Potential of thin stillage as a low-cost nutrient source for direct cellulose fermentation by <i>Clostridium thermocellum</i>

Utilization of thin stillage (TS), derived from grain-based ethanol production, was investigated as an alternative source for microbial growth nutrients during direct conversion of cellulose by <i>Clostridium thermocellum</i> DSM 1237. Fermentation end-products synthesized by <i>C. thermocellum</i> grown on media prepared with various concentrations (50-400 g/L) of TS were compared to those synthesized by <i>C. thermocellum</i> grown on reagent grade chemical (reference) medium. Cell-growth in TS media, monitored with the aid of quantitative polymerase chain reactions (qPCR) technique, showed prolonged growth with increasing TS concentration. Final fermentation end-product concentrations from TS media were comparable with those from the reference medium despite lower growth-rates. The volumetric H₂ production generated by <i>C. thermocellum</i> grown with medium containing a low concentration (50 g/L) of TS matched the volumetric H₂ production by <i>C. thermocellum</i> grown in the reference medium, while higher concentrations (200 g/L) of TS resulted in greater synthesis of ethanol. Supplementation of TS-media with Mg⁺⁺ enhanced ethanol production, while hydrogen production remained unchanged. These results suggest that TS, an attractive source of low-cost nutrients, is capable of supporting the growth of <i>C. thermocellum</i> and that high concentrations of TS favor synthesis of ethanol over hydrogen from cellulose