Ammonium sensitivity of biological nitrogen fixation by anaerobic diazotrophs in cultures and benthic marine sediments.

Biological nitrogen fixation (BNF) is a critical process for the N budget and productivity of marine ecosystems. Nitrogen-fixing organisms typically turn off BNF when less metabolically costly N sources, like ammonium (NH₄⁺), are available. Yet, several studies have reported BNF in benthic marine sediments despite high porewater NH₄⁺ concentrations (10-1,500 µM). These activities were generally linked to anaerobic sulfate-reducing bacteria (SRB) and fermenting firmicutes.

To better understand the regulation and importance of benthic marine BNF, we evaluate the sensitivity of BNF to NH₄⁺ in benthic diazotrophs using incubations of increasing complexity. We conduct our experiment with cultures of model anaerobic diazotrophs (sulfate-reducer Desulfovibrio vulgaris var. Hildenborough, fermenter Clostridium pasteurianum strain W5), sulfate-reducing sediment enrichment cultures, and slurry incubations of sediments from three Northeastern salt marshes (USA).

All our samples demonstrate high sensitivity to external NH₄⁺. BNF is inhibited by NH₄⁺ beyond an apparent threshold [NH₄⁺] of 2 µM in liquid cultures and 9 µM in sediment slurries. Consistent with other studies, we find SRB-like nitrogenase (nifH) gene and transcripts are prevalent in sediments. We compare our inhibition threshold value with a survey of porewater NH₄⁺ data from diverse sediments, suggesting the confinement of benthic BNF to surficial sediments.

Variations in the timing to onset BNF inhibition, NH₄⁺ uptake rate, and sediment composition and biophysics could affect measurements of the apparent sensitivity of benthic BNF to NH₄⁺. We propose a simple model based on NH₄⁺ transporter affinity as a fundamental mechanistic constraint on NH₄⁺ control of BNF to improve biogeochemical models of N cycling.