PS 40-116 - Evidence of the release and removal of natural nanoparticles by periphyton in stream mesocosms

Abstract: Research into the fate and impacts of engineered nanomaterials has revealed that natural nanoparticles may be important to understanding the biogeochemical cycling of metals and metalloids in ecosystems. Although water filtered with standard pore sizes (e.g., 200, 450, or 700 nm) is operationally defined as being dissolved, some elements in this size fraction may instead be found as nanoparticles and small colloids, and little is known about how these submicron particles are cycled in ecosystems. In this study, we conducted a 15-day stream mesocosm experiment to examine how natural nanoparticles are cycled in streams. Trays of cobbles were first colonized by periphyton and benthic macroinvertebrates for 28 days in the Clark Fork River in western Montana, US, before being transported to the lab and placed in stream mesocosms. These mesocosms mimic natural stream conditions with a recirculating flow and a water residence time of one day, with new water continuously added by a peristaltic pump. Four replicate mesocosms were exposed to either stream water with a colloidal fraction enriched by particles concentrated using tangential flow filtration or to stream water with an equivalent metal and metalloid concentration added as metal salts. On day 15 of the experiment, we used single particle inductively coupled plasma mass spectrometry (sp-ICP-MS) to quantify the concentration and size distribution of particles entering the mesocosms in the treatment water and leaving the mesocosms. Regardless of the treatment, we saw evidence of nanoparticles being released from the benthos containing Al, Fe, P, and Zn. In the treatment with increased concentrations of nanoparticles and colloids, we also saw evidence of nanoparticle removal by the benthos. These results suggest that our previous observations of relatively constant longitudinal concentrations of nanoparticles in the Clark Fork River represent the balance between the removal and release of nanoparticles by the benthos, similar to patterns commonly associated with dissolved nutrients. Further, this study suggests that nanoparticle forms of metals may be an important route of dietary exposure to benthic macroinvertebrates grazing on periphyton that has removed nanoparticles from the water column.