Extracellular electron uptake for CO2 fixation by Rhodopseudomonas palustris during electro-cultivation in darkness

Zero-valent iron as an alternative electron donor for extracellular electron uptake linked to CO2 fixation in Rhodopseudomonas palustris

Anoxygenic phototrophs oxidize both organic and inorganic electron donors for phototrophic fixation of CO2 without O2 generation, playing important roles in global carbon cycles. However, it remains unknown whether and how they can fix CO2 using zero-valent iron (ZVI) as solid-phase electron donor. This study investigated the feasibility of Fe0-driven CO2 fixation by the model bacteria Rhodopseudomonas palustris using nano- (nZVI) and micron-ZVI (mZVI). The results showed that ZVI could empower photoautotrophic and photoheterotrophic growth of R. palustris through iron biocorrosion, with CO2 fixation increased by up to 15%. The ZVI-driven CO2 fixation was attributed to H2-mediated extracellular electron uptake (EEU) and Fe(II) oxidation. The genes encoding EEU-associated pathways were up-regulated in the presence of ZVI, indicating that ZVI promoted CO2 fixation through direct transfer. However, ZVI cannot enable dark CO2 fixation. These findings highlighted the potential of ZVI as a solid electron donor for enhanced microbial CO2 fixation.

Electrochemical enhancement of the accumulation of photosensitive components in anoxygenic phototrophic bacteria extracellular: A new insight into the preparation of degradable microbial photosensitizer for water treatment

Extracellular polymeric substances (EPS) are promising biomaterials for environmental remediation, but their application is hindered by low production efficiency and limited pollutant degradation capacity. In this study, photosynthetic electron extraction enabled Rhodopseudomonas palustris (R. palustris) to efficiently produce EPS enriched with functionalized components. The enhanced EPS (0.2V-EPS), produced from electrically domesticated R. palustris, achieved an 82 % degradation rate of sulfamethoxazole (SMX) within 10 hours, an 18 % improvement compared to EPS produced under open-circuit conditions (OP-EPS). Mechanistic analysis revealed that photosynthetic electron extraction enriched EPS with photosensitive molecules, including tryptophan, humic acid, fulvic acid, which significantly promoted the generation of reactive species. The primary reactive species identified were triplet-excited EPS (³EPS*), ¹O₂, and •OH, with ¹O₂ as the dominant contributor to SMX degradation. The steady-state concentration of ³EPS*, ¹O₂, and •OH increased by 73 %, 34 % and 16 %, respectively, compared to the control. Structural modifications of 0.2V-EPS, including increased hydrophilicity, electronegativity, and aromaticity, enhance its physicochemical properties, thereby facilitating interactions with pollutants. Furthermore, an 88 % reduction in biofilm polysaccharides diminished free radical scavenging activity, promoting the generation of reactive species. This study provides a sustainable strategy for enhancing EPS functionality and offers insights into the metabolic regulation of microorganisms for pollutant degradation.

Microbial advanced oxidation aroused by bacteria-algae symbiosis induced abiotic methane production in anaerobic digestion

The slow decomposition of recalcitrant substrate limits the conversion efficiency of anaerobic digestion. Microbial advanced oxidation, capable of in-situ generating reactive oxygen species (ROS) with the microbial aerobic/anaerobic respiration, provided a potential way to strengthen the substrate-methane conversion in anaerobic digestion. In this study, microalgae were inoculated in anaerobic system and formed redox oscillation under the intermittent illumination, which ultimately increased the methane production by 27.4 %. With the redox oscillation, •OH, the typical ROS, showed a 6.27-fold increase in production (72.95 ± 9.06 μM vs. 10.03 ± 1.49 μM), facilitating the decomposition of lignocelluloses. Notably, abiotic methanation was observed in anaerobic digestion with the occurrence of microbial advanced oxidation. ROS quenching experiments revealed that abiotic methanation roughly accounted for 17.5 % of the total methane production. Microbial advanced oxidation formed by redox oscillation showed the potential to strengthen anaerobic digestion. Notably, for the first time, it was confirmed that abiotic methanation could be established in anaerobic digestion with the ROS generated by microbial advanced oxidation, which offered a new perspective to understand and improve the performances of natural and engineered ecosystems.

Photoheterotrophic extracellular reduction of ferrihydrite activates diverse intracellular metabolic pathways in Rhodopseudomonas palustris for enhanced antibiotic degradation

Anoxygenic photosynthetic bacteria (APB) have been frequently detected as a photoautotrophic Fe-carbon cycling drivers in photic and anoxic environment. However, the potential capacity of these bacteria for photoheterotrophic extracellular reduction of iron-containing minerals and their impact on the transformation of organic pollutants remain currently unknown. This study investigated the capacity of R. palustris, a purple non-sulfur anoxygenic photosynthetic bacterium, to reduce ferrihydrite (Fh) and its correlation with sulfamethazine (SDZ) degradation were firstly investigated. The results revealed that R. palustris could undergo photoheterotrophic extracellular reduction of Fh to form goethite through direct contact, facilitating the formation of conductive bands and enter the interior of cells with a maximum Fe(II)/Fe(T) ratio of up to 39 % within 8 days which led to 13 % increase in assimilation rate of acetate carbon and 53.2 % increase in SDZ degradation rates, as compared with those by R. palustris alone. Moreover, the intermediates generated during the degradation of SDZ by R. palustris-Fh exhibited relatively lower developmental toxicity compared with the original SDZ molecule. The extracellular reduction of Fh significantly up-regulated the expression of genes related to photosynthetic metabolic enzymes, extracellular electron transporters, and extracellular degrading enzymes in R. palustris. This enhancement promoted the photoheterotrophic metabolism and extracellular secretion of photosensitive active compounds in R. palustris, thereby enhancing both the biodegradation and photosensitive degradation of SDZ.

Acceleration of microbiologically influenced corrosion of 304 stainless steel caused by photolysis of riboflavin

This study investigates the effect of riboflavin on microbiologically influenced corrosion (MIC) of 304 stainless steel induced by Rhodopseudomonas palustris TIE-1. Riboflavin accelerated the MIC process, deepening and expanding corrosion pits. Electrochemical results showed a significant increase in corrosion rate, especially with the addition of 40 ppm riboflavin. X-ray photoelectron spectroscopy (XPS) analysis demonstrated that the passive film underwent a compositional transformation from Fe2O3/Fe3O4/Cr2O3 to FeOOH/Cr(OH)3, accompanied by oxidative conversion of Cr2O3 to CrO3 mediated by riboflavin-facilitated extracellular electron uptake (EEU). High performance liquid chromatography (HPLC) results confirmed riboflavin degradation into lumichrome, which accelerated extracellular electron transfer (EET). Scanning electrochemical microscopy (SECM) analysis demonstrated that lumichrome's redox cycling enhanced EEU efficacy, resulting in the degradation of passive film.

Revisiting Solar Energy Flow in Nanomaterial-Microorganism Hybrid Systems

Nanomaterial-microorganism hybrid systems (NMHSs), integrating semiconductor nanomaterials with microorganisms, present a promising platform for broadband solar energy harvesting, high-efficiency carbon reduction, and sustainable chemical production. While studies underscore its potential in diverse solar-to-chemical energy conversions, prevailing NMHSs grapple with suboptimal energy conversion efficiency. Such limitations stem predominantly from an insufficient systematic exploration of the mechanisms dictating solar energy flow. This review provides a systematic overview of the notable advancements in this nascent field, with a particular focus on the discussion of three pivotal steps of energy flow: solar energy capture, cross-membrane energy transport, and energy conversion into chemicals. While key challenges faced in each stage are independently identified and discussed, viable solutions are correspondingly postulated. In view of the interplay of the three steps in affecting the overall efficiency of solar-to-chemical energy conversion, subsequent discussions thus take an integrative and systematic viewpoint to comprehend, analyze and improve the solar energy flow in the current NMHSs of different configurations, and highlighting the contemporary techniques that can be employed to investigate various aspects of energy flow within NMHSs. Finally, a concluding section summarizes opportunities for future research, providing a roadmap for the continued development and optimization of NMHSs.

A novel alginate-embedded magnetic biochar-anoxygenic photosynthetic bacteria composite microspheres for multipollutant removal: Mechanisms of photo-bioelectrochemical enhancement and excellent reusability performance

Existing wastewater treatment technologies face the key challenge of simultaneously removing emerging contaminants and nutrients from wastewater efficiently, with a simplified technological process and minimized operational costs. In this study, a novel alginate-embedded magnetic biochar-anoxygenic photosynthetic bacteria composite microspheres (CA-MBC-PSB microspheres) was prepared for efficient, cost-effective and one-step removal of antibiotics and NH4+-N from wastewater. Our results demonstrated that the CA-MBC-PSB microspheres removed 97.23% of sulfadiazine (SDZ) within 7 h and 91% of NH4+-N within 12 h, which were 21.23% and 38% higher than those achieved by pure calcium alginate-Rhodopseudomonas palustris microspheres (53% and 45.7%), respectively. The enhanced SDZ and NH4+-N removal were attributed to the enhanced photoheterotrophic metabolism and excretion of extracellular photosensitive active substances from R. Palustris through the photo-bioelectrochemical interaction between R. Palustris and magnetic biochar. The long-term pollutants removal performance of the CA-MBC-PSB microspheres was not deteriorated but continuously improved with increasing ruse cycles with a simultaneous removal efficiency of 99% for SDZ and 92% for NH4+-N after three cycles. The excellent stability and reusability were due to the fact that calcium alginate acts as an encapsulating agent preventing the loss and contamination of R. palustris biomass. The CA-MBC-PSB microspheres also exhibited excellent performance for simultaneous removal of SDZ (89% in 7 h) and NH4+-N (90.7% in 12 h) from the secondary effluent of wastewater treatment plant, indicating the stable and efficient performance of CA-MBC-PSB microspheres in practical wastewater treatment.

Reason for the increased electroactivity of extracellular polymeric substances with electrical stimulation: Structural change of α-helix peptide of protein

Electroactivity is an important parameter to assess the ability of the extracellular polymeric substance (EPS) of microorganisms to participate in extracellular respiration. Many reports have found that the electroactivity of microbial sludge could be enhanced with electrical stimulation, but the reason remains unclear. The results of this study showed that the current generation of the three microbial electrolysis cells increased by 1.27-1.76 times during 49 days of electrical stimulation, but the typical electroactive microorganisms were not enriched. Meanwhile, the capacitance and conductivity of EPS of sludge after the electrical stimulation increased by 1.32-1.83 times and 1.27-1.32 times, respectively. In-situ FTIR analysis indicated that the electrical stimulation could lead to the polarization of amide groups in the protein, likely affecting the protein structure related to the electroactivity. Accordingly, the dipole moment of the α-helix peptide of protein of sludge increased from 220 D to 280 D after the electrical stimulation, which was conducive to electron transfer in the α-helix peptide. Moreover, the vertical ionization potential and E_LUMO-E_HOMO energy gap of the C-terminal in the α-helix peptide decreased from 4.43 eV to 4.10 eV and 0.41 eV to 0.24 eV, respectively, which indicated that the α-helix was easier to serve as the electron transfer site of electron hopping. These results meant that the enhancement of the dipole moment of the α-helix peptide unchoked the electron transfer chain of the protein, which was the main reason for the increased electroactivity of EPS protein.