Butyryl/Caproyl-CoA:Acetate CoA-transferase: cloning, expression and characterization of the key enzyme involved in medium-chain fatty acid biosynthesis.

Coenzyme A transferases (CoATs) are important enzymes involved in carbon chain elongation, contributing to medium-chain fatty acid (MCFA) biosynthesis. For example, butyryl-CoA:acetate CoA transferase (BCoAT) is responsible for the final step of butyrate synthesis from butyryl-CoA. However, little is known about caproyl-CoA:acetate CoA-transferase (CCoAT), which is responsible for the final step of caproate synthesis from caproyl-CoA. In the present study, two CoAT genes from Ruminococcaceae bacterium CPB6 and Clostridium tyrobutyricum BEY8 were identified by gene cloning and expression analysis. Enzyme assays and kinetic studies were carried out using butyryl-CoA or caproyl-CoA as the substrate. CPB6-CoAT can catalyze the conversion of both butyryl-CoA into butyrate and caproyl-CoA into caproate, but its catalytic efficiency with caproyl-CoA as the substrate was 3.8-times higher than that with butyryl-CoA. In contrast, BEY8-CoAT had only BCoAT activity, not CCoAT activity. This demonstrated the existence of a specific CCoAT involved in chain elongation via the reverse ß-oxidation pathway. Comparative bioinformatics analysis showed the presence of a highly conserved motif (GGQXDFXXGAXX) in CoATs, which is predicted to be the active center. Single point mutations in the conserved motif of CPB6-CoAT (Asp346 and Ala351) led to marked decreases in the activity for butyryl-CoA and caproyl-CoA, indicating that the conserved motif is the active center of CPB6-CoAT and that Asp346 and Ala351 have a significant impact on the enzymatic activity. This work provides insight into the function of CCoAT in caproic acid biosynthesis and improves understanding of the chain elongation pathway for MCFA production.

Electrode-dependent ammonium oxidation with different low C/N ratios in single-chambered microbial electrolysis cells.

Alternative method should be found to solve the ammonia accumulation in anaerobic digestion. Herein, electrode-dependent ammonium oxidation was successfully achieved in anaerobic single-chambered microbial electrolysis cells (MECs)under different low C/N ratios (0, 1, and 1.5), with an applied voltage of 0.6 V as well as an initial NH4+-N and NO3--N concentration of 500 and 300 mg/L. The nitrogen removal performance of MECs and the controls indicated that applying a voltage stimulated nitrogen removal under low C/N ratios of 0, 1, and 1.5. However, the remaining organic carbon in MEC with a relatively higher C/N ratio of 3 inhibited the ammonium oxidation. Current changes and cyclic voltammetry demonstrated that the bioanode with several bioelectrochemical activities could promote ammonium oxidation. The dominant genera Truepera, Aquamicrobium, Nitrosomonas, Arenimonas, Comamonas, and Cryobacterium enriched on both electrodes could be the key functional taxa in MECs with C/N ratios of 0, 1, and 1.5. The remaining sodium acetate in MEC with C/N ratio of 3 inhibits microbial community structure and relative abundance, which may adversely affected nitrogen removal. Further caculation showed that nitrogen balance was essentially achieved, while electron balance was disrupted since electrons may be consumed through NO3--N recycle and cell synthesis, and finally caused low coulombic efficiency.

Comparative evaluation of simultaneous nitritation/denitritation and energy recovery in air-cathode microbial fuel cells (ACMFCs) treating low C/N ratio wastewater.

Air-cathode microbial fuel cells (ACMFCs) can extract available electrons from the low C/N ratio wastewater (LCNW) for pollutant degradation and power generation. However, the multiple effects of operating parameters and their relationship between the performances and parameters are still lacking. In this study, several ACMFCs for simultaneous nitritation/denitritation (SND) and energy recovery were constructed and evaluated in terms of chemical oxygen demand (COD), NH4+-N, C/N ratio, phosphate buffer solution (PBS), and external resistance (Rext), and several derived parameters (e.g., organic loading rate (OLR), nitrogen loading rate (NLR)). Results indicated that ACMFCs could be used to treat LCNW successfully with high pollutant removal rates and sustainable current generation. Maximum removal efficiencies of 94% COD, 92% NH4+-N, and 92% total nitrogen (TN) were achieved. A maximum power density of 1400 mW m-2 and columbic efficiency of 69.2% were also obtained at a low C/N ratio of 1.7-2.6. Low C/N ratios promoted SND by balancing nitritation and denitritation. The microbial community and their predicated function results showed considerable nitrifiers and denitrificans were enriched in the ACMFCs, contributing to SND and power recovery. Further analyses showed that the NH4+-N could inhibit SND, but PBS and Rext had no obvious effects on this outcome. Co-occurrence network analysis demonstrated that power is positively correlated with COD and Rext; strong correlations between organic removal and COD, and between nitrogen removal and ammonia, conductivity, and C/N ratio were also noted. Overall, the appropriate control of such parameters is necessary to achieve efficient SND in ACMFCs for LCNW treatment.

Ammonia oxidation and denitrification in a bio-anode single-chambered microbial electrolysis cell.

In this study, anodic ammonia oxidation and denitrification were performed in single-chamber bioelectrochemical systems at a wide range of anodic potentials (-400 to +400 mV) versus Ag/AgCl. The low coulombic efficiencies (~30.84%) in reactors were mainly due to electrons being transferred to atmospheric oxygen through the electrode and reversal of the electrode. The removal efficiencies of acetate, ammonia, and total nitrogen were 100%, 100%, and 40.44% at +200 mV and 100%, 100%, and 50.24% at -200 mV, respectively. The nitrogen-removal mechanisms were nitrogen respiration/nitrate reduction at +200 mV and denitrification at -200 mV, and ammonia oxidation occurred by coupling with sulfate-reducing at -300 and -400 mV. Thauera, Comamonas, Alicycliphilus, Nitrosomonas, Desulforhabdus, Dethiosulfatibacter, and Desulfomicrobium were the dominant genera at the anode which participated in the nitrification/denitrification or sulfate-reducing processes. In summary, ammonia oxidation and denitrification could be coupled with carbon-removal or sulfur-reduction using a bio-anode with a suitable anodic potential.