Significance of archaeal nitrification in hypoxic waters of the Baltic Sea

Abstract

Ammonia-oxidizing archaea (AOA) of the phylum Thaumarchaeota are widespread, and their abundance in many terrestrial and aquatic ecosystems suggests a prominent role in nitrification. AOA also occur in high numbers in oxygen-deficient marine environments, such as the pelagic redox gradients of the central Baltic Sea; however, data on archaeal nitrification rates are scarce and little is known about the factors, for example sulfide, that regulate nitrification in this system. In the present work, we assessed the contribution of AOA to ammonia oxidation rates in Baltic deep basins and elucidated the impact of sulfide on this process. Rate measurements with 15 N-labeled ammonium, CO 2 dark fixation measurements and quantification of AOA by catalyzed reporter deposition-fluorescence in situ hybridization revealed that among the three investigated sites the highest potential nitrification rates (122-884 nmol l À 1 per day) were measured within gradients of decreasing oxygen, where thaumarchaeotal abundance was maximal (2.5-6.9 Â 10 5 cells per ml) and CO 2 fixation elevated. In the presence of the archaeal-specific inhibitor GC 7 , nitrification was reduced by 86-100%, confirming the assumed dominance of AOA in this process. In samples spiked with sulfide at concentrations similar to those of in situ conditions, nitrification activity was inhibited but persisted at reduced rates. This result together with the substantial nitrification potential detected in sulfidic waters suggests the tolerance of AOA to periodic mixing of anoxic and sulfidic waters. It begs the question of whether the globally distributed Thaumarchaeota respond similarly in other stratified water columns or whether the observed robustness against sulfide is a specific feature of the thaumarchaeotal subcluster present in the Baltic Deeps.

Nitrous oxide production by ammonia oxidizers: Physiological diversity, niche differentiation and potential mitigation strategies

“…The water column B (WCB) or low ammonium concentration group dominates at depths >300 m and currently has no cultivated representative. Correlations between oxygen and molecular data were similar to those from low-oxygen regions of the Baltic Sea (Berg, Vandieken, Thamdrup, & Jürgens, 2015), supporting AOA activity at low oxygen concentration. The basis of niche differentiation between these two groups, however, remains uncertain.…”

Nitrous oxide as a function of oxygen and archaeal gene abundance in the North Pacific

“…Admittedly, gene abundance (and by extrapolation cell number) does not necessarily confer a direct role on that gene for a measured process. Yet, similar positive relationships between Thaumarchaeota cell abundances and nitrification potentials are present in the low-oxygen waters of the Baltic, which, along with exponential increases in AamoA abundance below 100 μmol O 2  l −1 in the Atlantic, suggest maintenance of active populations of Thaumarchaeota in low-oxygen waters1319.…”

“…The Thaumarchaeota, the archaeal phylum that encompasses the ammonium oxidizing archaea (AOA18), are commonly found in low-oxygen waters at the margins of an OMZ. AOA abundance decreases as oxygen concentrations rise towards air saturation in the upper mixed layers of the ocean1319 and have been shown to also decrease as oxygen practically disappears at the oxic to anoxic interface at the core of an OMZ6. Thus, lower-oxygen waters appear to be an important niche for at least some Thaumarchaeota groups20.…”

Pathways and key intermediates required for obligate aerobic ammonia-dependent chemolithotrophy in bacteria and Thaumarchaeota

“…(Zhu-Barker et al, 2015). Due to the relatively high abundance and activity of Thaumarchaea across terrestrial, freshwater, and marine environments (Zhang et al, 2010;Pratscher et al, 2011;French et al, 2012;Berg et al, 2015) and their established tolerance of low ammonium and oxygen environments (Martens-Habbena et al, 2009), their contributions to NOx emissions is likely of high global significance (Babbin et al, 2015). For instance, marine Thaumarchaea may be essential in providing a substantial concentration of NO to denitrifying microorganisms within oxygen minimum zones, and in return, the denitrifiers could provide organic carbon to the Thaumarchaeota to establish a nitrifyingdenitrifying consortium (Karner et al, 2001;Beman et al, 2012;Ganesh et al, 2015).…”