Influence of temperature on selenium mobility under contrasting redox conditions: a sediment flow-through reactor experiment

Selenium (Se) biogeochemistry in boreal and permafrost-rich soils and sediments remains poorly constrained, despite its importance as both an essential micronutrient and potential contaminant. As climate change accelerates warming in northern ecosystems, the mobilization of vast carbon pools may significantly alter Se cycling and bioavailability, with cascading effects on aquatic food webs. In this context, we aim to investigate how temperature and organic matter (OM) lability influence Se redox dynamics in lake sediments, providing insights for predicting its behavior as these northern ecosystems continue to warm. We studied Se sequestration as a function of OM lability, temperature (4 and 23 °C) and Se speciation in minimally disturbed lacustrine sediments using flow-through reactors (FTRs). Initial sediments contained OM characterized as either labile (fresh) or recalcitrant (aged), and were fed with environmentally relevant, low Se concentrations and filtered lake water. We monitored Se concentration as well as speciation along with pH and the concentrations of dissolved OM, NO₃⁻, NO₂⁻, Fe(II), SO₄²⁻ and HS⁻ in the outflow of FTRs during 8 experimental phases. All sediments sequestered a large proportion of Se, with FTRs containing fresh OM removing 50% more Se than those containing aged OM. Along with a higher production of reduced species, such as ferrous Fe and sulfides, in the reactors with fresh OM, this result is consistent with reducing conditions promoting Se sequestration. Inflowing selenite was sequestered to a larger extent than inflowing selenate. Lastly, only selenate removal responded strongly to temperature. With an inflow concentration of 100 nM, selenate was sequestered at a rate of 92 pmol cm⁻³ d⁻¹ at 23 °C, which decreased to 80 pmol cm⁻³ d⁻¹ at 4 °C. In selenate removal experiments, outflow Se speciation consisted mostly of organic Se species at 23 °C and, in contrast, entirely of selenate at 4 °C. We hypothesize that selenate removal proceeded via microbial processes, consistent with temperature-dependent reactions catalyzed by enzymes. Overall, our findings suggest that the mobilization and warming of the boreal and permafrost carbon pools may increase the capacity of aquatic environments to sequester Se, lowering its bioavailability.