A new study by l'Helguen et al. brings interesting evidence concerning the inhibition of nitrate uptake by ammonium. From experiments conducted with open ocean phytoplankton communities using a 15N tracer technique to measure the uptake kinetics for two photoautotrophic size classes (<2µm and >2µm), together with a number of previous studies, the discussion provides an interesting and robust hypothesis. At ambient ammonium concentrations and more realistic environmental conditions, distinct differences in the uptake rates of nitrate were observed in all of the experiments with an iterative empirically-based relationship between a declining rate of nitrate uptake evident with elevated ammonium concentrations.
The smaller size fraction (< 2µm) of an open ocean phytoplankton community displayed reduced rates of nitrate uptake that conform to a reduced half-inhibition constant (Ki) and increased maximum inhibition (Imax) calculated by the use of the Michaelis-Menton formulation presented by Varela and Harrison (1999). An increased αi ratio (Imax/Ki) indicates an increased sensitivity for the smaller phytoplankton and implies a pronounced effect for small cell oligotrophic communities. For the larger size fraction these results were not only observed to be the contrary, but through comparisons with other studies, explanations are presented for the inhibition kinetics observed in other ocean regions. This is especially relevant to the general correlation between Ki and the Chl-a in the >2µm size fraction. When an increased autotrophic biomass was present in the >2µm size fraction a reduced inhibitory effect was observed and generally associated with generally larger cell-type diatoms and dinoflagellate species (Wheeler and Kokkinakis 1990). With a reduced autotrophic biomass (the smaller <2µm size fraction) inhibition is observed to increase, in agreement with the observations made under oligotropihc conditions by Harrison et al. (1996).
The study helps to elucidate our understanding of dissolved inorganic nitrogen uptake in the open ocean with results that can be used to parameterise more mechanistic photoautotrophic models and develop a better description for the uptake of nitrate and ammonium in larger scale global models.
In addition, there are implications for what is termed “new production” as used in the definition of the f-ratio. The study does not address whether the ammonium inhibition of nitrate uptake is compensated or not, but does indicate the inability for larger cell types to adapt, even if restricted by a reduced surface:volume.
This would however only add to the resulting effect that a decrease of nitrate uptake would have on new production and consequently the f-ratio, especially in areas where there is a low autotrophic biomass. More that this, the study parts with one main conclusion that is still often not accounted for; cell size is important and this is not often given enough consideration when discussing nitrogen metabolism in the ocean.
Review by Nicholas Stephens, European Institute of Marine Science, Brest, France
Stéphane L'Helguen, Jean-François Maguer, and Julien Caradec, 2008. Inhibition kinetics of nitrate uptake by ammonium in size-fractionated oceanic phytoplankton communities: implications for new production and f-ratio estimates, J. Plankton Res. 2008 30: 1179-1188; doi:10.1093/plankt/fbn072