Potential role of copper availability in nitrous oxide accumulation in a temperate lake

Abstract

Denitrifying bacteria require copper for the synthesis of nitrous oxide reductase. In the absence of sufficient bioavailable Cu, nitrous oxide (N 2 O) may accumulate in natural waters during denitrification. Cultures of Paracoccus denitrificans and natural bacterial assemblages collected from a mesotrophic lake (Linsley Pond) were grown at varying Cu concentrations to determine the Cu speciation that results in elevated N 2 O accumulation. P. denitrificans experienced Cu limitation beginning at inorganic Cu concentrations of 0.6 fmol L 21 (2log of inorganic Cu, pCu9, 5 15.2). The natural community did not show an effect until pCu9 was reduced to 23.7 with 8-hydroxyquinoline, resulting in an approximate 10-fold increase in N 2 O concentrations during denitrification. Additions of ethylenediaminetetraacetic acid alone to natural communities did not affect N 2 O concentrations. Natural copper-binding ligands detected with competitive ligand exchange-cathodic stripping voltammetry occurred at concentrations of 6 to 25 nmol L 21 with conditional stability constants (K Cu 2z L ) between 10 14.4 and 10 15.1 . Although more than 99% of total Cu in Linsley Pond was bound to these ligands, inorganic Cu concentrations remained 10 orders of magnitude above those found to increase N 2 O accumulation during denitrification incubations. Measurements of nitrogen species, dissolved oxygen, and sulfide in the water column of Linsley Pond over the spring growing season revealed that N 2 O was produced by assimilatory nitrate reduction and nitrification in addition to denitrification, with nitrification generating most of the N 2 O found in the surface waters of the lake. The results suggest that inorganic Cu concentrations in Linsley Pond are sufficient to support denitrification. Moreover, some denitrifying bacteria may be able to access organically bound Cu, reducing the potential for this metal to affect N 2 O production in other aquatic environments.

Denitrification alternates between a source and sink of nitrous oxide in the hypolimnion of a thermally stratified reservoir

“…Although several studies have documented N 2 O consumption via denitrification in hypoxic hypolimnion with limited NO { 3 availability (Mengis et al 1997;Twining et al 2007;Sasaki et al 2011), there are exceptions. For example, Deemer et al (2011) reported persistent N 2 O supersaturation in the anoxic hypolimnion of a small reservoir, despite NO { 3 depletion by late summer.…”

“…One explanation is that the labile carbon derived from the algal bloom stimulated sulfate reduction, resulting in a temporary accumulation of sulfide, a known inhibitor of N 2 O reductase (Knowles 1982). An alternative explanation is that DOC from the algal bloom scavenged inorganic copper from the water column, a key element of N 2 O reductase, thereby inhibiting the synthesis of this N 2 O reducing enzyme (Twining et al 2007). Other investigators have attributed temporary spikes of dissolved N 2 O to denitrification under conditions of moderate oxygen availability.…”

“…The most likely explanation for the loss of hypolimnetic N 2 O is reduction to N 2 via denitrification. The literature suggests that denitrification is most likely to reduce extracellular N 2 O after NO { 3 and oxygen have been completely consumed (Mengis et al 1997;Twining et al 2007;Sasaki et al 2011), which is consistent with the samples collected in the lower portion of the hypolimnion where N 2 O consumption began after NO The occurrence of N 2 O consumption via denitrification in anoxic hypolimnion has led to the hypothesis that eutrophic lakes function as a sink for atmospheric N 2 O (Lemon and Lemon 1981;Mengis et al 1997). Net N 2 O consumption could occur if atmospheric N 2 O diffused into the epilimnion, through the thermocline, and into the hypoxic hypolimnion where it was reduced to N 2 .…”

Nitrous oxide from chemodenitrification: A possible missing link in the Proterozoic greenhouse and the evolution of aerobic respiration

“…Buick (2007) posited that Proterozoic oceans had low dissolved Cu due to the low solubility of Cu sulfides (Emerson, Jacobs, & Tebo, 1983;Jacobs, Emerson, & Skei, 1985;Kremling, 1983;Morse & Luther, 1999) and that Cu limitation may have limited activity of the last enzyme in denitrification, nitrous oxide reductase (NOS), which requires Cu as a cofactor for N 2 O reduction to N 2 . Accumulation of N 2 O under Cu limitation occurs in denitrifying bacterial isolates (Granger & Ward, 2003), but not in natural waters (Twining, Mylon, & Benoit, 2007;Ward et al, 2008). Further, recent data suggest that marine Cu concentrations likely remained relatively stable throughout Earth history (Fru et al, 2016), making Cu limitation of Proterozoic N 2 O reduction less feasible as a N 2 O accumulation pathway.…”

Chelator‐induced inhibition of copper metalloenzymes in denitrifying bacteria

“…Ward et al (2008) evaluated the control of denitrification in incubations of water from the three major oxygen-minimum zones of the world ocean and concluded that organic carbon availability, rather than copper, limited denitrification at all sites. Twining et al (2007) induced Cu limitation of N 2 O reduction by a natural freshwater denitrifying assemblage by adding a strong Cu chelator but saw no evidence for Cu limitation in natural samples without added chelators, even those containing natural ligands. They also saw no inhibition of N 2 O reduction of natural assemblages at the same EDTA additions that were effective in marine cultures (Granger and Ward 2003).…”