitant increases in DOC concentrations from a background of 12 mg C L^-1 to 34 and 31 mg DOC L^-1 for the AgNO₃ and GA-AgNP treatments, respectively.  PVP coated AgNPs led to less dramatic plant dieback and more modest DOC increases to 19 mg DOC L^-1.  Watercolumn dissolved-oxygen decreased, with depressed oxygen persisting out to 30 days, despite the fact that plants had begun to recover. Dissolved concentrations of carbon dioxide, methane, and N₂O increased dramatically following dosing. Methane in particular rose from an average of 58 to an average of 1800ppm in two days. This methane response was short lived, though, decreasing back to the level of the controls by 8 days for AgNO₃ and GA-AgNPs, and by day 20 for the PVP-AgNPs.

The ecosystem showed an initial phase of acute toxicity of AgNPs to aquatic plants and release of DOC, which was then associated with an increase in microbial activity, decrease in O₂, and increase in trace gas production. While these responses decreased as watercolumn silver decreased, the long term impact depends on both the long term fate and bioavailability of silver in sediments.