## 4 Discussion
The same conclusion had been reached through exploration and application of temperature-adapted phytases, pectinases, proteases, amylases, xylanases and cellulases described in previous studies [38; 39; 40]. Besides, BL-SsEct also demonstrated a stronger tolerance to the substrate aspartate, and maintained 90% ectoines production at the concentration of 300 mM, under which _Escherichia coli_ K12/BW25113 and _E. coli_ MG1655 ECT2 only showed the approximately 55% and 40%, respectively [7; 19]. Given that aspartate also could be utilized by the glutamate-aspartate transamination system to form glutamate, which could act as an important osmoregulatory solute in vivo and circularly provide the amino donor group for the transamination catalyzed by \(\text{Et}\text{B}_{\text{S. salarius}}\) participating in the first reaction of ectoine synthesis [42; 43], thereby it was confirmed to be an ideal substrate for ectoine bioconversion, and increasing concentration of this dual functional substrate was thought to be rather advantageous for driving ectoine synthesis and accumulation in the BL-SsEct cells at maximum-tolerated concentrations. Finally, to investigate the potential of strain BL-SsEct to become a promising candidate for commercial production, bioconversion was performed in a 5 L fermentator under the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Microorganism** & **Source of Genes** & **Ectoine Titer** & **Optimal** & **Ptimal** & **Reactor** & **Reference** \\  & & **(g/L)** & **Temperature** & **pH** & **System** & **Reference** \\ \hline _Corynebacterium_ & & & & & & \\ _glutaminamin_ & _Pseudomonas stuttzeri_ & 65.3 & 30 & 7.0 & Fed-batch & [24] \\ _ectABC\({}^{\text{opt}}\)_ & & & & & & \\ _Escherichia coli_ & & & & & & \\ W3110 & _Halomonas elongata_ & 25.1 & 36 & 7.0 & Fed-batch & [30] \\ _Escherichia coli_ & _Halomonas elongata_ & & & & Whole-cell & [19] \\ K12/BW25113 & DSM 2581 & 25.1 & 30 & 7.0 &
\begin{tabular}{c} Whole-cell \\ catalysis \\ \end{tabular} & [19] \\ _Escherichia coli_ BL21 & _Salinicola salarius_ & 22.5 & 25 & 6.5 & Fed-batch & This study \\ _(DE3)_ & & & & & \\ _Escherichia coli_ & & & & & \\ MG1655 & _Halomonas elongata_ & 12.7 & 40 & 6.5 & Batch & [7] \\ _Escherichia coli_ & & & & & \\ DH5\(\alpha\) & & & & & \\ _Escherichia coli_ & _Acipliithicum cryptum_ & & & & \\ DH5\(\alpha\) & & DSM 2389 & 1.7 & 37 & 7.0 & Batch & [41] \\ _E. coli_ BL21 (DE3) & _Halomonas elongata_ & & & & \\ BK-AG25 & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of bioconversion conditions for ectoine heterologous production. optimum condition of 25 \({}^{\circ}\)C. By process controlling of the pH and substrate feeding, the concentration of the extracellular ectoine maintained a linear accumulation during the whole bioreactor cultivation of 24 h, and achieved highest yield of 22.5 g/L, of which exhibited a significant improvement of catalysis efficiency compared with the reaction in flask level. However, it was slightly lower than ectoine yield of 25.1 g/L produced by _Escherichia coli_ K12/BW25113 [19] and _Escherichia coli_ W3110 ECT05 [30] harboring the ectoine biosynthesis genes from _H. elongata_, BL-SsEct demonstrated a more excellent adaptability under the lower catalytic temperature of 25 \({}^{\circ}\)C. It is widely acknowledged that lower reaction temperature could lead to some obvious and unique advantages compared to mesophilic and thermophilic manufacture process, including reducing the energy consumption in the heating process, avoiding thermo-inactivation of the heat-labile enzymes and preventing the configural and functional transformations of the thermosensitive flavors and drugs [25; 38]. Accordingly, efficient bioconversion under lower catalytic temperature using this producer strain meant a competitive advantage for commercialized production of pharmaceutical ectoine.