Study of Succinic Acid Production by Actinobacillus Succinogenes

Succinic acid has recently emerged as an important chemical （commodity） because it can be used for the manufacturing of synthetic resins and biodegradable polymers and as an intermediate for chemical synthesis. Till date, succinic acid is mainly produced by chemical processes, however, due to the environmental concerns and the concepts of sustainability, researches are directed towards the production of succinic acid by microbial fermentation. The fact that carbon dioxide （CO2） is needed by the microorganisms for succinic acid production is another interesting feature. The fermentation was carried out with Actinobacillus succinogenes using a two-level fractional factorial design 2sl. The variables analyzed and their levels were： concentration of glucose, yeast extract, temperature, pH and agitation. The results show that the variables that more influenced on succinic acid production were pH, temperature and yeast extract.

Optimization of succinic acid fermentation with Actinobacillus succinogenes by response surface methodology(RSM)

Succinic acid is considered as an important platform chemical. Succinic acid fermentation with Actinobacillus succinogenes strain BE-1 was optimized by central composite design （CCD） using a response surface methodology （RSM）. The optimized production of succinic acid was predicted and the interactive effects between glucose, yeast extract, and magnesium carbonate were investigated. As a result, a model for predicting the concentration of succinic acid production was developed. The accuracy of the model was confirmed by the analysis of variance （ANOVA）, and the validity was further proved by verification experiments showing that percentage errors between actual and predicted values varied from 3.02% to 6.38%. In addition, it was observed that the interactive effect between yeast extract and magnesium carbonate was statistically significant. In conclusion, RSM is an effective and useful method for optimizing the medium components and investigating the interactive effects, and can provide valuable information for succinic acid scale-up fermentation using A. succinogenes strain BE-I.

Efect of the Gad system on Actinobacillus succinogenes during acid stress

The glutamate decarboxylase(Gad)system is an important amino acid-dependent acid resistance system commonly found in microorganisms.Actinobacillus succinogenes is one of the best natural producers of succinic acid(SA)but lacks glutamate decarboxylase.This study assessed the efects of Gad system introduction into A.succinogenes.The recombinant strains gadB-SW and gadBC-SW were constructed by heterologous expression of gadB alone,or gadB together with gadC,respectively.After 1.0 and 1.5 h of acid stress at pH 4.6,cell survival of gadBC-SW was greater than gadB-SW.The growth of gadB-SW and gadBC-SW was both afected by the expression of heterologous proteins and byγ-aminobutyric acid,with gadBC-SW growth reduced at a neutral pH.SA production in acidic conditions was evaluated by a shake fask and by 3-L bioreactor fermentation.The results showed gadBC-SW to increase SA production by 8.4%in shake fask compared to the parent strain,SW.For a 3-L bioreactor batch fermentation under acidic environment,the highest conversion rate of sugar to SA was observed for gadBC-SW,reaching 96%.However,SA concentration by gadBC-SW was only 47 g/L and 31 g/L at pH 6.5 and pH 6.0,respectively.In summary,the introduction of heterologous gadB and gadC into A.succinogenes not only improved acid tolerance but also infuenced the synthesis of SA and added a metabolic burden.

Enhanced direct gaseous CO_(2) fixation into higher bio-succinic acid production and selectivity

Utilizing CO_(2)for bio-succinic acid production is an attractive approach to achieve carbon capture and recycling(CCR)with simultaneous production of a useful platform chemical.Actinobacillus succinogenes and Basfia succiniciproducens were selected and investigated as microbial catalysts.Firstly,the type and concentration of inorganic carbon concentration and glucose concentration were evaluated.6 g C/L MgCO_(3)and 24 g C/L glucose were found to be the optimal basic operational conditions,with succinic acid production and carbon yield of over 30 g/L and over 40%,respectively.Then,for maximum gaseous CO_(2)fixation,carbonate was replaced with CO_(2)at different ratios.The“less carbonate more CO_(2)”condition of the inorganic carbon source was set as carbonate:CO_(2)=1:9(based on the mass of carbon).This condition presented the highest availability of CO_(2)by well-balanced chemical reaction equilibrium and phase equilibrium,showing the best performance with regarding CO_(2)fixation(about 15 mg C/(L·hr)),with suppressed lactic acid accumulation.According to key enzymes analysis,the ratio of phosphoenolpyruvate carboxykinase to lactic dehydrogenase was enhanced at high ratios of gaseous CO_(2),which could promote glucose conversion through the succinic acid path.To further increase gaseous CO_(2)fixation and succinic acid production and selectivity,stepwise CO_(2)addition was evaluated.50%-65%increase in inorganic carbon utilization was obtained coupled with 20%-30%increase in succinic acid selectivity.This was due to the promotion of the succinic acid branch of the glucose metabolism,while suppressing the pyruvate branch,along with the inhibition on the conversion from glucose to lactic acid.