Increased methane flux from wetlands due to differential toxicity of silver nanoparticle pollution

Background/Question/Methods:

Through discharge of sewage wastewater, wetland ecosystems are predicted to be exposed to engineered silver nanoparticles (AgNPs) from a variety of consumer products, ranging from clothing to cleaning products and supplements. In laboratory settings, these reactive silver particles (1-100nm) have been shown to be toxic across taxanomic boundaries, but what are their effects on the functioning of wetland ecosystems?  Here we report results from a 15 month 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 concentration of 2.5mg L^-1 Ag. Parallel laboratory studies were also employed to tease out mechanisms for observed patterns.

Results/Conclusions:

The abundant macrophytes, Egeria densa and Potamogeton diversifolius, died back within a day of exposure to both AgNO₃ and GA- AgNPs, leading to a marked increase 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 smaller plant dieback and more modest DOC increases to 19 mg DOC L^-1.  With an increase in microbial respiration and elimination of photosynthesis, watercolumn dissolved-oxygen plummeted, with depressed oxygen persisting out to 30 days, despite the fact that plants had begun to recover. The most dramatic result was a three order of magnitude increase in methane concentrations in all three silver treatments, despite the fact that only two of the three had the large DOC increase and strong O₂ decreases, conditions expected to favor methanogenesis, and limit aerobic methanotrophy. Parallel microcosm studies suggest that Ag is acting both directly and indirectly to stimulate this increase in methane concentrations by differentially affecting methanogenic, methanotrophic, and fermentative microorganisms, but that long term changes in methane fluxes are not expected under a one-time pulse scenario such as in this experiment.