Modular configurations of living biomaterials incorporating nano-based artificial mediators and synthetic biology to improve bioelectrocatalytic performance: A review

Dynamic membrane filtration accelerates electroactive biofilms in bioelectrochemical systems

Bioelectrochemical systems (BES) have emerged as a dual-function technology for treating wastewater and recovering energy. A vital element of BES is the rapid formation and maintenance of electroactive biofilms (EABs). Previous attempts to accelerate EAB formation and improve electroactivities focused on enhancing the bacterial adhesion process while neglecting the rate-limiting step of the bacterial transport process. Here, we introduce membrane filtration into BES, establishing a dynamic membrane filtration system that enhances overall performance. We observed that optimal membrane flux considerably reduced the startup time for EAB formation. Specifically, EABs established under a 25 L m-2 h-1 flux (EAB25 LMH) had a formation time of 43.8 ± 1.3 h, notably faster than the 51.4 ± 1.6 h in the static state (EAB0 LMH). Additionally, EAB25 LMH exhibited a significant increase in maximum current density, approximately 2.2 times higher than EAB0 LMH. Pearson correlation analysis indicated a positive relationship between current densities and biomass quantities and an inverse correlation with startup time. Microbial analysis revealed two critical findings: (i) variations in maximum current densities across different filtration conditions were associated with redox-active substances and biomass accumulation, and (ii) the incorporation of a filtration process in EAB formation enhanced the proportion of viable cells and encouraged a more diverse range of electroactive bacteria. Moreover, the novel electroactive membrane demonstrated sustained current production and effective solid-liquid separation during prolonged operation, indicating its potential as a viable alternative in membrane-based systems. This approach not only provides a new operational model for BES but also holds promise for expanding its application in future wastewater treatment solutions.

Electromagnetic Field Drives the Bioelectrocatalysis of γ-Fe2O3-Coated Shewanella putrefaciens CN32 to Boost Extracellular Electron Transfer

The microbial hybrid system modified by magnetic nanomaterials can enhance the interfacial electron transfer and energy conversion under the stimulation of a magnetic field. However, the bioelectrocatalytic performance of a hybrid system still needs to be improved, and the mechanism of magnetic field-induced bioelectrocatalytic enhancements is still unclear. In this work, γ-Fe2O3 magnetic nanoparticles were coated on a Shewanella putrefaciens CN32 cell surface and followed by placing in an electromagnetic field. The results showed that the electromagnetic field can greatly boost the extracellular electron transfer, and the oxidation peak current of CN32@γ-Fe2O3 increased to 2.24 times under an electromagnetic field. The enhancement mechanism is mainly due to the fact that the surface modified microorganism provides an elevated contact area for the high microbial catalytic activity of the outer cell membrane's cytochrome, while the magnetic nanoparticles provide a networked interface between the cytoplasm and the outer membrane for boosting the fast multidimensional electron transport path in the magnetic field. This work sheds fresh scientific light on the rational design of magnetic-field-coupled electroactive microorganisms and the fundamentals of an optimal interfacial structure for a fast electron transfer process toward an efficient bioenergy conversion.

Illuminated fulvic acid stimulates denitrification and As(III) immobilization in flooded paddy soils via an enhanced biophotoelectrochemical pathway

Fulvic acid (FA) is a representative photosensitive dissolved organic matter (DOM) compound that occurs naturally in paddy soils. In this study, the effect of a FA + nitrate treatment (0, 4 and 8 mg/L FA + 20 mmol/L nitrate) on denitrification and As(III) immobilization in flooded paddy soils was assessed under dark and intermittently illuminated conditions (12 h light+12 h dark). The FA input stimulated denitrification in illuminated soils (~100 % of nitrate removal within 6 days) compared to dark conditions (~92 % nitrate removal after 6 days). Meanwhile, As(III) (initial concentration of 0.1 mmol/L) was nearly completely immobilized (~100 %) under illuminated conditions after 4 days for the FA + nitrate treatment compared to 90- 93 % retention in the dark. Denitrification and As immobilization were positively related to the FA dosage in the illuminated assays. The stronger denitrification in illuminated soils was ascribed to denitrifiers harvesting photoelectrons from photosensitive substrates/semiconducting minerals. FA addition also increased the activities of denitrifying enzymes (e.g., NAR, NIR and NOR) and the denitrification electron transport system by nearly 0.6-0.7 and 1.5-1.8 times that of the nitrate-alone treatment under illuminated and dark conditions, thereby fostering stronger denitrification. Upon irradiation, As(III) immobilization was not only stimulated by the interactions with the denitrification pathway whereby As(III) acts as an electron donor for denitrifiers, but was also modulated by Fe(III)/oxidative reactive species-derived photooxidation of As(III). Moreover, the FA + nitrate treatment promoted the enrichment of metal-oxidizing bacteria (e.g., Stenotrophomonas and Acidovorax) that are responsible for nitrate-dependent As(III)/Fe(II) oxidation. The results of this study enhance our understanding of interactions among the biogeochemical cycles of As, Fe, N and C, which are intricately linked by a biophotoelectrochemical pathway in flooded paddy soils.

Self-assembled electrochemically active biofilms doped with carbon nanotubes: Electron exchange efficiency and cytotoxicity evaluation

Thick electrochemically active biofilms (EABs) will lead to insufficient extracellular electron transfer (EET) rate because of the limitation of both substrate diffusion and electron exchange. Herein, carbon nanotubes (CNTs)-doped EABs are developed through self-assembly. The highly conductive biofilms (internal resistance of ∼211 Ω) are efficiently enriched at CNTs dosage of 1 g L-1, with the stable power output of 0.568 W m-2 over three months. The embedded CNTs can act as electron tunnel to accelerate the EET rate in thick biofilm. Self-charging/discharging experiments and Nernst-Monod model stimulation demonstrate a higher net charge storage capacity (0.15 C m-2) and more negative half-saturation potential (-0.401 V) for the hybrid biofilms than that of the control (0.09 C m-2, and -0.378 V). Enzyme activity tests and the observation of confocal laser scanning microscopy by live/dead staining show a nearly negligible cytotoxicity of CNTs, and non-targeted metabonomics analysis reveals fourteen differential metabolites that do not play key roles in microbial central metabolic pathways according to KEGG compound database. The abundance of typical exoelectrogens Geobacter sp. is 2-fold of the control, resulting in a better bioelectrocatalytic activity. These finding provide a possible approach to prolong electron exchange and power output by developing a hybrid EABs doped with conductive material.

Novel approaches to energize microbial biocatalysts

An efficient and cheap energization of microbial biocatalysts is essential in current biotechnological processes. A promising alternative to the use of common organic or inorganic electron donors is the semiconductor nanoparticles (SNs) that absorb light and transfer electrons (photoelectrons) behaving as artificial photosynthetic systems (biohybrid systems). Excited photoelectrons generated by illuminated SNs are highly reductive and readily accepted by membrane-bound proteins and electron shuttles to drive specific cell reduction processes and energy generation in microbes. However, the operational mechanisms of these hybrid systems are still poorly understood, especially at the material-microbe interface, and therefore the design and production of efficient biohybrids are challenging. Some major limitations/challenges and future prospects of SNs as microbial energization systems are discussed.

Inward-to-outward assembly of amine-functionalized carbon dots and polydopamine to Shewanella oneidensis MR-1 for high-efficiency, microbial-photoreduction of Cr(VI)

A novel photosensitized living biohybrid was fabricated by inward-to-outward assembly of amine-functionalized carbon dots (NCDs) and polydopamine (PDA) to Shewanella oneidensis MR-1 and applied for high-efficiency, microbial-photoreduction of Cr(VI). Within a 72 h test period, biohybrids achieved a pronounced catalytic reduction capacity (100%) for 100 mg/L Cr(VI) under visible illumination, greatly surpassing the poor capacity (only 2.5%) displayed by the wild strain under dark conditions. Modular configurations of NCDs and PDA afforded biohybrids with a large electron flux by harvesting extracellular photoelectrons generated from illuminated NCDs and increasing reducing equivalents released from an enlarged intracellular NADH/NAD⁺ pool. Further, increased production of intracellular c-type cytochromes and extracellular flavins resulting from the modular configuration enhanced the biohybrid electron transport ability. The enhancement of electron transport was also attributed to more conductive conduits at NCDs-PDA junction interfaces. Moreover, because NCDs are highly reductive, the enhanced Cr(VI) reduction was also attributed to direct reduction by the NCDs and the direct Cr(VI) reduction by sterile NCDs-assembled biohybrid was up to 20% in the dark. Overall, a highly efficient strategy for removal/transformation of Cr(VI) by using NCD-assembled photosensitized biohybrids was proposed in this work, which greatly exceeded the performance of Cr(VI)-remediation strategies based on conventional microbial technologies.