Nitrous oxide from nitrification and denitrification by archaea

Archaea have been detected in almost all oceanic regions throughout the water column and in the sediments (see for example Karner et al, 2001; Sinninghe Damste et al, 2002; Francis et al, 2005; Teira et al, 2006; Varela et al, 2008). The successful isolation of an NH4+ -oxidizing archaeon (Konneke et al, 2005) raised the question of whether we have overlooked the role of ammonium-oxidizing archaea (AOA) in the oceanic nitrogen cycle. Meanwhile the gene amoA, which is commonly used as a marker gene for the ammonium-oxidizing enzyme ammonia mono-oxygenase in Crenarchaeota (the dominant group of mesophilic archaea in the ocean), has been detected in the North Atlantic Ocean, the North Sea, the Black Sea and in sediments (see for example Francis et al, 2005; Wuchter et al, 2006; Lam et al, 2007). On the basis of the dominant abundance of the AOA amoA compared to the bacterial amoA, it has been suggested that Crenarchaeota in the uppermost 1000m of the North Atlantic Ocean were mainly responsible for NH4+ oxidation (i.e. the first step of nitrification), whereas nitrifying bacteria seem to play only a minor role (Wuchter et al, 2006). Similar results were found in estuarine sediments where AOA seem to play a major role as NH4+ oxidizers (Caffrey et al, 2007). Archaea are also capable of performing the classical denitrification pathway, including N2O formation and its subsequent reduction to N2 (see for example the overview article by Cabello et al, 2004, and references therein). Despite the fact that archaea perform the same nitrogen transformation processes as bacteria, there are 'significant differences in the structure and regulation of some enzymes involved in the nitrogen metabolism in archaea' as stated by Cabello et al (2004). This might be especially important for interpreting N2O isotopic signatures. However, N2O production and/or consumption by archaea have not been studied yet.

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