Abstract
Abstract. Nitrate pollution of freshwaters and methane emissions into the atmosphere are crucial factors in deteriorating the quality of drinking water and in contributing to global climate change. The n-damo (nitrite-dependent anaerobic methane oxidation), nitrate-dependent anaerobic methane oxidation and the anaerobic oxidation of ammonium (anammox) represent two microbially mediated processes that can reduce nitrogen loading of aquatic ecosystems and associated methane emissions to the atmosphere. Here, we report vertical concentration and stable-isotope profiles of CH4, NO3-, NO2-, and NH4+ in the water column of Fohnsee (lake in southern Bavaria, Germany) that may indicate linkages between denitrification, anaerobic oxidation of methane (AOM), and anammox. At a water depth from 12 to 20 m, a methane–nitrate transition zone (NMTZ) was observed, where δ13C values of methane and δ15N and δ18O of dissolved nitrate markedly increased in concert with decreasing concentrations of methane and nitrate. These data patterns, together with the results of a simple 1-D diffusion model linked with a degradation term, show that the nonlinear methane concentration profile cannot be explained by diffusion and that microbial oxidation of methane coupled with denitrification under anaerobic conditions is the most parsimonious explanation for these data trends. In the methane zone at the bottom of the NMTZ (20 to 22 m) δ15N of ammonium increased by 4 ‰, while ammonium concentrations decreased. In addition, a strong 15N enrichment of dissolved nitrate was observed at a water depth of 20 m, suggesting that anammox is occurring together with denitrification. The conversion of nitrite to N2 and nitrate during anammox is associated with an inverse N isotope fractionation and may explain the observed increasing offset (Δδ15N) of 26 ‰ between δ15N values of dissolved nitrate and nitrite at a water depth of 20 m compared to the Δδ15Nnitrate-nitrite of 11 ‰ obtained in the NMTZ at a water depth between 16 and 18 m. The associated methane concentration and stable-isotope profiles indicate that some of the denitrification may be coupled to AOM, an observation supported by an increased concentration of bacteria known to be involved in n-damo/denitrification with AOM (NC10 and Crenothrix) and anammox (“Candidatus Anammoximicrobium”) whose concentrations were highest in the methane and ammonium oxidation zones, respectively. This study shows the potential for a coupling of microbially mediated nitrate-dependent methane oxidation with anammox in stratified freshwater ecosystems, which may be important for affecting both methane emissions and nitrogen concentrations in lakes.
Highlights
Methane is a more potent greenhouse gas than CO2 and is responsible for 20 % of global warming (IPCC Working Group I, 2001). Bastviken et al (2004) have shown that lacustrine ecosystems may be responsible for 6–16 % of natural methane emissions
The associated methane concentration and stable-isotope profiles indicate that some of the denitrification may be coupled to anaerobic oxidation of methane (AOM), an observation supported by an increased concentration of bacteria known to be involved in n-damo/denitrification with AOM (NC10 and Crenothrix) and anammox (“Candidatus Anammoximicrobium”) whose concentrations were highest in the methane and ammonium oxidation zones, respectively
Experiments have shown that ndamo bacteria that are members of the candidate phylum NC10 use nitrite for the anaerobic oxidation of methane (Ettwig et al, 2010), while archaea such as ANME-2d prefer nitrate as an electron acceptor (Haroon et al, 2013)
Summary
Methane is a more potent greenhouse gas than CO2 and is responsible for 20 % of global warming (IPCC Working Group I, 2001). Bastviken et al (2004) have shown that lacustrine ecosystems may be responsible for 6–16 % of natural methane emissions. With the discovery of the anaerobic oxidation of methane (AOM) coupled to nitrate or nitrite reduction more than 10 years ago a new process was suggested that has the potential to reduce emissions of greenhouse gases of lacustrine environments by oxidizing CH4 to CO2 under anoxic conditions (Ettwig et al, 2010; Haroon et al, 2013; Raghoebarsing et al, 2006). Experiments have shown that ndamo (nitrite-dependent anaerobic methane oxidation) bacteria that are members of the candidate phylum NC10 use nitrite for the anaerobic oxidation of methane (Ettwig et al, 2010), while archaea such as ANME-2d prefer nitrate as an electron acceptor (Haroon et al, 2013). Evidence of archaeal AOM coupled with bacterial denitrification was first reported from culture experiments with two microorganisms, “Candidatus Methylomirabilis oxyfera”, which belongs to the phylum NC10 and reduces nitrite to N2, and ANME-2d lineage that uses methane to reduce nitrate to nitrite (Raghoebarsing et al, 2006)
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