Capnophilic lactic fermentation and hydrogen synthesis by Thermotoga neapolitana: An unexpected deviation from the dark fermentation model

•Thermotoga neapolitana possesses a novel CO2-requiring process of lactic fermentation.•This process occurs to the detriment of acetic acid with no change in carbon balance.•CO2-requiring lactic fermentation does not affect production of bio-hydrogen.•Absence of correlation between lactic and hydrogen production questions the DF model.

The heterotrophic bacterium Thermotoga neapolitana produces hydrogen by fermentation of organic substrates. The process is referred to as dark fermentation and is typically complemented by production of acetic acid. Here we show that synthesis of products derived by reductive metabolism of pyruvate, mainly lactic acid, occurs to the detriment of acetic acid fermentation when the cultures of the thermophilic bacterium are flushed by saturating level of CO2. Sodium bicarbonate in a very narrow range of concentrations (∼14 mM) also causes the same metabolic shift. The capnophilic (CO2-requiring) re-orientation of the fermentative process toward lactic acid does not affect hydrogen productivity thus challenging the currently accepted dark fermentation model that predicts reduction of this gas when glucose is converted into organic products different from acetate.