Physics of Extracellular Electron Transfer via Protein Nanowires by A Single Microbe and Biofilms

Electron transfer (ET) is central to all life processes. Although electrons follow quantum physics, quantum phenomena in living systems are generally assumed to arise primarily through interactions with light in photosynthetic or fluorescent proteins. Protein nanowires will provide non-photosynthetic protein analogs for flexible testbeds for studies of quantum processes, with structures and assemblies that can be controlled at the atomic and mesoscale. In contrast to most synthetic quantum materials that require very low temperatures to function, these proteins evolved to optimize ultrafast electron transfer in aqueous environments at physiological temperatures. Understanding how to optimize systems' electron transfer will have implications for designing protein nanowire-based quantum sensors and developing new quantum computing applications. This work will evaluate the role of quantum effects in extracellular electron transfer (EET) via nanowire, the role of nanowire structure on conductivity, and the details of metabolic components and pathways in bacteria. Applications of the findings of this project will include the modeling of biogeochemical cycles and ecosystem dynamics, improved understanding of the human microbiome's respiration, and enhanced performance of bioremediation systems and bioreactors. The findings will also inform engineering innovations in self-assembling electronic materials, targeted drug delivery, and biosensors. Furthermore, new techniques developed here will help build and validate a physical model of a fundamental biological process. The proposed methods will provide new insights into EET by identifying the nanowires that drive it and elucidating its underlying mechanisms. These studies will provide a foundation for exploring how EET is used to control entropy, energy, and information flow across diverse microbial systems, impacting fields ranging from Biophysics to Quantum Information Sciences. These studies will prepare the next generation of interdisciplinary scientists for education and workforce development, with a primary focus on recruiting, training, and retaining students with diverse research backgrounds. Recently, quantum computations have found coherent ET exceeding 60 Å and spin polarization at 300K for non-photosynthetic protein "nanowires" with closely stacked (3.5–6 Å) cytochrome hemes. Long-range (> 10 µm) extracellular electron transfer via nanowire" enables bacterial respiration, communication, and adhesion in environments that lack O2-like soluble molecules. EET drives various global environmental phenomena, including biogeochemical cycling of carbon, nutrients, and minerals; bioremediation of toxic contaminants; corrosion; the gut microbiome; and the conversion of waste into methane or electricity. Coherence could explain up to a billion-fold lower ET computed using semi-classical adiabatic ET models than measured as ultrafast (10 S/cm) and mobility (0.25 cm2/Vs) ever reported in proteins that confers metal-like temperature dependence of conductivity to living biofilms. This proposal aims to advance the study of microbial bioenergetics by bridging theories of ET through experimental insights. The PI will apply a nanoscopic approach to study EET mechanisms, develop a robust physical model of microbial bioenergetics, and directly visualize and quantify EET in single live cells and microbial communities to identify EET components and pathways, and to measure electrical and electrochemical properties of living cells. Particular focus will be placed on understanding how diverse bacterial species use "nanowire" filaments, which play a fundamental role in driving long-range EET. The projects objectives are to: (1) Observe the mechanism of EET via nanowires in vivo with single-molecule imaging; and (2) Measure the electron conductivity and redox potentials of intact nanowires attached to EET-performing cells and biofilms using nanoelectrodes under physiological conditions. (3) Probe the physics of EET by measuring EET and electron conductivity of biofilms and nanowires as a function of key physiological probes: temperature, pH, photoexcitation, gate voltage, electric and magnetic fields. Data will be used to build a quantitative model of bioenergetics. Because more is known about the molecular genetics of nanowire synthesis in Geobacter sulfurreducens (Gs) than in any other species, they will be used for initial in vivo studies. Other nanowire-producing species, such as G. metallireducens (Gm) and Geoalkalibacter subterraneus (Ga) will also be characterized, to understand how diverse species perform EET. This may reveal how biofilm cells respire by performing EET over a distance of ~500 µm. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.