Ambient nitrate switches the ammonium consumption pathway in the euphotic ocean

Abstract

Phytoplankton assimilation and microbial oxidation of ammonium are two critical conversion pathways in the marine nitrogen cycle. The underlying regulatory mechanisms of these two competing processes remain unclear. Here we show that ambient nitrate acts as a key variable to bifurcate ammonium flow through assimilation or oxidation, and the depth of the nitracline represents a robust spatial boundary between ammonium assimilators and oxidizers in the stratified ocean. Profiles of ammonium utilization show that phytoplankton assemblages in nitrate-depleted regimes have higher ammonium affinity than nitrifiers. In nitrate replete conditions, by contrast, phytoplankton reduce their ammonium reliance and thus enhance the success of nitrifiers. This finding helps to explain existing discrepancies in the understanding of light inhibition of surface nitrification in the global ocean, and provides further insights into the spatial linkages between oceanic nitrification and new production.

Enhanced Ammonia Oxidation Caused by Lateral Kuroshio Intrusion in the Boundary Zone of the Northern South China Sea

“…In Figure 2b, we present vertical profiles of in situ AO rate. AO was slow in surface layers (0.001-0.6 nmol · L À1 · day À1 ) with high values ranging from 3.5 to 133.5 nmol · L À1 · d À1 at depths of~40 to 100 m. Such downward increasing trend had been observed in previous studies and been attributed to light inhibition (Ward, 2005) and ambient nitrate concentration (Wan et al, 2018). Here in this study, AO appeared at around nitracline being consistent to previous observations.…”

“…Xu, M. N., Zhang, W., Zhu, Y., Liu, L., Zheng, Z., Wan, X. S., et al (2018 Hayward, 1996). Some model results show that horizontal advection of such new nutrients is comparable or exceeded the vertical transport of inorganic nutrients in at least some regions (Anderson et al, 2015, and references therein;Letscher et al, 2016).…”

Factors Regulating Nitrification in the Arctic Ocean: Potential Impact of Sea Ice Reduction and Ocean Acidification

“…AOA are known to have high ammonia affinity, allowing successful competition with phytoplankton (Martens‐Habbena et al, ). However, above the nitracline depth, AOOs could be outcompeted by phytoplankton (Wan et al, ). In the surface layer, the nitrate concentration was very low, except near the bottom in the shelf region (Figure ).…”

“…Ammonia oxidation, the first rate‐limiting step in nitrification, is known to be susceptible to changes in ammonium concentration, pH, and light in oxygenated waters (Beman et al, ; Guerrero & Jones, , ; Horak et al, ; Kitidis et al, ; Martens‐Habbena et al, ; Merbt et al, ; Qin et al, ). Ammonia is the substrate for ammonia oxidation and, thus, its availability limits nitrification (Horak et al, ; Smith et al, ; Wan et al, ; Ward, ). The ammonia/ammonium equilibrium is pH dependent (NH 3 + H + ⇄ NH 4 + ) and substrate availability for ammonia oxidation is therefore influenced by changes in pH (Beman et al, ).…”

Coupled effect of substrate and light on assimilation and oxidation of regenerated nitrogen in the euphotic ocean

“…Besides light inhibition, phytoplankton competition for substrates is also a plausible cause of suppressing euphotic NH 4 + oxidation in the euphotic zone (Ward ; Christman et al ; Beman et al ; Smith et al ). Recently, the concentration of ambient NO 3 − was found to be an effective factor additionally to influencing the vertical distribution of NH 4 + oxidation (Wan et al ), as increased ambient NO 3 − level may modulate the phytoplankton community and then down regulate the affinity of phytoplankton toward NH 4 + , leaving more opportunity for nitrifiers to thrive. Besides NH 4 + , urea has long been shown to be a reactive N source for phytoplankton growth (e.g., McCarthy ; Antia et al ; Berman and Bronk ; Casey et al ; Painter et al ; Solomon et al ).…”