Abstract
We investigated the effects of trace metal additions on microbial nitrogen (N) and carbon (C) cycling using freshwater wetland sediment microcosms amended with micromolar concentrations of copper (Cu), molybdenum (Mo), iron (Fe), and all combinations thereof. In addition to monitoring inorganic N transformations (NO3–, NO2–, N2O, NH4+) and carbon mineralization (CO2, CH4), we tracked changes in functional gene abundance associated with denitrification (nirS, nirK, nosZ), dissimilatory nitrate reduction to ammonium (DNRA; nrfA), and methanogenesis (mcrA). With regards to N cycling, greater availability of Cu led to more complete denitrification (i.e., less N2O accumulation) and a higher abundance of the nirK and nosZ genes, which encode for Cu-dependent reductases. In contrast, we found sparse biochemical evidence of DNRA activity and no consistent effect of the trace metal additions on nrfA gene abundance. With regards to C mineralization, CO2 production was unaffected, but the amendments stimulated net CH4 production and Mo additions led to increased mcrA gene abundance. These findings demonstrate that trace metal effects on sediment microbial physiology can impact community-level function. We observed direct and indirect effects on both N and C biogeochemistry that resulted in increased production of greenhouse gasses, which may have been mediated through the documented changes in microbial community composition and shifts in functional group abundance. Overall, this work supports a more nuanced consideration of metal effects on environmental microbial communities that recognizes the key role that metal limitation plays in microbial physiology.
Highlights
Wetland microbes are important for removing anthropogenic pollutants from surface waters, effectively preventing the contaminants from entering downstream coastal and marine ecosystems
We investigated the effects of trace metal additions on the microbial biogeochemistry of freshwater wetland sediments focusing primarily on NO3−/NO2− reduction and greenhouse gas (GHG) kinetics, and used quantitative polymerase chain reaction to assess changes in the abundance of key microbial functional groups associated with these processes
None of the trace metal additions had an effect on NO3− removal (Figure 1), which is somewhat surprising since all bacterial nitrate reductases contain a Mo cofactor at their active sites (Moreno-Vivian et al, 1999)
Summary
Wetland microbes are important for removing anthropogenic pollutants from surface waters, effectively preventing the contaminants from entering downstream coastal and marine ecosystems. ∼10% of the human-derived N input is returned to the atmosphere as N2 via denitrification from wetlands and terrestrial ecosystems globally (Schlesinger, 2009). Wetlands are important for their role in carbon (C) cycling Despite their relatively small coverage (∼10% of global land area), these ecosystems store large amounts of organic C and emit considerable amounts of methane (CH4) at an estimated rate of 144 Tg CH4 y−1 (IPCC, 2014). These net CH4 emissions represent a balance between the microbial processes of methanogenesis and methanotrophy
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