Biokinetics of protein degrading Clostridium cadaveris and Clostridium sporogenes in batch and continuous mode of operations

A quantitative real-time polymerase chain reaction (QPCR) was applied to estimate biokinetic coefficients of Clostridium cadaveris and Clostridium sporogenes, which utilize protein as carbon source. Experimental data of changes in peptone concentration and 16S rRNA gene copy numbers of C. cadaveris and C. sporogenes were fitted to model. The fourth-order Runge-Kutta approximation with non-linear least squared analysis was employed to solve the ordinary differential equations to estimate biokinetic coefficients. The maximum specific growth rate (μ_max), half saturation concentration (K_s), growth yield (Y), and decay coefficient (K_d) of C. cadaveris and C.sporogenes were 0.73 ± 0.05 and 1.35 ± 0.32 h^-1, 6.07 ± 1.52 and 5.67 ± 1.53 g/L, 2.25 ± 0.75 × 10¹⁰ and 7.92 ± 3.71 × 10⁹ copies/g, 0.002 ± 0.003 and 0.002 ± 0.001 h^-1, respectively. The theoretical specific growth rate of C. sporogenes always exceeded than that of C. cadaveris at peptone concentration higher than 3.62 g/L. When the influent peptone concentration was 5.0 g/L, the concentration of C.cadaveris gradually decreased to the steady value of 2.9 × 10¹⁰ copies/mL at 4 hours HRT, which indicates 67.1% of the initial population reduction, but the wash out occurred at 1.9 and 3.2 hours HRTs. The 16S rRNA gene copy numbers of C. sporogenes gradually decreased to steady values ranging from 1.1 × 10¹⁰ to 2.9 × 10¹⁰ copies/mL. C. sporogenes species was predicted to wash out at an HRT of 1.6 h.

Biokinetic Characterization and Activities of N2O-Reducing Bacteria in Response to Various Oxygen Levels

Nitrous oxide (N₂O)-reducing bacteria, which reduce N₂O to nitrogen in the absence of oxygen, are phylogenetically spread throughout various taxa and have a potential role as N₂O sinks in the environment. However, research on their physiological traits has been limited. In particular, their activities under microaerophilic and aerobic conditions, which severely inhibit N₂O reduction, remain poorly understood. We used an O₂ and N₂O micro-respirometric system to compare the N₂O reduction kinetics of four strains, i.e., two strains of an Azospira sp., harboring clade II type nosZ, and Pseudomonas stutzeri and Paracoccus denitrificans, harboring clade I type nosZ, in the presence and absence of oxygen. In the absence of oxygen, the highest N₂O-reducing activity, V_m,N2O, was 5.80 ± 1.78 × 10^-3 pmol/h/cell of Azospira sp. I13, and the highest and lowest half-saturation constants were 34.8 ± 10.2 μM for Pa. denitirificans and 0.866 ± 0.29 μM for Azospira sp. I09. Only Azospira sp. I09 showed N₂O-reducing activity under microaerophilic conditions at oxygen concentrations below 110 μM, although the activity was low (10% of V_m,N2O). This trait is represented by the higher O₂ inhibition coefficient than those of the other strains. The activation rates of N₂O reductase, which describe the resilience of the N₂O reduction activity after O₂ exposure, differ for the two strains of Azospira sp. (0.319 ± 0.028 h^-1 for strain I09 and 0.397 ± 0.064 h^-1 for strain I13) and Ps. stutzeri (0.200 ± 0.013 h^-1), suggesting that Azospira sp. has a potential for rapid recovery of N₂O reduction and tolerance against O₂ inhibition. These physiological characteristics of Azospira sp. can be of promise for mitigation of N₂O emission in industrial applications.