## 3 Results and Discussion
### Hydrogen Production Performance of C. populeti FZ10
#### 3.2.2 Hydrogen Production Using Natural Lignocellulosic Biomass
In addition to the observation of surface morphology, the changes in the chemical components of CS and the enzyme activity of the fermentation broth were also analyzed. As presented in Figure 7a, the hemicellulose content decreased significantly after cultivation for 25 h, while the cellulose content did not drop until cultivation for 35 h. The reason might be that hemicellulose, with its lower polymerization degree, is more easily degraded than cellulose by _C. populeti_ FZ10. We noticed that the lignin content had no obvious change, indicating that _C. populeti_ FZ10 could not degrade lignin efficiently. At the end of fermentation, 47.2 \(\pm\) 2.3% of cellulose and 58.1 \(\pm\) 2.0% of hemicelluloses in corn stalk were degraded by _C. populeti_ FZ10. Consistent with the degradation of cellulose and hemicellulose, the activity of exoglucanase, endoglucanase and xylanase increased rapidly after cultivation for 25 h, reaching the maximum values of 0.37 \(\pm\) 0.02 U/mL, 0.46 \(\pm\) 0.01 U/mL and 0.54 \(\pm\) 0.02 U/mL, respectively (Figure 7b). \(\beta\)-glucosidase activity was not obviously detected until cultivation for 45 h, achieving a maximum activity of 0.13 \(\pm\) 0.02 U/mL. These results further indicated the degradation of cellulose and hemicellulose in untreated corn stalk by _C. populeti_ FZ10. The hydrogen production capability from various untreated cellululosic substrates (pure cellulose and lignocellulosic biomasses) by _C. populeti_ FZ10 was also compared with that of several other _Clostridium_ species. As illustrated in Table 6, when using pure cellulose as the fermentation substrate for hydrogen production, the hydrogen yield of _C. populeti_ FZ10 was much higher than that found in the existing literature. This may be due to microbial acclimation by MCC, making strain _C. populeti_ FZ10 more adaptable to cellulose than other strains. When using various other lignocellulosic biomasses as substrates, the strain  
Figure 6: SEM image of corn stalk. (**a**) Unfermented corn stalk, (**b**) corn stalk after fermentation. C. populeti_ FZ10 displayed an excellent hydrogen production performance. Particularly when compared with the previous isolated strain _C. sartaggorne_ FZ11, _C. populeti_ FZ10 demonstrated a higher degradation of cellulose and hemicellulose in raw corn stalk [26]. Although [PERSON] et al. reported that _C. butyricum_ FS3 displayed a slightly higher hydrogen yield for raw corn stalk than the present strain _C. populeti_ FZ10, it could not directly utilize pure cellulose to produce hydrogen [34]. Moreover, the hydrogen yield from raw corn stalk by _C. populeti_ FZ10 was comparable to some reported data on the hydrogen fermentation of pretreated corn stalk. For example, [PERSON] et al. reported that _C. butyricum_ AS1.209 could utilize steam-exploded corn stalk to generate hydrogen, and the hydrogen yield reached 68 mL/g [39]. [PERSON] et al. introduced _C. paraputritritum_ into the hydrolysis stage of corn stalk under acidic conditions, and the hydrogen yield was only 63.7 mL/g [40]. Many studies have also reported on hydrogen production with _C. thermocellum_ under thermophilic conditions. For example, [PERSON] et al. reported that the maximum hydrogen yield was only 23.97 mL/g from dried distillers grain using _C. thermocellum_ 27,405 [25]. [PERSON] et al. used _C. thermocellum_ 7072 to degrade corn stalk, and the maximum hydrogen yield was able to reach 74.4 mL/g from corn stalk [46]. Compared with _C. thermocellum_, the present mesophilic strain _C. populeti_ FZ10 had a higher hydrogen yield and hydrogen production rate from raw corn stalk. Due to the different fermentation conditions, it is