The kinetics of nitrogen utilization in the oceanic mixed layer: Nitrate and ammonium interactions at nanomolar concentrations

Abstract

The concentration-dependent uptake of nitrate (NO,) and its inhibit ion by ammonium (NH,) were surveyed in surface waters of the North Atlantic Ocean. Low-level NO, determinations combined with conventional tracer methods provided the first comprehensive data set on NO, utlization kinetics at nanomolar concentrations. Uptake followed saturation kinetics described by the Michaeli s-Menten equation. The half-saturation parameter for uptake (KN) ranged 2-3 orders of magnitude, covarying with ambient NO, concentrations. KN concentrations in oceanic waters averaged -20-30 nM. NH, half-saturation parameters could only be approximated (i.e. KA + A), but observations suggested that KA and K,, were of similar magnitude in oceanic waters. Maximum uptake rates of nitrate, prntN), and ammonium, prncA), covaried, but prncA) almost always exceeded prncN); in oceanic waters, the disparity was an order of magnitude or greater. Most of the variability in A,,(~ and prncA) could be explained by variations in phytoplankton biomass and temperature. The slope of the uptake vs. concentration relationship, CX, was also investigated but was highly variable and could not be related to any of the oceanographic properties observed; cr, was generally greater than ayN. Kinetics analysis showed that NH4 is preferentially utilized over NO3 over the full spectrum of nitrogen concentrations, nanomolar to micromolar.The inhibition of NO3 uptake by NH,, was also parameterized using the Michaelis-Menten expression. The inhibition half-saturation parameter (KJ covaried with KN, but Kj concentrations in oceanic waters (N 40-50 nM) always exceeded KW Maximum inhibition (1,) was rarely complete (i.e. I,,., < I), even at 2,000 nM ammonium. Overall, results suggest that nitrogen utilization parameters currently used in ecosystem models of the open ocean should be re-examined.

Nitrate uptake and diffusive nitrate supply in the Central Atlantic

“…Glibert et al 1998Glibert et al 1998Metzler et al 1997Lewis et al 1986LeBouteiller 1986 This study ¶ 0.001 h Ϫ1 (overall mean Ϯ SE ϭ 0.0013 Ϯ 0.0002 h Ϫ1 ), three times slower than that for ammonium uptake (overall mean Ϯ SE ϭ 0.0038 Ϯ 0.0002 h Ϫ1 ). The dominance of ammonium over nitrate uptake is indicative of a system dominated by recycled production, as described for other similarly oligotrophic regions of the tropical ocean (Eppley 1981;Murray et al 1989;Dugdale et al 1992;Peña et al 1992;Harrison et al 1996). The dominance of ammonium uptake was associated with a major contribution of picoautotrophs to the photosynthetic production in the area (Agustí unpubl.…”

“…Studies in the Atlantic Ocean are largely restricted to the North Atlantic (Lewis et al 1986;Glibert et al 1988;Eckman 1994;Harrison et al 1996), and there is a remarkable paucity of information on the Central and South Atlantic (Le Bouteiller 1986;Metzler et al 1997). These studies have provided some evidence of imbalances between the diffusive nitrogen supply and new production (McGillicuddy et al 1998;Oschlies and Garçon 1998).…”

Evidence for efficient regenerated production and dinitrogen fixation in nitrogen-deficient waters of the South Pacific Ocean: impact on new and export production estimates

“…In these nutrient conditions, the tracer addition violates the general assumption that the tracer addition does not disturb the steady-state of the system and could well have evoked a major perturbation in the nitrate and ammonium uptake (Allen et al, 2002;Harrison et al, 1996). Despite this, the use of kinetic parameters, described by Harrison et al (1996), enabled us to account for the uptake rate enhancement according to the following equation given by Rees et al (1999),…”

“…To quantify the affinity constant Ks, two kinetic studies were performed with 6 graduated additions of 15 N-labeled substrate ( [V =V mS/(Ks+S)] was assumed, where V is the uptake rate for substrate concentration S, V m is the saturated uptake rate and K S , the affinity constant, e.g. the substrate concentration at half V m. The kinetics constant K S , needed for the use of Rees et al's model, was derived from the Wolf plot linear transformation of S/V vs. S. Ks obtained during 4 experiments performed in nitrogen-depleted waters (12.9 and 15 nmoles l −1 for nitrate and ammonium, respectively) were a little lower than those measured in other oceanic waters (around 25-30 nmoles l −1 ) by Sahlsten (1987) and Harrison et al (1996). As a first approximation, we assumed that DIN losses as DON (ρ loss DIN ) and nitrification (ρ NIT ) could be activated in the same way as net uptake rates and we applied the same procedure of correction to these processes.…”

Modeling the seasonal autochthonous sources of dissolved organic carbon and nitrogen in the upper Chesapeake Bay

“…This formulation allows phytoplankton cells to take up ammonium and labile DON preferentially, with nitrate uptake inhibited in the presence of significant concentrations of ammonium Harrison et al, 1996) and labile DON. DON uptake by phytoplankton was included in this model because recent research has shown that DON can be a significant nitrogen source (up to 50%) for phytoplankton (Fan et al, 2003;Mulholland et al, 2002Mulholland et al, , 2003.…”