Understanding Nitrogen Fixation in Phototrophic Diazotrophs: Insights from a Stoichiometric model

Nitrogen (N) plays a pivotal role in regulating productivity and imposes constraints on the adaptability of ecosystems to climate change and biogeochemical cycles.  Most N exists in the inert form of di-nitrogen gas (N2), inaccessible to many organisms except diazotrophs, which convert N2 into bioavailable ammonia (NH3) using the nitrogenase enzyme. While the stoichiometry of N2-fixation reaction is well understood, the organismal stoichiometric controls on N2-fixation remain elusive. Here, we developed a mechanistic model, representing a chemostat culture, to explore population and nutrient stoichiometry dynamics of phototrophic diazotrophs. It tracks biomass in terms of carbon and energetic ATP, and the organismal (cellular) content of the essential nutrients N, phosphorus (P), iron (Fe), and molybdenum (Mo). Our model tracks multiple N components including ammonium and nitrate, nitrogenase, nitrate reductase, and other protein forms, allowing us to explore stoichiometric constraints on C fixation, biosynthesis, nutrient uptake, and N2 fixation rates. Our model examines how these factors interact and predicts when any given potentially limiting factor takes prominence over others across a gradient of N:P:Fe input concentration ratios. Through model analyses, we can predict how these limiting elements affect diazotrophs with different physiological traits. Our model is a novel approach that betters our understanding of phototrophic diazotrophs, N2 fixation, and biogeochemical cycling.

Presentation Preference: Oral

Primary Presenter: Nicole Wagner, Oakland University (nicolegouldingwagner@gmail.com)

Authors:

Nicole Wagner, Oakland University  (nicolewagner@oakland.edu)

Angela Peace, Texas Tech University  (a.peace@ttu.edu)

Angela Knapp, Texas A&M University  (aknapp@tamu.edu)

Robert Sterner, University of Minnesota Duluth  (stern007@d.umn.edu)

Robert McManus, University of Michigan  (rmcman@umich.edu)

Na Li, GEOMAR Helmholtz Centre for Ocean Research Kiel  (nli@geomar.de)

Ana Fávaro, University of Minnesota-Twin Cities  (fvaro001@umn.edu)

Audrey Goeckner, University of Florida  (agoeckner@ufl.edu)

Elena Litchman, Michigan State University  (litchman@msu.edu)

Patrick Kelly, Wisconsin Department of Natural Resources  (patrick.kelly@wisconsin.gov)

Megan Berberich, Michigan Technological University  (meberber@mtu.edu)

Robinson Fulweiler, Boston University  (rwf@bu.edu)

Thad Scott, Baylor University  (thad_scott@baylor.edu)

Amy Marcarelli, Michigan Technological University  (ammarcar@mtu.edu)