Hydrogen Production and Utilization of Agricultural Residues by Thermotoga Species
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Abstract: Hydrogen can be a renewable energy source to replace conventional fossil fuels. Compared to current hydrogen production processes by consuming fossil fuels, biological hydrogen production has the advantage of being environmentally friendly because of the use of renewable and low value biological materials. Some hyperthermophiles, such as Thermotoga species, are capable of producing hydrogen during growth. In this study, Thermotoga maritima, Thermotoga neapolitana DSM 4359 and DSM 5068, were used to investigate their potential in converting selected sugars (glucose and xylose) and complex carbon sources (cellulose, starch, xylan and agricultural residues, such as barley straw, corn stover, soybean straw, wheat straw and corn husk) to hydrogen. In addition, factors which influenced growth and hydrogen production were studied, and optimal conditions for hydrogen production were obtained. All three Thermotoga species could grow in the presence of mono sugars (glucose, xylose) and complex carbohydrates (starch, xylan, milled corn husk). They all could produce hydrogen in the presence of micro-molar level of oxygen without addition of any reducing agents in the growth medium. Compared to the slight inhibition caused by L-lactate accumulation during the growth, gradual pH decreases were the main reasons to inhibit both growth and hydrogen production of T. neapolitana species. Increasing the initial pH of the growth medium to 8.5 and stabilizing the pH by 50 mM Triz buffer resulted in higher growth and hydrogen production of T. neapolitana strains. Adjusting the medium pH at early stationary phase also increased the hydrogen production, and fewer enhancements to the growth. The pH control methods also resulted in higher conversion efficiency (converting glucose to H2) of T. neapolitana strains from 2.2 to 3.6 (H2/glucose), which was approximately 90% of the theoretical efficiency (4 moles H2 produced from 1 mole glucose). The expression of hydrogenases of T. neapolitana strains could also be increased by the pH control methods. Thermotoga species could grow and produce hydrogen using agricultural residues, such as corn husk, achieving 60% growth and hydrogen production as compared to that iii from glucose. With pH control methods, hydrogen production by T. neapolitana strains from corn husk was higher than that from glucose without pH control. These results indicated that the pH was the main factor to affect both hydrogen production and growth of T. neapolitana species, and optimal conditions for hydrogen production could be achieved by using pH control methods. Selected agricultural residues could be utilized for biological hydrogen production by Thermotoga species with minimum pre-treatment, and the pH control methods could result in a higher hydrogen production compared to that from glucose. Further studies on the continuous growth and hydrogenases of Thermotoga species are needed for better understanding of the hydrogen production mechanisms.