Abstract
Abstract. In the last decade our understanding of the marine nitrogen cycle has improved considerably thanks to the discovery of two novel groups of microorganisms: ammonia-oxidizing archaea (AOA) and anaerobic ammonia-oxidizing (anammox) bacteria. Both groups are important in oxygen-deficient zones (ODZs), where they substantially affect the marine N budget. These two groups of microbes are also well known for producing specific membrane lipids, which can be used as biomarkers to trace their presence in the environment. We investigated the occurrence and distribution of AOA and anammox bacteria in the water column of the eastern tropical North Pacific (ETNP) ODZ, one of the most prominent ODZs worldwide. Suspended particulate matter (SPM) was collected at different depths of the water column in high resolution, at both a coastal and an open-ocean setting. The SPM was analyzed for AOA- and anammox bacteria-specific intact polar lipids (IPLs), i.e., hexose-phosphohexose (HPH)-crenarchaeol and phosphatidylcholine (PC)-monoether ladderane. Comparison with oxygen profiles reveals that both the microbial groups are able to thrive at low (< 1 μM) concentrations of oxygen. Our results indicate a clear niche segregation of AOA and anammox bacteria in the coastal waters of the ETNP but a partial overlap of the two niches of these microbial species in the open-water setting. The latter distribution suggests the potential for an interaction between the two microbial groups at the open-ocean site, although the nature of this hypothetical interaction (i.e., either competition or cooperation) remains unclear.
Highlights
The marine nitrogen cycle has been widely investigated, as nitrogen is one of the main limiting factors of primary production in the upper sunlit layers of the oceans (Arrigo, 2005; Codispoti, 1997) and the ocean accounts for about half of the global net primary production (Field et al, 1998; Gruber and Galloway, 2008)
During the eastern tropical North Pacific (ETNP) (TN278) cruise in March–April 2012 the water column of the ETNP oxygendeficient zones (ODZs) was sampled at high resolution for Suspended particulate matter (SPM), at depths from 20 to 2000 m in coastal waters and from 50 to 2500 m in open-ocean waters at a number of geographically nearby stations (Fig. 2; Table 1)
For the other coastal stations the oxycline occurs in shallow waters at a σθ of 25.0, i.e., ∼ 30 m depth, while for the in open-ocean waters it is located at σθ 25.5, which corresponds to ∼ 100 m depth
Summary
The marine nitrogen cycle has been widely investigated, as nitrogen is one of the main limiting factors of primary production in the upper sunlit layers of the oceans (Arrigo, 2005; Codispoti, 1997) and the ocean accounts for about half of the global net primary production (Field et al, 1998; Gruber and Galloway, 2008). Nitrogen fixation is mediated by few microorganisms, including cyanobacteria, while denitrification is performed by a wide range of microorganisms with different metabolic features, able to switch from aerobic to anaerobic nitrate (NO−3 )-dependent respiration modes (Lam and Kuypers, 2011). In this classical view, nitrification, representing the major oxidative part of the cycle, connecting organic nitrogen to NO−3 (Codispoti et al, 2001; Lam and Kuypers, 2011), was seen exclusively as an aerobic process carried out by ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria, members of the β- and γ proteobacteria. Oxygen serves as the electron acceptor, AOB have
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