The kinetics and implications of nitrification in the Southern Ocean mixed layer

Abstract

The Southern Ocean (SO. is an important sink for anthropogenic carbon dioxide, which depends in part on phytoplankton and bacterial cycling of nutrients in the surface mixed layer (ML.. Nitrogen (N. limits primary production in much of the ocean, yet SO surface waters are perennially high in nitrate (NO3-. due to combined iron and light limitation of phytoplankton. Nonetheless, the seasonal drawdown of NO3-, supplied to ML waters largely through winter mixing, can be quantitatively related to carbon export provided that nitrification, the oxidation of ammonium (NH4+. to nitrite (NO2-. and NO3-, occurs in the ML at a rate that is insignificant relative to autotrophic NO3- uptake. Our work in the Atlantic SO has shown strong seasonality in ML nitrification, with wintertime rates that are 30-70 times higher than those measured in summer when nitrification accounts for <5% of the NO3- consumed by phytoplankton. This suggests that summertime NO3- drawdown from surface waters is a reasonable proxy for SO carbon export. However, high winter ML nitrification rates affect the NO3- pool available for consumption in the following spring/summer season because the winter ML evolves into both the spring/summer ML and the subsurface “winter water” layer that underlies and exchanges with the spring/summer ML. The implications of this for SO carbon export are not yet clear. To address this uncertainty, we conducted a series of high depth-resolution nitrification experiments across the Indian sector of the SO in winter 2017. In addition, we investigated the physiological capabilities of NH4+ and NO2- oxidizing bacteria through kinetic (i.e., Michaelis-Menten. experiments that deduce the theoretical maximum oxidation rate (Vmax. and half saturation constant (Ks; an indicator of substrate affinity. parameters. These results will yield insights into the controls on nitrification under varying conditions. For example, our work-to-date suggests that NH4+ oxidation is the rate-limiting step for nitrification, but the controls on this remain unknown. - Abstract as displayed in the - Abstract booklet. There was no presentation.