Unpacking drivers of heterotrophic N2 fixation across aquatic redox gradients: A mathematical model with bioenergetic and stoichiometric constraints

N2 fixation by aquatic organoheterotrophs supplies bioavailable nitrogen to the biosphere and thereby supports ecosystem production; yet the factors which drive this activity are poorly understood. Here, we present a generalized chemostat model to investigate stoichiometric and energetic constraints on free-living heterotrophic diazotrophs across gradients in redox state, resource quantity/quality, and resource stoichiometry. The model couples nutrient uptake and allocation functions to predict elemental fluxes with an energy dissipation model to calculate growth efficiency. After constraining model parameters with relevant culturing (e.g., Azotobacter) and environmental literature, we assessed model sensitivity to these parameters using Latin Hypercube Sampling and the statistical Partial Rank Correlation Coefficient technique. Consistent with the limited observational data available, the results showed energy acquisition and respiratory efficiency as two major controls on N2 fixation. The model predicted the presence of N2 fixation under both N-limiting and N-replete conditions, as seen in nature. N2 fixation under N-replete conditions increased with increasing resource C:N, and was least sensitive to exogenous N under eutrophic conditions with an energy-rich C source. While N2 fixation under N-replete conditions represented a relatively small (<4 %) contribution to community N demand, absolute rates under these conditions were on par with field observations under N-limitation due to higher overall heterotrophic production rates. Resolving physiological, stoichiometric, and energetic constraints on diazotrophic growth, this model unpacks drivers of N2 fixation by metabolically diverse heterotrophs.