A diazotrophy-ammoniotrophy dual growth model for the sulfate reducing bacterium Desulfovibrio vulgaris var. Hildenborough

Sulfate reducing bacteria (SRB) comprise one of the few prokaryotic groups in which biological nitrogen fixation (BNF) is common. Recent studies have highlighted SRB roles in N cycling, particularly in oligotrophic coastal and benthic environments where they could contribute significantly to N input. Most studies of SRB have focused on sulfur cycling and SRB growth models have primarily aimed at understanding the effects of electron sources, with N usually provided as fixed-N (nitrate, ammonium). Mechanistic links between SRB nitrogen-fixing metabolism and growth are not well understood, particularly in environments where fixed-N fluctuates. Here, we investigate diazotrophic growth of the model sulfate reducer Desulfovibrio vulgaris var. Hildenborough under anaerobic heterotrophic conditions and contrasting N availabilities using a simple cellular model with dual ammoniotrophic and diazotrophic modes. The model was calibrated using batch culture experiments with varying initial ammonium concentrations (0-3000 & mu;M) and acetylene reduction assays of BNF activity. The model confirmed the preferential usage of ammonium over BNF for growth and successfully reproduces experimental data, with notably clear biphasic growth curves showing an initial ammoniotrophic phase followed by onset of BNF. Our model enables quantification of the energetic cost of each N acquisition strategy and indicates the existence of a BNFspecific limiting phenomenon, not directly linked to micronutrient (Mo, Fe, Ni) concentrations, by-products (hydrogen, hydrogen sulfide), or fundamental model metabolic parameters (death rate, electron acceptor stoichiometry). By providing quantitative predictions of environment and metabolism, this study contributes to a better understanding of anaerobic heterotrophic diazotrophs in environments with fluctuating N conditions. & COPY; 2023 The Authors. Published by Elsevier B.V. on behalf of Research Network of Computational and Structural Biotechnology. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

Recent studies have shown that sulfate reducing bacteria play an important role in nitrogen cycling, especially in oligotrophic coastal and benthic environments. A cellular model was developed to study the growth of sulfate reducers under different nitrogen availabilities. The model confirmed that these bacteria prefer to use ammonium rather than fix nitrogen for growth. This study contributes to a better understanding of anaerobic heterotrophic diazotrophs in environments with fluctuating nitrogen conditions.