Yongze Lu

Metronidazole (MNZ) is a nitroimidazole antibiotic that is poorly removed by conventional biological wastewater treatment, motivating interest in microalgae-based tertiary treatment strategies. This study characterizes the concentration-dependent toxicity, degradation kinetics, and removal mechanism of MNZ by Tetradesmus obliquus monoculture, and examines whether external carbon supplementation can enhance its degradation capacity. Across a concentration range of 1–9 mg/L, MNZ inhibited T. obliquus growth and photosynthetic pigment content in a dose-dependent manner, while degradation efficiency declined correspondingly, from 25.5% at 1 mg/L to 12.8% at 9 mg/L. Dark-condition controls combined with reactive oxygen species quenching showed that MNZ removal was dominated by extracellular organic matter (EOM)-mediated indirect photodegradation, with triplet-state EOMs (³EOM*) identified as the dominant reactive species. Supplementation with exogenous glucose (0–200 mg/L) induced a heterotrophic metabolic shift that enhanced MNZ photodegradation in a dose-dependent manner; EEM-PARAFAC fluorescence analysis linked this enhancement to selective enrichment of protein-like, tryptophan/tyrosine-associated EOM components rather than a uniform increase in total EOM content. These findings indicate that the carbon-responsive enhancement of EOM photoreactivity previously reported for a methanotroph–T. obliquus co-culture system reflects a conserved algal metabolic response rather than a phenomenon contingent on bacterial partnership, and establish a mechanistic and kinetic baseline for algal-mediated antibiotic photodegradation independent of bacterial co-culture.

Upgrading biogas to pipeline-quality methane requires the efficient removal of CO₂, yet conventional physicochemical routes remain energy-intensive. Coupling a CO₂-selective membrane with microalgal photosynthetic fixation offers a green alternative, but is constrained by the low CO₂/CH₄ selectivity of common membranes and the poor adhesion of microalgae to hydrophobic membrane surfaces. Here, a dual-functional composite membrane was developed that simultaneously provides CO₂/CH₄ sieving and a biocompatible interface for microalgal attachment, and was integrated into a microalgae membrane bioreactor (MMBR). A cellulose acetate mixed-matrix membrane incorporating polyethyleneimine-grafted ZIF-8 (CA/PZIF-8(15)) achieved a mixed-gas CO₂ permeability of 122.3 Barrer and a CO₂/CH₄ selectivity of 41.17. An ionic-liquid-modified chitosan (CS/IL) coating, first optimized on a commercial flat-sheet polyethersulfone (PES) membrane used as a model surface for the adhesion study, reversed the surface charge from −30.8 to +3.75 mV, lowered the water contact angle to 51.2°, and increased the day-7 adhesion of Scenedesmus obliquus by ~108%. Transferring the coating onto CA/PZIF-8(15) further raised the permeability to 138 Barrer and the selectivity to 57.31, placing the composite above the 2008 Robeson upper bound. In the MMBR, CH₄ purity reached 95.13% after 48 h; a mass balance on the recirculating gas volume indicated that essentially all of the CO₂ removed from the gas phase permeated the membrane, of which an estimated 2% was fixed into microalgal biomass while the remainder was retained in the liquid phase. This work offers a membrane-design strategy that bridges gas-separation functionality and microalgal carbon fixation for sustainable biogas upgrading.

Methanotrophs are of fundamental importance in the global methane cycle and hold promise for the bioconversion of methane into valuable products. This study elucidates the impact of alternating aerobic-anoxic conditions on metabolism of the two phylogenetically distinct groups of methanotrophs: Methylomonas koyamae (type I, γ-proteobacteria) and Methylocystis bryophila (type II, α-proteobacteria). Using batch cultivations with single and repeated oxygen pulses, we demonstrated that cyclic aerobic-anoxic alternation significantly increased the secretion of organic compounds. Notably, acetate secretion by the type II methanotroph M. bryophila was observed for the first time. Genome-scale metabolic modeling and flux balance analysis revealed that acetate secretion serves as an auxiliary ATP-generating pathway for M. bryophila under oxygen limitation. We further uncovered a dynamic mechanism: aerobic phases trigger methane uptake and build up intracellular organic carbon pools, while anoxic phases redirect these pools toward acetate and/or hydrogen release to sustain survival under oxygen limitation. This cyclic regimen effectively channels more methane-derived carbon into extracellular acetate. Our findings underscore that redox conditions critically shape methanotrophic metabolism, influencing both the biological methane cycle and the global climate.

Highly permeable and selective membranes are crucial for energy-efficient gas separation. Two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) has attracted significant attention due to its unique structural characteristics, including ultra-thin thickness, inherent surface porosity, and abundant amine groups. However, the interfacial defects caused by poor compatibility between g-C₃N₄ and polymers deteriorate the separation performance of membrane materials. In this study, amino-functionalized g-C₃N₄ nanosheets (CN@PEI) was prepared by a post-synthesis method, then blended with the polymer Pebax to fabricate Pebax/CN@PEI mixed matrix membranes (MMMs). Compared to g-C₃N₄, MMMs with CN@PEI loading of 20 wt% as nanofiller exhibited a CO₂ permeance of 241 Barrer as well as the CO₂/CH₄ and CO₂/N₂ selectivity of 39.7 and 61.2, respectively, at the feed gas pressure of 2 bar, which approaches the 2008 Robeson upper bound and exceeded the 1991 Robeson upper bound. The Pebax/CN@PEI (20) membrane showed robust stability performance over 70 h continuous gas permeability testing, and no significant decline was observed. SEM characterization revealed a uniform dispersion of CN@PEI throughout the Pebax matrix, demonstrating excellent interfacial compatibility between the components. The increased free volume fraction, enhanced solubility, and higher diffusion coefficient demonstrated that the incorporation of CN@PEI nanosheets introduced more CO₂-philic amino groups and disrupted the chain packing of the Pebax matrix, thereby creating additional diffusion channels and facilitating CO₂ transport.