3.2 Efficient bioconversion of xylose into succinic acid byA. succinogenes 130Z
Previous studies have shown that A. succinogenes 130Z could consume a broad range of substrates including C6 (e.g., glucose) and C5 sugars (e.g., xylose) (Salvachúa et al., 2016). Especially, A. succinogenes 130Z showed comparable xylose utilization efficiency as glucose. However,A. succinogenes 130Z could not directly utilize xylan due to the lack of related hydrolases. As seen in Fig 3A,A. succinogenes 130Z could generate approximately 45.56 g/L of succinic acid from 80 g/L xylose with yield and productivity of 0.57 g/g and 0.63 g/L/h, respectively. The highest xylose utilization rate reached 1.93 g/L/h, which is comparable to that using glucose as the substrate (data not shown). When xylose concentration was further increased to 100 g/L, succinic acid production was also decreased. It should be noticed that succinic aid yield was decreased when xylose concentration exceeded 40 g/L.
As mentioned, T. thermosaccharolyticum M5 would be a good partner to achieve succinic acid production from hemicellulosic materials owning to its efficient xylan degradation capability. The xylose production rate ofT. thermosaccharolyticum M5 and xylose consumption rate ofA. succinogenes 130Z were accordingly compared. As shown in Fig 3B, the xylose consumption rate of A. succinogenes 130Z reached the highest 1.93 g/L/h during the first 24 h and tends to be stabilized after 72 h. While the xylose production rate of T. thermosaccharolyticum M5 was basically stable at 0.60 g/L/h, suggesting that xylose was continuously secreted. Therefore, the higher xylose consumption rate of A. succinogenes 130Z would eliminate enzymes activities inhibition ofT. thermosaccharolyticum M5, as xylose could be simultaneously consumed when A. succinogenes 130Z and T. thermosaccharolyticum M5 were co-cultivated together. Indeed, xylanase and β-xylosidase activities could maintain at 0.38-0.43 U/mL and 6.02-7.34 U/mL during the fermentation period.