Abstract
The bacterial production of acetate via reductive acetogenesis along the Wood–Ljungdahl metabolic pathway is an important source of this molecule in several environments, ranging from industrial bioreactors to the human gastrointestinal tract. Here, we contributed to the study of reductive acetogens by considering mathematical modelling techniques for the prediction of bacterial growth and acetate production. We found that the incorporation of a hydrogen uptake concentration threshold into the models improves their predictions and we calculated this threshold as 86.2 mM (95% confidence interval 6.1–132.6 mM). Monod kinetics and first-order kinetics models, with the inclusion of two candidate threshold terms or reversible Michaelis–Menten kinetics, were compared to experimental data and the optimal formulation for predicting both growth and metabolism was found. The models were then used to compare the efficacy of two growth media for acetogens. We found that the recently described general acetogen medium was superior to the DSMZ medium in terms of unbiased estimation of acetogen growth and investigated the contribution of yeast extract concentration to acetate production and bacterial growth in culture. The models and their predictions will be useful to those studying both industrially and environmentally relevant reductive acetogenesis and allow for straightforward adaptation to similar cases with different organisms.
Highlights
The short-chain fatty acid acetate may be formed via a number of bacterial metabolic pathways, either from more complex organic compounds, or from the combination of two single carbon molecules
Model fitting of the T1 model gave an estimate of 86.2 mM (95% confidence interval 6.1–132.6 mM) for the threshold hydrogen concentration, which is comparable to the 70 ± 12.7 mM estimate of Leclerc et al [14]
The best fits to the data were obtained using the T2 and T3 models, for which the R2 values for acetate, hydrogen and growth are all above 0.9
Summary
The short-chain fatty acid acetate may be formed via a number of bacterial metabolic pathways, either from more complex organic compounds, or from the combination of two single carbon molecules. Reductive acetogenesis via the Wood–Ljungdahl metabolic pathway is an example of the latter, in which acetate may be formed from the combination of CO2 and/or CO with hydrogen [1]. Over 100 species of acetogenic bacteria that use this pathway have been isolated from a range of anaerobic habitats, such as the human and bovine gastrointestinal tract (GIT), oil fields and freshwater sediments [2]. In the human GIT, acetogens, alongside methanogens, may cross-feed on the hydrogen and CO2 produced by saccharolytic members of the GIT microbial population (for a review, see Smith et al [3]).
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