Potential competition between marine heterotrophic prokaryotes and autotrophic picoplankton for nitrogen substrates

Abstract

Heterotrophic prokaryotes have the capacity to uptake inorganic nitrogen (N) substrates. However, it remains unclear what the potential competition is between heterotrophic prokaryotes and autotrophic plankton for N in the ocean, which would shunt the flow of N supporting primary production. To date, it has been difficult to distinguish heterotrophic prokaryotic N uptake from that of autotrophic picoplankton, especially in oligotrophic oceans dominated by cyanobacteria. We carried out field-based DNA stable isotope probing incubation experiments in the South China Sea combining measurements of uptake rates of ammonium, nitrate, nitrite, and urea to estimate the taxon-specific potential N assimilation. The results indicate that phylogenetically diverse heterotrophic prokaryotes significantly incorporated multiple N sources, contributing approximately 17-41% and 19-55% of total N uptake potential in the euphotic zone of the South China Sea continental shelf and open ocean, respectively, potentially competing with cyanobacteria (mainly Prochlorococcus). Notably, heterotrophic prokaryotes made a higher contribution to bulk uptake of nitrate in the incubation systems of the open ocean relative to regenerated N, and thus there was a tendency to overestimate the f-ratio. Extrapolating our results to the oligotrophic, low-latitude ocean via a global model suggests the f-ratio would decrease~18%. This suggests a more complicated biogeochemical role of heterotrophic prokaryotes in the biological carbon pump than hitherto assumed, with important implications for N and carbon cycling in the vast open ocean. Nitrogen (N) limits the primary productivity of autotrophic plankton that, together with subsequent carbon export into deeper ocean waters, drives marine carbon sequestration by the so-called biological pump. Ammonium (NH þ 4 ), nitrate (NO À3 ), nitrite (NO À 2 ), and urea are the major nitrogenous substrates ("N substrate[s]" hereinafter only refers to these four substrates) supporting oceanic primary production; thus, measuring their uptake is of primary concern (Mulholland and Lomas 2008). Nitrogen imported into the euphotic zone supports new production, which can be exported into the deep ocean (Eppley and Peterson 1979). The primary source of "new" N to the euphotic zone in the open ocean is thought to be upward diffusion and convection of NO À 3 (Dugdale and Goering 1967). Ammonium and urea are the main N compounds internally recycled within the system, supporting "regenerated" primary production (Dugdale and Goering 1967). Ammonium oxidation and assimilatory reduction of NO À 3 by phytoplankton are two dominant sources of NO À 2 in the euphotic zone (Lomas and Lipschultz 2006;Buchwald and Casciotti 2013) and, therefore, NO À 2 uptake can contribute to new or regenerated primary production.Conventionally, autotrophic plankton are primary consumers of dissolved inorganic nitrogen (DIN) (NH þ 4 , NO À 3 , NO À2 ) in the euphotic zone, while bacterial heterotrophs are primary consumers of organic compounds. However, t...

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