IRON UPTAKE LEADS TO DIVERGENT RESPONSES IN NITROGEN FIXING MICROORGANISMS IN THE OLIGOTROPHIC OCEAN

Biological nitrogen (N₂) fixation represents the primary source of new nitrogen (N) in the ocean, playing a crucial role in controlling ocean productivity. The nitrogenase enzyme responsible for N₂ fixation has a high iron (Fe) requirement, in addition to Fe in the photosystem, and evidence from experiments on cultivated microorganisms suggests that Fe availability may regulate N₂ fixation activity in the ocean. Furthermore, regions with elevated dust (i.e., Fe) fluxes in the ocean exhibit some of the highest N₂ fixation rates. Despite this, there is limited evidence supporting a direct link between Fe input and N₂ fixation activity by microorganisms in the environment. Utilizing stable isotope incubations (⁵⁷Fe, ¹³C-DIC, and ¹⁵N₂) followed by single-cell uptake measurements (using nanoSIMS) in Northern Atlantic surface waters, we demonstrated that key N₂-fixing microorganisms incorporated the added Fe. The assimilated Fe did not exhibit a strong correlation with either N₂ and CO₂ fixation rates of N₂-fixing microorganisms. The addition of Fe stimulated both N₂ and CO₂ fixation rates in symbiotic Richelia, while globally important Trichodesmium spp and Crocosphaera directed the assimilated Fe towards fueling CO₂ fixation, simultaneously reducing their N₂ fixation. The divergent responses of various N₂-fixing microorganisms to the added Fe were responsible for the observed lack of major responses at the bulk level. This has implications for global biogeochemical models. Predicting N₂ fixation in the contemporary and future ocean may require knowledge not only of the structure of the N₂-fixing microbial community but also an understanding of the degree of nutrient stress.

Primary Presenter: Abiel Kidane, Max Planck Institute for Marine Microbiology, Bremen (akidane@mpi-bremen.de)

Authors:

Jan von Arx, Max Planck Institute for Marine Microbiology, Bremen  (jarx@mpi-bremen.de)

Miriam Philippi, AWI  (miriam.philippi@awi.de)

Bernhard Tschitschko, Max Planck Institute for Marine Microbiology, Bremen  (btschits@mpi-bremen.de)

Sten Littmann, Max Planck Institute for Marine Microbiology, Bremen  (slittman@mpi-bremen.de)

Jana Milucka, Max Planck Institute for Marine Microbiology, Bremen  (jmilucka@mpi-bremen.de)

Peter Croot, University of Galway, University Road, Galway, H91 TK33, Ireland  (peter.croot@nuigalway.ie)

Tim Ferdelman, Max Planck Institute for Marine Microbiology, Bremen  (tferdelm@mpi-bremen.de)

Wiebke Mohr, Max Planck Institute for Marine Microbiology, Bremen  (wmohr@mpi-bremen.de)

Marcel Kuypers, Max Planck Institute for Marine Microbiology, Bremen  (mkuypers@mpi-bremen.de)