Capnophilic lactic fermentation and hydrogen synthesis by Thermotoga neapolitana: An unexpected deviation from the dark fermentation 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 (w14 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.