Parallel cycling of nitrogen and iron in the upper Southern Ocean: implications for biological CO2 drawdown and global ocean fertility

Abstract

Thirty years ago, John Martin proposed that the high nutrient concentrations left unconsumed in Southern Ocean (SO) surface waters result from iron (Fe)-limitation of phytoplankton growth. Extensive culture and field work have since confirmed the central role of Fe in photosynthesis and nitrate assimilation. Given the implications for CO2, most studies of coupled Fe and nitrogen (N) cycling have focused on nitrate assimilation, with little attention paid to the role of Fe in mixed-layer N (re-)cycling. Our preliminary data suggest that Fe exerts a strong control on N regeneration; if verified, this has implications for our mechanistic understanding of the N cycle, now and in the future as the Fe supply to SO surface waters changes. To interrogate the role of Fe in the upper SO N cycle, we propose to measure 1) N uptake, regeneration, and oxidation rates at varying initial Fe concentrations ([Fe]); 2) coupled N and Fe uptake by different phytoplankton groups, separated via size fractionation and flow cytometry, at variable [Fe]; 3) kinetic parameters associated with N uptake, regeneration, and oxidation as a function of [Fe]; 4) the taxonomy and functioning of the associated planktonic (auto-, mixo- and heterotrophic) community. Diverse aspects of SO physics and chemistry are already undergoing climate-induced changes; a major motivation for this proposal is thus to develop expectations for SO fertility and ecology in response to such changes. Our work is also relevant for the nutrient supply to the low-latitude ocean, which is controlled by both the extent of nutrient uptake in SO surface waters and the ratios in which these nutrients are consumed; the latter is strongly affected by Fe availability and species composition. Through this new collaboration, we will train postgraduate students in the application of novel, cutting-edge techniques, thus building research capacity in analytical biogeochemistry that can be exploited beyond the specific aims of our proposal.