Denitrification is an indispensable part of most sewage treatment systems. The biological denitrification process has attracted much attention in the past decades due to the advantages such as cost-effectiveness, process simplicity, and absence of secondary pollution. This review summarized the advances on biological denitrification processes in recent years according to the different physiological characteristics and denitrification mechanisms of denitrification microorganisms. The pros and cons of different biological denitrification processes developed based on nitrifying bacteria, denitrifying bacteria, and anaerobic ammonia-oxidizing bacteria were compared with the aim to identify the best strategy for denitrification in a complex wastewater environment. The rapid development of synthetic biology provides possibilities to develop highly-efficient denitrifying strains based on mechanistic understandings. Combined with the applications of automatic simulation to obtain the optimal denitrification conditions, cost-effective and highly-efficient denitrification processed can be envisioned in the foreseeable future.
Aerobiosis ; Denitrification ; Nitrification ; Nitrogen ; Waste Water

Biological denitrification is the most widely used technology for nitrate removal in wastewater treatment. Conventional denitrification requires long hydraulic retention time, and the nitrate removal efficiency in winter is low due to the low temperature. Therefore, it is expected to develop new approaches to enhance the denitrification process. In this paper, the effect of adding different concentrations of Fe₃O₄ nanoparticles on the denitrification catalyzed by Pseudomonas stutzeri was investigated. The maximum specific degradation rate of nitrate nitrogen improved from 18.0 h⁻¹ to 23.7 h⁻¹ when the concentration of Fe₃O₄ increased from 0 mg/L to 4 000 mg/L. Total proteins and intracellular iron content also increased along with increasing the concentration of Fe₃O₄. RT-qPCR and label-free proteomics analyses showed that the relative expression level of denitrifying genes napA, narJ, nirB, norR, nosZ of P. stutzeri increased by 55.7%, 24.9%, 24.5%, 36.5%, 120% upon addition of Fe₃O₄, and that of denitrifying reductase Nap, Nar, Nir, Nor, Nos increased by 85.0%, 147%, 16.5%, 47.1%, 95.9%, respectively. No significant difference was observed on the relative expression level of denitrifying genes and denitrifying reductases between the bacteria suspended and the bacteria adhered to Fe₃O₄. Interestingly, the relative expression level of electron transfer proteins of bacteria adhered to Fe₃O₄ was higher than that of the bacteria suspended. The results indicated that Fe₃O₄ promoted cell growth and metabolism through direct contact with bacteria, thereby improving the denitrification. These findings may provide theoretical support for the development of enhanced denitrification.
Aerobiosis ; Denitrification ; Nitrates ; Nitrogen ; Pseudomonas stutzeri/genetics*

Heterotrophic nitrification-aerobic denitrification (HN-AD) is an enrichment and breakthrough theory of traditional autotrophic nitrification heterotrophic denitrification. Heterotrophic nitrification-aerobic denitrifiers with the feature of wide distribution, strong adaptability and unique metabolic mechanism have many special advantages, including fast-growing, rapid biodegradability and long lasting activity, which can rapidly remove ammonia nitrogen, nitrate nitrogen (NO₃⁻-N) and nitrite nitrogen (NO₂⁻-N) under aerobic conditions simultaneously. Therefore, HN-AD bacteria show the important potential for denitrification under extreme conditions with high-salt, low-temperature or high-ammonia nitrogen environment, and HN-AD bacteria attract extensive attention in the field of biological denitrification of wastewater. In this review, we first introduce the previously reported HN-AD bacterial species which have denitrification performance in the extreme environments and state their typical metabolic mechanism. Then, we systematically analyze the nitrogen removal characteristics and potential under extreme conditions. We also briefly describe the progress in the application of HN-AD bacterial. Finally, we outlook the application prospects and research directions of HN-AD denitrification technology.
Aerobiosis ; Bacteria ; Denitrification ; Heterotrophic Processes ; Nitrification ; Nitrites ; Nitrogen

OBJECTIVETo investigate the effect of environmental oxygen on the inhibition between Streptococcus oligofermentans (So) and Streptococcus mutans (Sm) and the producibilities of hydrogen peroxide by So.

METHODSThe aerobic and anaerobic environment was established by the carbon dioxide cultivation. The inhibition between So and Sm was observed by plating method. The production and synthesis rates of hydrogen peroxide by So were determined in both aerobic and anaerobic environment by 4-ATTP-horseradish peroxidase method at A(510).

RESULTSWhen both Sm and So were inoculated at the same time, Sm was not inhibited under the anaerobic environment, vice versa. Sm was slightly inhibited by So under the aerobic environment, the inhibition area was 1/5 of all bacterial membrane. When So was cultivated first and then Sm applied, So could inhibite Sm growth under both anaerobic and aerobic conditions. The inhibition area was 1/5 of bacterial membrane under the anaerobic environment, and 4/5 under the aerobic environment. When Sm was cultivated first and then So applied, So was unable to proliferate under both conditions. During the logarithmic phase, the production of H2O2 by So under the aerobic environment was higher than under the anaerobic environment (P < 0.05). The initial synthesis rate of H2O2 by So during growth cycle under the anaerobic condition was (11.84 ± 3.97) µmol/L per minute, which was only 49% of that under the aerobic environment [(24.13 ± 4.46) µmol/L per minute].

CONCLUSIONSThe oxygen has the effect on the inhibition between So and Sm, and the inhibition in the aerobic environment is much stronger than in the anaerobic environment. The synthesis ability of hydrogen peroxide by So under the aerobic environment is higher than under the anaerobic environment.

Aerobiosis ; Hydrogen Peroxide ; metabolism ; Oxygen ; metabolism ; Streptococcus ; growth & development ; metabolism ; Streptococcus mutans ; growth & development ; metabolism

Xylitol, a five-carbon sugar alcohol, has many interesting applications in the food, pharmaceutical, and odontological industries, owing to its high sweetening power, its anticariogenic properties, and its insulin-independent metabolism. The bioconversion of detoxified hemicellulosic hydrolysate to xylitol by microorganisms could be a cheaper alternative to the current chemical process, since it is a simple process, with great specificity and low energy requirements. However, the success of fermentations for xylitol production depends on the productivity of the strain and its tolerance to different toxic or inhibitory compounds existing in the hydrolysates. In addition, a number of culture process parameters proved to have significant effects on xylitol production in hemicellulosic hydrolysate media. One of the most important control variables in this bioconversion is the aeration level, which affects the biochemical pathways in the xylose metabolism. The production of biomass is favored by aerobic conditions, while under anaerobic conditions xylose cannot be assimilated by yeast, whereas xylitol is formed in oxygen-limited incubation conditions. An adapted Candida sp. with enhanced resistance to the inhibitors in the hydrolysate can directly ferment the simply detoxified corn cob hemicellulosic hydrolysate to xylitol. In the present study, the combined effects of shaking speed, C/ N ratio, initial pH, and inoculum level on the fermentation of corn cob hemicellulosic hydrolysate to xylitol by an adapted Candida sp. were investigated using an orthogonal experimental design in flask. As a result, the optimum fermentation conditions were as follows: 180 r/min, a C/N ratio of 50, initial pH 5.5, and an inoculum level of 5% (volume ratio). Moreover, the optimum concentration factor of hydrolysate varied between 3.0 and 3.72 was obtained. Based on these results, in order to evaluate the effect of aeration rate on the fermentation of corn cob hemicellulosic hydrolysate to xylitol in fermentor, batch fermentations were carried out in a 3.7 L stirred fermentor using four different aeration strategies, including three kind of two-stage aeration strategies, which provided relatively high aeration rate in the early stage but reduced it in the later stage, and including a one-stage aeration strategy provided a constant aeration rate. With respect to xylitol yield, the results indicated that two-stage aeration strategy was significantly superior to one-stage aeration strategy. The highest xylitol yield (0.75 g/g) was obtained with oxygen supply strategy C (3.75 L/min for first 24 h, then lowered it to 1.25 L/min, 2.5 L fermentation medium was employed). In this process, without extensive detoxification of hydrolysate, an adapted Candida sp. can efficiently ferment the simply treated corn cob hemicellulosic hydrolysate to xylitol under the optimized fermentation conditions. This work should help the development of an efficient process for producing xylitol from corn cob hemicellulosic hydrolysate on a larger scale by bioconversion.
Aerobiosis ; Candida tropicalis ; metabolism ; Fermentation ; Hydrolysis ; Polysaccharides ; metabolism ; Xylitol ; biosynthesis ; Zea mays ; metabolism

Heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria are aerobic microorganisms that can remove nitrogen under high-salt conditions, but their performance in practical applications are not satisfactory. As a compatible solute, trehalose helps microorganisms to cope with high salt stress by participating in the regulation of cellular osmotic pressure, and plays an important role in promoting the nitrogen removal efficiency of microbial populations in the high-salt environment. We investigated the mechanism of exogenous-trehalose-enhanced metabolism of HN-AD community under high-salt stress by starting up a membrane aerobic biofilm reactor (MABR) to enrich HN-AD bacteria, and designed a C₁₅₀ experimental group with 150 μmol/L trehalose addition and a C₀ control group without trehalose. The reactor performance and the community structure showed that NH₄⁺-N, total nitrogen (TN) and chemical oxygen demand (COD) removal efficiency were increased by 29.7%, 28.0% and 29.1%, respectively. The total relative abundance of salt-tolerant HN-AD bacteria (with Acinetobacter and Pseudofulvimonas as the dominant genus) in the C₁₅₀ group reached 66.8%, an 18.2% increase compared with that of the C₀ group. This demonstrated that trehalose addition promoted the enrichment of salt-tolerant HN-AD bacteria in the high-salt environment to enhance the nitrogen removal performance of the system. In-depth metabolomics analysis showed that the exogenous trehalose was utilized by microorganisms to improve proline synthesis to increase resistance to high-salt stress. By regulating the activity of cell proliferation signaling pathways (cGMP-PKG, PI3K-Akt), phospholipid metabolism pathway and aminoacyl-tRNA synthesis pathway, the abundances of phosphoethanolamine, which was one of the glycerophospholipid metabolites, and purine and pyrimidine were up-regulated to stimulate bacterial aggregation and cell proliferation to promote the growth of HN-AD bacteria in the high-salt environment. Meanwhile, the addition of trehalose accelerated the tricarboxylic acid (TCA) cycle, which might provide more electron donors and energy to the carbon and nitrogen metabolisms of HN-AD bacteria and promote the nitrogen removal performance of the system. These results may facilitate using HN-AD bacteria in the treatment of high-salt and high-nitrogen wastewater.
Nitrification ; Denitrification ; Trehalose ; Phosphatidylinositol 3-Kinases/metabolism* ; Heterotrophic Processes ; Salt Stress ; Nitrogen/metabolism* ; Aerobiosis ; Bioreactors/microbiology*

During the anaerobic fermentation by Escherichia coli AFP111 for succinic acid production, the viable cell concentration and productivity were decreased with the raising of succinic acid concentration. In order to restore cellular succinic acid productivity and prolong fermentation time, we collected strains and refreshed medium for repetitive succinic acid production. However, productivity is lower than that in the anaerobic fermentation before reusing strains. To enhance the productivity, strains were aerobically cultivated for 3 h in pure water before anaerobic fermentation. The activities of key enzymes were enhanced for better performance in producing succinic acid at anaerobic stage. After three rounds of repetitive fermentations, succinic acid concentration and yield reached to 56.50 g/L and 90% respectively. The succinic acid productivity was 0.81 g/(L x h), which was 13% higher than the repetitive fermentations without aerobic activation of the strains.
Aerobiosis ; Anaerobiosis ; Culture Media ; Escherichia coli ; genetics ; metabolism ; Fermentation ; Genetic Engineering ; Glucose ; metabolism ; Industrial Microbiology ; Succinic Acid ; metabolism

Glycerol from oil hydrolysis industry is being considered as one of the abundent raw materials for fermentation industry. In present study, the aerobic and anaerobic metabolism and growth properties on glycerol by Esherichia coli CICIM B0013-070, a D-lactate over-producing strain constructed previously, at different temperatures were investigated, followed by a novel fermentation process, named temperature-switched process, was established for D-lactate production from glycerol. Under the optimal condition, lactate yield was increased from 64.0% to 82.6%. Subsequently, the yield of D-lactate from glycerol was reached up to 88.9% while a thermo-inducible promoter was used to regulate D-lactate dehydrogenase transcription.
Aerobiosis ; Anaerobiosis ; Escherichia coli ; genetics ; metabolism ; Fermentation ; Glycerol ; metabolism ; L-Lactate Dehydrogenase ; metabolism ; Lactic Acid ; biosynthesis ; Promoter Regions, Genetic ; genetics ; Temperature

Based on carbon metabolic pathway analysis of Escherichia coli MG1655, an aerobic succinate fermentation platform was constructed by knocking out five genes (ptsG, poxB, pta, iclR and sdhA), which was named E. coli QZ1111. Flask cultivation results showed that E. coli QZ1111 could accumulate succinate with a concentration of 26.4 g/L under aerobic conditions. The byproduct acetate was only 2.3 g/L. The production ratio of succinate and acetate reached 11.5:1.
Aerobiosis ; Escherichia coli ; genetics ; metabolism ; Fermentation ; Gene Expression Regulation, Bacterial ; Gene Knockout Techniques ; Protein Engineering ; methods ; Recombinant Proteins ; biosynthesis ; genetics ; Succinic Acid ; metabolism

To understand the effects of sugar whose uptake is dependent or independent on the phosphotransferase system (PTS), two-stage culture of Escherichia coli strain NZN111 that was constructed by disruption of IdhA and pflB encoding the fermentative lactate dehydrogenase (LDH) and pyruvate: formate lyase (PFL) of E. coli W1485, was carried out for organic acids production. When NZN111 was aerobically cultured on fructose (PTS dependent) or maltose (PTS independent), it fermented glucose with succinic acid and pyruvic acid as the major products in subsequent anaerobic culture. The experiments were also performed in a 5-L fermentor. The yields of succinic acid by the fructose-and maltose-grown NZN111 were 0.84 and 0.75 mol/mol, whereas the yields of pyruvic acid were 0.65 and 0.83 mol/mol, respectively. The final ratio of succinic acid to pyruvic acid in the anaerobic stage reached 1.73:1 and 1.21:1, respectively. The different behaviors in anaerobic fermentation by the fructose-, maltose- and glucose-grown NZN111 were likely caused by the regulation of catabolite repression in the aerobic culture stage.
Aerobiosis ; Anaerobiosis ; Carbon ; metabolism ; Escherichia coli ; classification ; metabolism ; Fermentation ; Fructose ; metabolism ; Maltose ; metabolism ; Phosphotransferases ; metabolism ; Pyruvic Acid ; metabolism ; Succinic Acid ; metabolism