IS ANAEROBIC AMMONIUM OXIDATION (“ANAMMOX”) IMPORTANT FOR NITROGEN CYCLING IN GROUNDWATER?

Anaerobic ammonium oxidation (anammox) is a process that couples the oxidation of ammonium with the reduction of nitrite, producing nitrogen gas as an end product. Anammox has been shown to compete effectively with denitrification in wastewater treatment and in marine sediments, two environments where nitrate and ammonium often co-exist. However, little is known about the potential importance of anammox in groundwater, particularly relative to denitrification. A collaborative study of anammox in groundwater is being conducted at the USGS Cape Cod Toxic Substances Hydrology research site, which features a wastewater-impacted contaminant plume containing zones of elevated nitrate, ammonium, and dissolved organic carbon (DOC) concentrations and where previous studies have shown denitrification to be an active electron-accepting process. In the present study, molecular analyses indicate that anammox bacteria are a substantial component of the groundwater microbial community within the contaminant plume; relative abundances of different anammox bacteria vary with differing locations and geochemistry. Incubation experiments with groundwater microorganisms collected on filters and combinations of ¹⁴N and ¹⁵N nitrite and ammonium demonstrated that anammox contributed up to 62% of potential N₂ production in this aquifer. Likewise, natural-gradient tracer tests with ¹⁵N-enriched nitrite produced both ²⁹N₂ and ³⁰N₂, demonstrating that anammox was active in situ, even when background ammonium concentrations were low. The added nitrite tracer stimulated in situ expression of hzo and nosZ, genes that encode for hydrazine oxidase (anammox) and nitrous oxide reductase (denitrification), respectively. This study indicates that anammox can occur in groundwater and that excess N₂ is not necessarily produced only by denitrification. Differential transport rates for nitrate and ammonium, the potential for ammonium production by DOC degradation, and the competition with denitrification suggest that subsurface nitrogen cycling is more complex than is commonly considered.