Hydrogen production by the hyperthermophilic bacterium part I: effects of sulfured nutriments, with thiosulfate as model, on hydrogen production and growth.

BACKGROUND

and are hyperthermophile bacteria chosen by many research teams to produce bio-hydrogen because of their potential to ferment a wide variety of sugars with the highest theoretical H/glucose yields. However, to develop economically sustainable bio-processes, the culture medium formulation remained to be optimized. The main aim of this study was to quantify accurately and specifically the effect of thiosulfate, used as sulfured nutriment model, on growth, yields and productivities of hydrogen. The results were obtained from batch cultures, performed into a bioreactor, carefully controlled, and specifically designed to prevent the back-inhibition by hydrogen.

RESULTS

Among sulfured nutriments tested, thiosulfate, cysteine, and sulfide were found to be the most efficient to stimulate growth and hydrogen production. In particular, under our experimental conditions (glucose 60 mmol L and yeast extract 1 g L), the cellular growth was limited by thiosulfate concentrations lower than 0.06 mmol L. Under these conditions, the cellular yield on thiosulfate () could be determined at 3617 mg mmol. In addition, it has been shown that the limitations of growth by thiosulfate lead to metabolic stress marked by a significant metabolic shift of glucose towards the production of extracellular polysaccharides (EPS). Finally, it has been estimated that the presence of thiosulfate in the culture medium significantly increased the cellular and hydrogen productivities by a factor 6 without detectable sulfide production.

CONCLUSIONS

The stimulant effects of thiosulfate at very low concentrations on growth have forced us to reconsider its role in this species and more probably also in all thiosulfato-reducer hyperthermophiles. Henceforth, thiosulfate should be considered in as (1) an essential sulfur source for cellular materials when it is present at low concentrations (about 0.3 mmol g of cells), and (2) as both sulfur source and detoxifying agent for H when thiosulfate is present at higher concentrations and, when, simultaneously, the pH is high. Finally, to improve the hydrogen production in bio-processes using species, it should be recommended to incorporate thiosulfate in the culture medium.