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"Bacteria-zooxanthellae-coral" is a pair of typical triangular relationships in the marine ecosystem. There are complex flows of material, information, and energy in this system. The balance and stability of the symbionts is an important guarantee for maintaining the health of coral reef ecosystems. Many studies have been conducted on the interaction of coral symbionts in the past 20 years, which help clarify the material metabolism and nutrient exchange between "bacteria-zooxanthellae-coral" and their interaction with the environment. Due to the complexity of this symbiotic system, the mechanisms of some phenomena are still not well understood, especially for the communication among the symbionts. The interaction mediated by signal molecules is the internal driving force for the homeostatic maintenance and efficient operation of coral symbionts. In this review, we tried to summarize the latest research progress by focusing on the chemical signaling molecules in coral symbiotic system, including the communications between the bacteria and bacteria, bacteria and corals, bacteria and zooxanthellae, and zooxanthellae and corals. The main signals molecules include quorum sensing (QS) molecules, dimethylsulfoniopropionate (DMSP), glycans signals, lipid signals, and the noncoding RNAs. We focused on the functional mode and ecological significance of signal molecules in symbionts, and selectively exemplified microbial cooperation and competition mediated by QS signals, the interaction between bacteria and corals under the regulation of DMSP, and the response process of corals and zooxanthellae to noncoding RNAs under environmental stresses. We proposed the future research focus and possible directions, including the expansion of research dimensions, the application of new technologies and new methods, and the construction of ecological models. This work would help improve the understanding of interactions between "bacteria-zooxanthellae-coral". The exploration about the ways based on communication language would provide new ideas for the restoration and protection of coral reef ecosystems.

Transcriptome analysis expands the potential roles of quorum sensing in biodegradation and physiological responses to microcystin.

Bacterial degradation is one of the most efficient ways to remove microcystins (MCs), the most frequently detected toxin in cyanobacterial blooms. Using Novosphingobium sp. ERW19 as a representative strain, our laboratory previously demonstrated that quorum sensing (QS), the cell density-dependent gene regulation system, positively regulates biodegradation of MCs via transcriptional activation of mlr-pathway-associated genes. Increasing evidence indicates that QS is involved in a wide spectrum of regulatory circuits, but it remains unclear which physiological processes in MC degradation besides the expression of MC-degrading genes are also subject to QS-dependent regulation. This study used transcriptome analysis to identify QS-regulated genes during degradation of MCs. A large percentage (up to 32.6%) of the genome of the MC-degrading bacterial strain Novosphingobium sp. ERW19 was significantly differentially expressed in the corresponding QS mutants. Pathway enrichment analysis of QS-regulated genes revealed that QS mainly influenced metabolism-associated pathways, particularly those related to amino acid metabolism, carbohydrate metabolism, and biodegradation and metabolism of xenobiotics. In-depth functional interpretation of QS-regulated genes indicated a variety of pathways were potentially associated with bacterial degradation or physiological responses to MCs, including genes involved in cell motility, cytochrome P450-dependent metabolism of xenobiotics, glutathione S-transferase (GST), envelope stress response, and ribosomes. Furthermore, QS may be involved in regulating the initial and final steps of the catabolic pathway of phenylacetic acid, an intermediate product of MC degradation. Collectively, this global survey of QS-regulated genes in a MC-degrading bacterial strain facilitates a deeper understanding of QS-controlled processes that may be important for bacterial degradation of MCs or may contribute to the physiological responses of bacteria to MCs.

Co-production of acetoin and succinic acid by metabolically engineered Enterobacter cloacae.

BACKGROUND: Renewable chemicals have attracted attention due to increasing interest in environmental concerns and resource utilization. Biobased production of industrial compounds from nonfood biomass has become increasingly important as a sustainable replacement for traditional petroleum-based production processes depending on fossil resources. Therefore, we engineered an Enterobacter cloacae budC and ldhA double-deletion strain (namely, EC∆budC∆ldhA) to redirect carbon fluxes and optimized the culture conditions to co-produce succinic acid and acetoin. RESULTS: In this work, E. cloacae was metabolically engineered to enhance its combined succinic acid and acetoin production during fermentation. Strain EC∆budC∆ldhA was constructed by deleting 2,3-butanediol dehydrogenase (budC), which is involved in 2,3-butanediol production, and lactate dehydrogenase (ldhA), which is involved in lactic acid production, from the E. cloacae genome. After redirecting and fine-tuning the E. cloacae metabolic flux, succinic acid and acetoin production was enhanced, and the combined production titers of acetoin and succinic acid from glucose were 17.75 and 2.75 g L-1, respectively. Moreover, to further improve acetoin and succinic acid production, glucose and NaHCO3 modes and times of feeding were optimized during fermentation of the EC∆budC∆ldhA strain. The maximum titers of acetoin and succinic acid were 39.5 and 20.3 g L-1 at 72 h, respectively. CONCLUSIONS: The engineered strain EC∆budC∆ldhA is useful for the co-production of acetoin and succinic acid and for reducing microbial fermentation costs by combining processes into a single step.

Screening of a highly inhibitor-tolerant bacterial strain for 2,3-BDO and organic acid production from non-detoxified corncob acid hydrolysate.

Fermentation of chemicals from lignocellulose hydrolysate is an effective way to alleviate environmental and energy problems. However, fermentation inhibitors in hydrolysate and weak inhibitor tolerance of microorganisms limit its development. In this study, atmospheric and room temperature plasma mutation technology was utilized to generate mutant strains of Enterobacter cloacae and screen for mutants with high inhibitor tolerance to acid hydrolysate of corncobs. A highly inhibitor-tolerant strain, Enterobacter cloacae M22, was obtained after fermentation with non-detoxified hydrolysate, and this strain produced 24.32 g/L 2,3-butanediol and 14.93 g/L organic acids. Compared with that of the wild-type strain, inhibitor tolerance was enhanced twofold with M22, resulting in improvement of 2,3-butanediol and organic acid production by 114% and 90%, respectively. This work presents an efficient method to screen for highly inhibitor-tolerant strains and evidence of a novel strain that can produce 2,3-butanediol and organic acids using non-detoxified acid hydrolysate of corncobs.

Model-based temperature control for improving lactic acid production from glycerol.

To maximize the final lactic acid productivity and concentration, temperature control was optimized using a mathematical modelling approach. A kinetic model, including cell growth, product formation and substrate consumption equations, was proposed to describe the lactic acid production process by Escherichia coli AC-521 with glycerol as the substrate. By constructing four functions, the temperature effect was introduced on the fermentation process, where four parameters (X max, µ max, Y ps and ß) were observed to be significantly affected by the temperature. For the convenience of application, the temperature control strategies were simplified by dividing the whole fermentation process into several units. In each unit, the temperature was controlled constantly. Based on the model, the optimal temperature for each unit was determined to maximize the final lactate productivity. This temperature control strategy can be effectively applied in batch and fed-batch cultures, and the verified experimental evaluation showed a good correlation with the model data. Under improved temperature control conditions, a maximal lactic acid concentration of 90.4 g L-1 was obtained after 80 h of fed-batch fermentation, giving a productivity of 1.13 g L-1 h-1, which is 1.2 times more than that in the conventional constant temperature during the cultivation course.

Aerobic and sequential anaerobic fermentation to produce xylitol and ethanol using non-detoxified acid pretreated corncob.

BACKGROUND: For economical bioethanol production from lignocellulosic materials, the major technical challenges to lower the production cost are as follows: (1) The microorganism should use efficiently all glucose and xylose in the lignocellulose hydrolysate. (2) The microorganism should have high tolerance to the inhibitors present in the lignocellulose hydrolysate. The aim of the present work was to combine inhibitor degradation, xylitol fermentation, and ethanol production using a single yeast strain. RESULTS: A new process of integrated aerobic xylitol production and anaerobic ethanol fermentation using non-detoxified acid pretreated corncob by Candida tropicalis W103 was proposed. C. tropicalis W103 is able to degrade acetate, furfural, and 5-hydromethylfurfural and metabolite xylose to xylitol under aerobic conditions, and the aerobic fermentation residue was used as the substrate for ethanol production by anaerobic simultaneous saccharification and fermentation. With 20% substrate loading, furfural and 5-hydroxymethylfurfural were degraded totally after 60 h aerobic incubation. A maximal xylitol concentration of 17.1 g l(-1) was obtained with a yield of 0.32 g g(-1) xylose. Then under anaerobic conditions with the addition of cellulase, 25.3 g l(-1) ethanol was produced after 72 h anaerobic fermentation, corresponding to 82% of the theoretical yield. CONCLUSIONS: Xylitol and ethanol were produced in Candida tropicalis W103 using dual-phase fermentations, which comprise a changing from aerobic conditions (inhibitor degradation and xylitol production) to anaerobic simultaneous saccharification and ethanol fermentation. This is the first report of integrated xylitol and ethanol production from non-detoxified acid pretreated corncob using a single microorganism.

Improved succinate production by metabolic engineering.

Succinate is a promising chemical which has wide applications and can be produced by biological route. The history of the biosuccinate production shows that the joint effort of different metabolic engineering approaches brings successful results. In order to enhance the succinate production, multiple metabolical strategies have been sought. In this review, different overproducers for succinate production, including natural succinate overproducers and metabolic engineered overproducers, are examined and the metabolic engineering strategies and performances are discussed. Modification of the mechanism of substrate transportation, knocking-out genes responsible for by-products accumulation, overexpression of the genes directly involved in the pathway, and improvement of internal NADH and ATP formation are some of the strategies applied. Combination of the appropriate genes from homologous and heterologous hosts, extension of substrate, integrated production of succinate, and other high-value-added products are expected to bring a desired objective of producing succinate from renewable resources economically and efficiently.

Effects of pH and dissolved CO2 level on simultaneous production of 2,3-butanediol and succinic acid using Klebsiella pneumoniae.

The influences of pH and dissolved CO2 level on the regulation of growth and formation of catabolic end products have been investigated in Klebsiella pneumoniae. With increasing CO2 levels, there were no apparent changes in 2,3-butanediol production but succinic acid productions were enhanced significantly. A novel strategy for co-production of 2,3-butanediol and succinic acid using K. pneumoniae was developed by controlling pH and dissolved CO2 concentration in fermentation medium. Under the optimum condition, maximal 77.1 g l(-1) 2,3-butanediol and 28.7 g l(-1) succinic acid were obtained after 60 h of fed-batch fermentation, giving a 2,3-butanediol+succinic acid yield of 1.03 mol mol(-1) glucose. This type of fermentation producing two commercial interests at the same fermentation process might be considered for a promising biological production process which will decrease the production cost by sharing the operation and recovery cost.

Downstream processing of biotechnological produced succinic acid.

Succinic acid is a promising chemical which has a wide range of applications and can be biologically produced. The separation of succinic acid from fermentation broth makes more than 50 % of the total costs in their microbial production. This review summarizes the present state of methods studied for the recovery and purification of biologically produced succinate. Previous studies on the separation of succinic acid primarily include direct crystallization, precipitation, membrane separation, extraction, chromatography, and in situ separation. No single method has proved to be simple and efficient, and improvements are especially needed with regard to yield, purity, and energy consumption. It is argued that separation technologies coupled with upstream technology, in situ product removal, and biorefining strategy deserve more attentions in the future.

Statistical optimization of sulfite pretreatment of corncob residues for high concentration ethanol production.

In this study, a central composite design of response surface method was used to optimize sulfite pretreatment of corncob residues, in respect to sulfite charge (5-10%), treatment time (1-2h), liquid/solid (l/s) ratio (6:1-10:1) and temperature (150-180°C) for maximizing glucose production in enzymatic hydrolysis process. The relative optimum condition was obtained as follows: sulfite charge 7.1%, l/s ratio 7.6:1, temperature 156°C for 1.4h, corresponding to 79.3% total glucan converted to glucose+cellobiose. In the subsequent simultaneous saccharification and fermentation (SSF) experiments using 15% glucan substrates pretreated under this kind of conditions, 60.8 g ethanol l(-1) with 72.2% theoretical yield was obtained.

Integrated production of xylitol and ethanol using corncob.

Xylitol production from corncob hemicellulose is a popular process in China. Microbial conversion of xylose to xylitol, as a biological process with many advantages, has drawn increasing attention. As a by-product from the manufacturing of xylitol, corncob cellulosic residues are produced in very large amounts and represent an environmental problem. As a result, considering the large amount of xylitol production in China, the conversion of corncob cellulosic residues has become a widespread issue having to be tackled. After the hemicellulose in corncob has been hydrolyzed for xylitol production, the corncob cellulosic residue is porous and can easily be hydrolyzed by cellulases into glucose and further converted to ethanol, another high-added-value chemical. Based on the latest technology advancements in xylitol, cellulase, and ethanol production, the integrated production of ethanol from corncob cellulosic residues appears as a promising way to improve the profit of the whole xylitol production process.

Statistical optimization of recycled-paper enzymatic hydrolysis for simultaneous saccharification and fermentation via central composite design.

A central composite design of the response surface methodology (RSM) was employed to study the effects of temperature, enzyme concentration, and stirring rate on recycled-paper enzymatic hydrolysis. Among the three variables, temperature and enzyme concentration significantly affected the conversion efficiency of substrate, whereas stirring rate was not effective. A quadratic polynomial equation was obtained for enzymatic hydrolysis by multiple regression analysis using RSM. The results of validation experiments were coincident with the predicted model. The optimum conditions for enzymatic hydrolysis were temperature, enzyme concentration, and stirring rate of 43.1 degrees C, 20 FPU g(-1) substrate, and 145 rpm, respectively. In the subsequent simultaneous saccharification and fermentation (SSF) experiment under the optimum conditions, the highest 28.7 g ethanol l(-1) was reached in the fed-batch SSF when 5% (w/v) substrate concentration was used initially, and another 5% added after 12 h fermentation. This ethanol output corresponded to 77.7% of the theoretical yield based on the glucose content in the raw material.

Sugarcane bagasse mild alkaline/oxidative pretreatment for ethanol production by alkaline recycle process.

In order to decrease the alkali and water consumptions in the sugarcane bagasse alkaline/oxidative pretreatment for ethanol production, an alkaline recycle process was carried out. Two recycles of NaOH/H2O2 pretreatment did not decrease the pretreatment and enzymatic hydrolysis efficiencies and the consumptions of NaOH and water would be saved by 26% and 40%, respectively. A simultaneous saccharification and fermentation (SSF) culture with pretreated bagasse as substrate was developed giving 25 g ethanol l(-1) with a yield of 0.2 g g(-1) bagasse and productivity of 0.52 g l(-1) h(-1).

Fermentation of pretreated sugarcane bagasse hemicellulose hydrolysate to ethanol by Pachysolen tannophilus.

Sugarcane bagasse hemicellulose hydrolysates, pretreated by either over-liming or electrodialysis and, supplemented with nutrient materials, were fermented to ethanol using Pachysolen tannophilus DW06. Compared with detoxification by over-liming, detoxification by electrodialysis decreased the loss of sugar and increased the acetic acid removal, leading to better fermentability. A batch culture with electrodialytically pretreated hydrolysate as substrate was developed giving 21 g ethanol l(-1) with a yield of 0.35 g g(-1) sugar and productivity of 0.59 g l(-1) h(-1).

Production of 1,3-propanediol by Klebsiella pneumoniae from glycerol broth.

Broth containing 152 g glycerol l(-1) from Candida krusei culture was converted to 1,3-propanediol by Klebsiella pneumoniae. Residual glucose in the broth promoted growth of K. pneumoniae while acetate was inhibitory. After desalination treatment of glycerol broth by electrodialysis, the acetate in the broth was removed. A fed-batch culture with electrodialytically pretreated broth as substrate was developed giving 53 g 1,3-propanediol l(-1) with a yield of 0.41 g g(-1) glycerol and a productivity of 0.94 g l(-1) h(-1).

Multiple growth inhibition of Klebsiella pneumoniae in 1,3-propanediol fermentation.

The inhibition of substrate and product on the growth of Klebsiella pneumoniae in anaerobic and aerobic batch fermentation for the production of 1,3-propanediol was studied. The cells under anaerobic conditions had a higher maximum specific growth rate of 0.19 h(-1) and lower tolerance to 110 g glycerol l(-1), compared to the maximum specific growth rate of 0.17 h(-1) and tolerance to 133 g glycerol l(-1) under aerobic conditions. Acetate was the main inhibitory metabolite during the fermentation under anaerobic conditions, with lactate and ethanol the next most inhibitory. The critical concentrations of acetate, lactate and ethanol were assessed to be 15, 19, 26 g l(-1), respectively. However, cells grown under aerobic conditions were more resistant to acetate and lactate but less resistant to ethanol. The critical concentrations of acetate, lactate and ethanol were assessed to be 24, 26, and 17 g l(-1), respectively.

1,3-Propanediol production by Klebsiella pneumoniae under different aeration strategies.

1,3-Propanediol production by Klebsiella pneumoniae was studied in batch cultures under N2 flow and four airflow systems. Different byproducts were formed under different aeration conditions. An anaerobic/aerobic combined fed-batch culture was developed giving 70 g 1,3-propanediol l(-1) and 16 g 2,3-butanediol l(-1) with total diol yield of 0.6 mol(-1) glycerol.