Model development and experimental validation of capnophilic lactic fermentation and hydrogen synthesis by Thermotoga neapolitana

The aim of the present study was to develop a kinetic model for a recently proposed unique and novel metabolic process called capnophilic (CO₂-requiring) lactic fermentation (CLF) pathway in Thermotoga neapolitana. The model was based on Monod kinetics and the mathematical expressions were developed to enable the simulation of biomass growth, substrate consumption and product formation. The calibrated kinetic parameters such as maximum specific uptake rate (k), semi-saturation constant (k_S), biomass yield coefficient (Y) and endogenous decay rate (k_d) were 1.30 h⁻¹, 1.42 g/L, 0.1195 and 0.0205 h⁻¹, respectively. A high correlation (>0.98) was obtained between the experimental data and model predictions for both model validation and cross validation processes. An increase of the lactate production in the range of 40–80% was obtained through CLF pathway compared to the classic dark fermentation model. The proposed kinetic model is the first mechanistically based model for the CLF pathway. This model provides useful information to improve the knowledge about how acetate and CO₂ are recycled back by Thermotoga neapolitana to produce lactate without compromising the overall hydrogen yield.