## 2 Results and Discussion
### The Impact of Fed-Batch Fermentation on the ORP-Controlled MEC System  
The fed-batch fermentation strategy is a widely acknowledged technique for achieving elevated concentrations of target metabolites in various bioprocesses. In our current study, we integrated the fed-batch methodology with an ORP-controlled MEC system to enhance the production of succininic acid.  
During the course of our 48 h fermentation period, the implementation of fed-batch fermentation in the ORP-controlled MEC system resulted in a substantial increase in the succininic acid concentration, reaching an impressive 74.43 g/L, as depicted in Figure 3. This represents an increase of 10.81% and 12.44% over the succininic acid yields obtained from the non-electric and potentiostatic (constant \(-\)1 V) MEC systems, respectively. Notably, the application of a cathode-mediated potential in the ORP-controlled MEC system was correlated with accelerated microbial growth rates, whereas the continuous exposure to the potential was observed to have a detrimental effect on cell proliferation.  
Further analysis, as illustrated in Figure 3c, revealed that the ORP-controlled MEC system achieved a higher rate of succinic acid productivity, quantified at 0.735 g L\({}^{-1}\) h\({}^{-1}\) g\({}^{-1}\)DCw, when compared to the control setup. This enhanced productivity can be attributed to the optimized extracellular conditions provided by the ORP control, which are conducive to microbial activity and succinic acid biosynthesis.  
Comparative studies indicated that the potentiostatic (constant \(-\)1 V) system, which subjects the microbial culture to a sustained high voltage, led to lower succinic acid concentrations due to the inhibitory effects of prolonged high-voltage application on cell growth. Such conditions can negatively impact the bacterial cells, undermining their viability and metabolic capacity for succinic acid production [7].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**ORP Level** & **Succinic Acid** & \begin{tabular}{c} **Succinic Acid/** \\ **Acetic Acid** \\ **(g/L)** \\ \end{tabular} & \begin{tabular}{c} **Acetic Acid (g/L)** \\ **(10 h)** \\ \end{tabular} & \
\begin{tabular}{c} **Electric Charge (C)** \\ \end{tabular} &
\begin{tabular}{c} **Electronic** \\ **Utilization** \\ **Efficiency** \\ \end{tabular} \\ \hline Control & 25.99 \(\pm\) 0.33  & / & / \\ CPMEC & 29.03 \(\pm\) 1.17  & 1076.42 \(\pm\) 91.22  & 2.03 & 8.39 \(\pm\) 0.14  & 135.64\% \\ \(-\)400 mV & 29.39 \(\pm\) 0.14  & 562.88 \(\pm\) 16.02  & 1.94 & 8.40 \(\pm\) 0.16  & 68.80\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Effects of ORP optimization in ORP-controlled MEC system.  
The superior performance of the fed-batch ORP-controlled MEC system was further substantiated by its energy consumption and electronic utilization efficiency. The system utilized 1748.62 coulombs of electricity, which was only 55.30% of the energy consumed by the potentiostatic system. Moreover, the electronic utilization efficiency of the ORP-controlled system reached 192.02%, a staggering 15.19-fold increase over the potentiostatic system.  
In conclusion, our study demonstrates that the high-voltage pulse-based ORP-controlled MEC system is not only more effective in providing the necessary electrons for microbial metabolism but also does so with a greater energy efficiency, significantly boosting succinic acid production. These compelling findings highlight the practical applicability and advantages of an ORP-controlled MEC system in industrial-scale fermentation processes, offering a promising avenue for sustainable and cost-effective organic acid production.