The release of methane by wetlands accounts for 55% of natural methane emissions, with factors such as the degree of inundation, vegetation type, and productivity driving those fluxes. Recent work looking at the ecosystem impacts of silver nanoparticles as an emerging contaminant indicated that such contaminants could tilt the balance toward enhanced methane release. Questions remained as to the mechanisms driving this phenomenon and its generalizability to real world scenarios given that reported observations were from experiments using onetime high-concentration additions of zero-valent silver nanoparticles (Ag0-NPs) while real world releases are likely to be chronic low-concentration additions of weathered and less toxic silver sulfide nanoparticles (Ag2S-NPs). To examine the underlying mechanisms a microcosm study was conducted with microcosms containing sediment, sediment and submersed vegetation, submersed vegetation, or just water with or without Ag0-NPs to examine the role of plant derived DOC, and whether methane production or consumption were the likely drivers. To test the impacts of the rate of addition or form of silver nanoparticles added, a yearlong mesocosm experiment was conducted comparing a one-time pulse addition of pristine silver nanoparticles (Pulse Ag0NPs) to chronic low-concentration additions of either pristine (Chronic Ag0-NPs) or weathered (Chronic Ag2S-NPs) silver nanoparticles.
Results/Conclusions
The microcosm study suggested that underlying mechanism was methane production fueled by the release of DOC released by submersed vegetation. The mesocosm study showed that while the Pulse Ag0-NP treatment had a rapid increase in methane concentration in the first week, the magnitude decreased over time converging with controls within a week. While the chronic treatments had no initial increase in methane, they did significantly increase relative to controls during weeks 28-34 and 42-45 in the Chronic Ag0-NPs treatment, and during weeks 28-30 and 42 in the Chronic Ag2S-NPs treatment, suggesting that even chronic addition of weathered particles could cause an increase in methane. Additionally, we observed that in one of our replicate mesocosms in the Chronic Ag2S-NPs treatment, we saw methane concentrations that were 10-fold higher than the others in that treatment. In this mesocosm, the extirpation of predatory fishes (Gambusia holbrookii) by crayfish may have caused a trophic cascade wherin increased grazing by zooplankton depleted the abundance of methane consuming microbes leading to elevated methane fluxes. In total, these studies show evidence of direct effects of contaminants on methane fluxes, and provide evidence supporting potential indirect effects that may exacerbate the underlying direct effects.