## 3 Results and Discussion
### Hydrogen Production Performance of C. populeti FZ10
#### 3.2.2 Hydrogen Production Using Natural Lignocellulosic Biomass  
In light of the above findings, the ability to utilize natural lignocellulosic biomasses was further investigated for _C. populeti_ FZ10. The hydrogen production capacities of three different untreated lignocellulosic substrates (10 g/L), i.e., corn stalk (CS), wheat straw (WS) and rice straw (RS), were assessed. As can be seen in Figure 5, the maximum hydrogen yield (_Ps_) for all three substrates appeared at 70-90 h. Consistent with the degradation of the substrates, the _Ps_ of CS was slightly higher than that of WS and RS. It should be noted that hydrogen could be detected within 10 h of inoculation for corn stalk (\(\lambda\) = 9), while no hydrogen was detected until 15 h after inoculation for WS and RS (Table 5). These phenomena show that CS is more favorable for hydrogen-producing bacteria growth than other substrates during the start-up phase of fermentation. The hydrogen production rates (_Rm_) of WS and RS were similar, but somewhat lower than that of CS. At the end of fermentation, the degradation of CS, WS and RS were 61.5 \(\pm\) 3.4%, 57.4 \(\pm\) 2.8% and 52.8 \(\pm\) 2.6%, respectively. Similar to the  
Figure 3: Comparison of H\({}_{2}\) production ability against different carbohydrates produced by _C. populeti_ FZ10. growth on the defined substrates, both acetate and butyrate were the main liquid metabolic by-products during the fermentation process. Figure 4: Characteristics of H\({}_{2}\) fermentation from MCC by C. _populeti_ FZ10. (**a**) Cumulative H\({}_{2}\) yield, H\({}_{2}\) production rate and medium pH, (**b**) cell protein, cellulase activity and cellulose degradation, (**c**) liquid fermentation products. Figure 5: H\({}_{2}\) fermentation from lignocellulosic biomasses by _C. populeti_ FZ10. (**a**) Cumulative H\({}_{2}\) yield, (**b**) substrate concentration, (**c**) liquid fermentation products. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Substrate** & _Ps_ **(mL/g)** & \(A\) **(h)** & _Rm_ **(mL-g-1.h-1)** & _R_\({}^{2}\) \\ \hline Corn stalk & 92.5 \(\pm\) 3.7 & 9 & 5.9 \(\pm\) 0.2 & 0.993 \\ Wheat straw & 86.2 \(\pm\) 3.5 & 15 & 5.6 \(\pm\) 0.3 & 0.995 \\ Rice straw & 88.5 \(\pm\) 4.3 & 16.5 & 5.7 \(\pm\) 0.2 & 0.997 \\ \hline \hline \end{tabular} Note: _Ps_ represents the H\({}_{2}\) yield; \(A\) represents the lag time; _Rm_ represents the maximum H\({}_{2}\) production rate.  
\end{table} 
Table 5: Kinetic parameters for hydrogen production from lignocellulosic biomasses. It can be seen from the above results that _C. populeti_ FZ10 could directly utilize untreated lignocellulosic biomasses (such as CS, WS and RS) to produce hydrogen. However, compared with the pure celluloses (such as MCC or CMC), the hydrogen yields of raw lignocellulosic biomasses were lower and the fermentation processes were slower. The main components of lignocellulosic biomasses are lignin, cellulose and hemicellulose. Lignin connects with hemicellulose through covalent bonds and encases cellulose to form a dense barrier, which hinders the degradation of cellulose and hemicellulose by fermentative microbes, resulting in low hydrogen production during the fermentation process [38]. To further elucidate the degradation characteristics of _C. populeti_ FZ10 on lignocellulosic substrates, corn stalk was used as a representative for batch fermentation tests. The changes in the surface morphology and microstructure of corn stalk before and after fermentation by _C. populeti_ FZ10 were determined with SEM. Before fermentation, a smooth and complete surface could be observed on the epidermis of corn stalk (Figure 6). After fermentation by _C. populeti_ FZ10, the surface morphology of CS was changed dramatically. The initial dense and regular structure was destroyed and replaced by a loose and porous structure. The waxy layer on the surface was almost removed, and pits and cracks could be observed clearly. These results indicated that the barrier structure of lignin was disintegrated.