Carbon source-dependent lactate formation during biohydogen production

Hydrogen is considered one of the potential candidates as an alternative future energy carrier. Microorganisms offer a sustainable, environmentally friendly route for producing the so-called “biohydrogen”. This is simply formed as a by-product during carbohydrate fermentation by heterotrophic anaerobes. Producing the maximum yield of hydrogen necessitates that minimum or no reduced end products, such as lactate or propionate, are formed due to the accompanying loss of some of the reducing power of pyruvate. In our study, we found that the extent of lactate formation, and consequently hydrogen yield, in certain extreme thermophiles of the genus Caldicellulosiruptor is greatly dependent on the nature of the carbon source fermented. In addition, the growth phase of the organism as well as the partial hydrogen pressure (PH2) in the bioreactor is strongly involved. Comparative fermentations were carried out in stirred-tank reactors using different carbon sources, including glucose, fructose, xylose and sucrose. Metabolite formation patterns together with the activities of involved enzymes were followed and correlated with the theoretical energy gain of the cells on different sugars. This work aims at a deeper understanding of the physiology behind biohydrogen production that should benefit the application of metabolic engineering in that field.