Effects of Seasonal Anoxia on Trace Level Natural and Anthropogenic Element Cycling in a Monomictic Pond
Fanny Coutelot1, Nimisha Edayilam1, Nancy Merino2, Naomi Wasserman2, Reid Williams1, Yongqin Jiao2, Daniel I. Kaplan3, Annie B. Kersting2, Brian A. Powell1*, Mavrik Zavarin2 (email@example.com)
1Clemson University, Clemson, SC; 2Lawrence Livermore National Laboratory, Livermore, CA; 3Savannah River Ecology Laboratory, University of Georgia, Aiken, SC
This study investigated the influence of seasonal anoxia in a warm monomictic pond on the composition of the microbial community and the cycling of natural and anthropogenic elements. Pond B at the Savannah River Site (Aiken, SC) received cooling water from the R-reactor that resulted in contamination of anthropogenic radionuclides plutonium-239 and cesium-137, which can be used as tracers to monitor natural hydro-biogeochemical processes. Seasonal stratification leads to seasonally anaerobic hypolimnion and profound changes in geochemical conditions that can impact the cycling of trace elements and organic matter through the system.
Two consecutive years of monitoring demonstrated the occurrence of highly correlated concentration profiles of arsenic, iron, aluminum, plutonium, and dissolved organic matter, all of which increased in concentration by 1–2 orders of magnitude within the anaerobic hypolimnion. Plutonium (Pu) appears to have become incorporated into the natural iron and carbon cycles with the highest concentrations in water observed at the start of stratification, with the majority released from shallow waters associated with iron(III)-POM (particulate organic matter). This finding is consistent with depth discrete sediment analyses, which demonstrated elevated concentrations of Pu in sediments with high organic matter. Thus, organic matter cycling likely plays a role in retaining Pu and other trace elements within the pond by causing seasonal redistribution between sediment and overlying pond water and deposition in organic rich sediments accumulating near the outlet. Conversely, cesium concentrations were highest in the pond inlet and appear to be controlled by particulate input from the influent canal, dominated by clay, silt, and sand minerals bearing iron (Fe).
Characterization of the microbial community provided additional insights regarding the observed cycling of Fe, Pu, and organic carbon. The microbial community varied with seasonal thermal stratification with Fe(III) reducers (e.g., Geothrix and Geobacter) dominating the deep, anoxic zone and sulfate reducers and methanogens present in the anoxic layer, with all likely contributing to Fe and Pu cycling. Microbiome analyses revealed potential for three impacts on the Pu and Fe biogeochemical cycles: (1) Pu bioaccumulation throughout the water column; (2) Pu-Fe-OM-aggregate formation by Fe(II) oxidizers under microaerophilic and aerobic conditions; and (3) Pu-Fe-OM-aggregate or sediment reductive dissolution in the deep, anoxic waters. Additionally, microcosm experiments revealed that the biosorption capacity of plutonium to washed bacterial and algal cells (loosely bound exudates removed) is similar.