Response relationships between CO2, CH4 and N2O emissions and microbial functional groups in wetland sediments after trace metal addition.

We investigated the effects of trace metal additions on microbial nitrogen and carbon cycling using freshwater wetland sediment microcosms amended with M concentrations of copper (Cu), molybdenum (Mo), iron (Fe), and all combinations. In addition to monitoring inorganic nitrogen transformations (NO3-, NO2-, N2O, NH4+) and carbon mineralization (CO2, CH4), we tracked changes in functional gene abundance associated with denitrification (nirS, nirK, nosZ), DNRA (nrfA), and methanogenesis (mcrA). 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. We found sparse evidence of DNRA activity and no consistent effect on CO2 production. Contrary, net CH4 production was stimulated by the trace metal amendments and the Mo additions, in particular, led to increased mcrA gene abundance. Taken together, these findings demonstrate that trace metal effects on microbial physiology, which have heretofore only been studied in pure culture, can impact community-level function. We observed direct and indirect effects on both nitrogen and carbon biogeochemistry that culminated in increased production of greenhouse gasses, and the shifts in functional group abundance that we documented suggest these responses may have been mediated through changes in microbial community composition. Overall, this work supports a more holistic consideration of metal effects on environmental microbial communities that recognizes the key role that metal limitation plays in microbial physiology.