## 3 Results and Discussion
### Hydrogen Production Performance of C. populeti FZ10  
#### 3.2.1 Hydrogen Production Using Defined Carbohydrates  
In order to assess the hydrogen production performance of _C. populeti_ FZ10, various defined carbohydrates present in cellulosic biomasses, including microcrystalline cellulose (MCC), carboxymethyl cellulose (CMC), xylan, xylose, cellobiose and glucose could be used as feedstock. As shown in Figure 3 and Table 4, _C. populeti_ FZ10 could effectively utilize glucose, xylose and cellobiose to generate hydrogen, which resulted in the complete degradation of these simple sugars. Xylan could also promote intensive cell growth and hydrogen production by _C. populeti_ FZ10. At the end of fermentation, almost all the xylan was degraded. It is worth noting that the strain _C. populeti_ FZ10 could also directly utilize microcrystalline cellulose (MCC) and carboxymethyl cellulose (CMC), and a considerable amount of hydrogen was obtained during the fermentation process. In all the substrates, acetate and butyrate were the main liquid metabolites of hydrogen fermentation, followed by a small amount of ethanol and butanol. These observations indicated that _C. populeti_ FZ10 had a great capability for utilizing various polymers presented in the lignocellulosic biomasses to produce hydrogen under mesophilic conditions. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Substrate** **(10 g/L)**} & \multirow{2}{*}{**Degradation** **(\%)**} & \multicolumn{4}{c}{**Fermentation Products (mg/L)**} \\ \cline{3-6}  & & **Acetate** & **Butyrate** & **Ethanol** & **Butanol** \\ \hline MCC & \(72\pm 2.4\) & \(1065.25\pm 34.55\) & \(1721.11\pm 76.73\) & \(57.61\pm 3.22\) & \(93.24\pm 4.11\) \\ CMC & \(68\pm 2.5\) & \(907.12\pm 41.23\) & \(1435.30\pm 70.15\) & \(75.52\pm 3.13\) & \(182.64\pm 7.81\) \\ Glucose & \(100\) & \(1174.75\pm 56.17\) & \(1846.89\pm 85.44\) & \(81.10\pm 2.54\) & \(56.41\pm 2.23\) \\ Xylose & \(100\) & \(1102.43\pm 43.76\) & \(1701.80\pm 68.52\) & \(90.71\pm 3.61\) & \(104.34\pm 4.41\) \\ Cellobiose & \(100\) & \(992.51\pm 40.15\) & \(1517.10\pm 75.31\) & \(73.14\pm 2.88\) & \(102.11\pm 3.71\) \\ Xylan & \(98.2\pm 4.3\) & \(1091.16\pm 53.61\) & \(1810.12\pm 68.51\) & \(66.07\pm 2.67\) & \(48.55\pm 2.52\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Liquid fermentation products of _C. populeti_ FZ10 from different carbohydrates. Figure 1: Scanning electron microscopy (SEM) image of strain FZ10. Figure 2: Phylogenetic tree between strain FZ10 and closely related species, according to 165 rDNA sequences. Based on the results of hydrogen production from various carbohydrates, the dynamics of hydrogen fermentation by _C. populeti_ FZ10 with 10 g/L MCC were further investigated. As presented in Figure 4a, a lag time of 15 h was observed. When the fermentation time was between 20 and 45 h, the cumulative hydrogen yield (_Ps_) and hydrogen production rate (_Rm_) increased rapidly. Thereafter, _Ps_ remained essentially constant, while _Rm_ decreased sharply. The maximum _Ps_ and _Rm_ were 177.5 \(\pm\) 4.8 mL/g and 7.7 \(\pm\) 0.2 mL-g-1.h-1, respectively. As fermentation progressed, the medium pH changed significantly. The pH value dropped from an initial 6.98 to 5.12 during the first 50 h. Subsequently, alongside hydrogen production and the formation of acidic metabolites, the pH value gradually stabilized at about 4.91. These results were similar to previous reports, in which cellulose degradation and hydrogen production did not occur when the medium pH was lower than 5.5 [35]. Furthermore, an exponential growth of cells was detected after cultivation for 24 h, and cell protein peaked at 231 mg/L at 60 h (Figure 4b).