A diazotrophy-ammoniotrophy dual growth model for the sulfate reducing bacterium

Area of Science:

• Microbiology
• Biogeochemistry
• Environmental Science

Background:

• Sulfate-reducing bacteria (SRB) are key players in nitrogen (N) cycling, particularly in N-limited environments.
• Biological nitrogen fixation (BNF) is common in SRB, yet its integration with growth models under varying N availability is poorly understood.
• Existing SRB models often focus on sulfur cycling and assume fixed-N availability.

Purpose of the Study:

• To investigate the diazotrophic growth of *Desulfovibrio vulgaris* under varying nitrogen conditions.
• To develop and calibrate a cellular model simulating dual ammoniotrophic and diazotrophic growth modes.
• To quantify the energetic costs and identify limiting factors for BNF in SRB.

Main Methods:

• Utilized a simple cellular model with dual ammoniotrophic and diazotrophic modes.
• Calibrated the model using batch culture experiments with controlled ammonium concentrations (0-3000 µM).
• Employed acetylene reduction assays to measure BNF activity.

Main Results:

• The model confirmed preferential ammonium assimilation over BNF for growth.
• Observed distinct bi-phasic growth curves: an initial ammoniotrophic phase followed by BNF onset.
• Identified a novel BNF-specific limiting phenomenon unrelated to common factors like micronutrients or by-products.

Conclusions:

• SRB exhibit a clear preference for ammonium, utilizing BNF only when ammonium is depleted.
• The developed model accurately predicts SRB growth and BNF activity under fluctuating N conditions.
• This research enhances understanding of anaerobic heterotrophic diazotrophs in dynamic N environments.