Isolation of Pb(II)-reducing bacteria and demonstration of biological Pb(II) reduction to metallic Pb

Minimizing the Lag Phase of Cupriavidus necator Growth under Autotrophic, Heterotrophic, and Mixotrophic Conditions

Cupriavidus necator has the unique metabolic capability to grow under heterotrophic, autotrophic, and mixotrophic conditions. In the current work, we examined the effect of growth conditions on the metabolic responses of C. necator. In our lab-scale experiments, autotrophic growth was rapid, with a short lag phase as the exponential growth stage was initiated in 6 to 12 h. The lag phase extended significantly (>22 h) at elevated O₂ and CO₂ partial pressures, while the duration of the lag phase was independent of the H₂ or N₂ partial pressure. Under heterotrophic conditions with acetate as the organic substrate, the lag phase length was short (<12 h), but it increased with increasing acetate concentrations. When glucose and glycerol were provided as the organic substrate, the lag phase was consistently long (>12 h) regardless of the examined substrate concentrations (up to 10.0 g/L). In the transition experiments, C. necator cells showed rapid transitions from autotrophic to heterotrophic growth in less than 12 h and vice versa. Our experimental results indicate that C. necator can rapidly grow with both autotrophic and heterotrophic substrates, while the lag time substantially increases with nonacetate organic substrates (e.g., glucose or glycerol), high acetate concentrations, and high O₂ and CO₂ partial pressures. IMPORTANCE The current work investigated the inhibition of organic and gaseous substrates on the microbial adaption of Cupriavidus necator under several metabolic conditions commonly employed for commercial polyhydroxyalkanoate production. We also proposed a two-stage cultivation system to minimize the lag time required to change over between the heterotrophic, autotrophic, and mixotrophic pathways.

Estimation of kinetic constants in high-density polyethylene bead degradation using hydrolytic enzymes

Microplastic beads are an emerging contaminant that can cause serious environmental and public health problems. Potential bypass of microplastic beads from wastewater to sludge treatment systems is a key challenge in the conventional wastewater treatment process. Moreover, there are no systematic studies on microplastic bead degradation by hydrolytic enzymes that are rich in concentration within wastewater and sludge treatment processes (e.g., anaerobic digestion (AD)). In this study, lab-scale experiments were conducted to investigate the degradation of high-density polyethylene beads by hydrolytic enzymes (e.g., lipase) under various experimental conditions (e.g., temperature). In a 3-day batch experiment, protease was most effective in polyethylene bead degradation as 4.0% of the initial bead mass was removed at an enzyme concentration of 88 mg/L under thermophilic temperature (55 °C). It was also found that the increasing enzyme concentration and high temperature enhanced the polyethylene bead degradation. In a separate 7-day experiment with repeated doses of protease, 23.3% of the initial mass of beads was removed at thermophilic temperature, indicating that AD with a long retention time (e.g., 20 days) and heated temperature has a significant potential for polyethylene bead degradation. A mathematical model was developed and calibrated using the experimental results to estimate the kinetic constant of the high-density polyethylene bead reduction by an enzyme (k_1,i) and enzyme self-decay constant (k_2,ii). The calibrated k_1,i ranged from 5.0 to 8.1× 10^-4 L/mg/hr while k_2,ii was 0.44-1.10 L/mg/hr. Using the calibrated model, degradation of polyethylene beads using a mixture of cellulase and protease was simulated, considering an interactive-decay reaction between the two enzymes. The calibrated model was used to simulate the polyethylene bead degradation in AD where 70-95% of the initial bead mass was removed at typical retention time under mesophilic digestion (37.5 °C). Based on the experimental and simulation results, it can be concluded that hydrolytic enzymes can be an efficient technology for large-scale high-density polyethylene bead removal applications.