Ammonium Sensitivity of Biological Nitrogen Fixation by Anaerobic Diazotrophsin Cultures and Benthic Marine Sediments

Publication Year
2022

Type

Journal Article
Abstract

New bioavailable nitrogen (N) from biological nitrogen fixation (BNF) is critical for the N budget and productivity of marine ecosystems. Nitrogen-fixing organisms typically inactivate BNF when less metabolically costly N sources, like ammonium (NH 4 +), are available. Yet, several studies have observed BNF in benthic marine sediments linked to anaerobic sulfate-reducing bacteria and fermenting firmicutes despite high porewater NH 4 + concentrations (10–1,500 μM). To better understand the regulating controls and importance of benthic marine BNF, we evaluate BNF sensitivity to NH 4 + in benthic diazotrophs using incubations of increasing biogeochemical complexity. BNF by cultures of model anaerobic diazotrophs (sulfate-reducer Desulfovibrio vulgaris var. Hildenborough, fermenter Clostridium pasteurianum strain W5), sulfate-reducing sediment enrichment cultures, and sediments from three Northeastern salt marshes (USA) is highly sensitive to external NH 4 + . BNF is inhibited by NH4 + beyond an apparent threshold [NH4 + ] of 2 μM in liquid cultures, most closely reflecting the true cellular sensitivity of BNF to NH4 +. Sediment slurries exhibited an apparent threshold [NH 4 + ] of 9 μM. Consistent with other studies, we find SRB-like nitrogenase (nifH) gene and transcripts are prevalent in sediments. Our survey of porewater NH 4 + data from diverse sediments suggests the broad applicability of inhibition thresholds measured here and confinement of benthic BNF to surficial sediments. Variations in BNF inhibition timing, NH 4 + uptake rate, sediment composition, and biophysics could affect measurements of the apparent sensitivity of benthic BNF to NH 4 + . We propose NH4 + transporter affinity as a fundamental mechanistic constraint on NH4 + control of cellular BNF to improve biogeochemical models of N cycling.

Journal
Journal of Geophysical Research: Biogeosciences
Volume
127
Issue
7
Date Published
07/2022
Type of Article
Open Access
Other Numbers
e2021JG006596