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

Nitrogen (N) plays a pivotal role in regulating productivity but also imposes constraints on the adaptability of ecosystems to climate change and biogeochemical cycles.  Most N exists in the inert form of N₂, accessible to specific groups of organisms capable of metabolizing it. Diazotrophic bacteria can introduce reactive N into the ecosystem through fixation, a process in which the nitrogenase enzyme converts N₂ into more biologically available species. There are unknowns concerning what factors limit the growth of photodiazotrophs or more specifically limit N fixation. We developed a model to explore population and nutrient dynamics of phototrophic diazotrophs. It tracks biomass, in terms of carbon and energetic ATP, and the organismal content of several essential nutrients (N, phosphorus, iron, and molybdenum). It includes several forms of N; ammonium, nitrate, nitrogenase, and other forms of proteins, which allows us to explore stoichiometric constraints on C fixation, biosynthesis, nutrient uptake, and N fixation rates.  The model examines how these factors interact and predicts when any given potentially limiting factor takes prominence over others.  Our stoichiometric approach widens the types of questions that models can help address, such as: When organisms invest in producing nitrogenase? What environmental conditions, such as N:P ratios and concentrations, oxygen, and light levels, support N₂ fixation? What is the fate of newly fixed N? It is a promising and novel approach that betters our understanding of phototrophic diazotrophs, N₂ fixation, and N cycling.

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

Authors:

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

Megan Berberich, Department of Biological Sciences, Michigan Technological University  (meberber@mtu.edu)

Ana Fávaro, Department of Ecology, Evolution and Behavior, University of Minnesota-Twin Cities, Saint Paul, Minnesota, USA  (fvaro001@umn.edu)

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

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

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

Angela Knapp, Earth, Ocean, and Atmospheric Science Dept., Florida State University  (anknapp@fsu.edu)

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

Elena Litchman, Department of Global Ecology, Carnegie Institution for Science, Stanford  (elitchman@carnegiescience.edu)

Amy Marcarelli, Department of Biological Sciences, Michigan Technological University  (ammarcar@mtu.edu)

Robert McManus, Department of Biology University of Minnesota Duluth  (mcman247@d.umn.edu)

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

Robert Sterner, Large Lakes Observatory and Department of Biology, University of Minnesota Duluth  (stern007@d.umn.edu)

Nicole Wagner, Department of Biological Sciences, Oakland University  (nicolewagner@oakland.edu)