Solar Hunter System

The recent work based upon Bacterial (M. thermoacetica)-quantum dots hybrid system to harvest value-based products has suggested great future for artificial photosynthesis system [1]. Despite important advances, the current efficiency and scope of application have been limited due to the damage of quantum dots on biological systems arising from direct contact of quantum dots with cell membrane, less efficient integration between bio-abiotic interfaces as well as poor conductivity of most biological systems. To address these issues, we develop a solar hunter platform that can seamlessly integrate conductive bacteria biofilms, high-efficiency photon-electron transformation of quantum dots with efficient metabolic pathways of biological systems.

The biofilm system that came into our sight is type IV pili in Geobacter sulfurreducens, which is conductive microbial nanowire [2]. The wire can be expressed in genetically manipulated strains as long wires with binding sites for quantum dots and efficiently conduct electrons. With the more surface area of biofilms for quantum dots and indirect contact between quantum dots with cell membrane, we expect a significant boost in the energy of light harvested by our Solar Hunter without sacrificing normal cell growth and regeneration.

Specifically, we propose three demo examples here based on our newly developed artificial photosynthesis. The first one is a simple artificial photosynthesis based on non-conductive biofilm; to increase system complexity and promote the efficiency of electrons transferring, we design the other two systems in which we use conductive biofilm of G. sulfurreducens to develop the electrons transferring tracks and connect the quantum dots with microorganisms.

1) At first, we want to establish the Solar Hunter system on E. Coli, whose biofilm serves as a synthetic nonconductive biological platform for self-assembling function materials. The amyloid protein CsgA , which is the dominant component in E. Coli, can be programmed to append small peptide domain and successfully secreted with biological functions. Then we propose that our Hunter family member can be an enzyme. Nitrogenase complex is the central enzyme in the natural nitrogen-fixing process. Previous researches have demonstrated the viability of using semiconductor CdS nanorod to harvest light and supply the electrons as a substitute for the Fe protein in the complex where electrons are generated from ATP [3]. The heterotetrameric MoFe protein, the other part in nitrogenase complex, will use the electrons provided to reduce N2 to NH3. We will explore the possibility of an increase in the efficiency of the semiconductor-enzyme system usingE.Coli’s biofilm, on which biofilm subunit are engineered with SpyTag and SpyCatcher system from FbaB protein to provide binding sites for proteins [4].

2) The solar source in the solar-chemical system is, in its essence, energy with electrons. In an attempt to apply our quantum dots-pili hybrid to a wider extent, we decide to try out this model on another amazing archaea, Methanosaeta harundinacea, which is likely to have a pathway to simply use carbon dioxide, electrons and protons for the biosynthesis of methane [5]. Geobacter can naturally express nanowires and transfer electrons between each other or do direct interspecies electron transfer(DIET) with other microorganisms; for our project, Methanosarcina. Extern light is absorbed by the Geobacter and is transferred into electrons. Semi-conductors are bind to the biofilm of Geobacter to enhance its conductivity. The electrons are then transported to Methanosarcina in the form of succinate and fumarate, used as the input material to produce value-added products like methane.

3) In addition, solar hunters will include a pathway for leucine synthesis from acetate (acetyl-coenzyme A) [6], since leucine is of higher value. They use carbon dioxide as a carbon source to synthesize isoleucine via a combination of two pathways. The first pathway is the acetyl-coenzyme A (acetyl-CoA) pathway [7], gaining electrons to reduce carbon dioxide and synthesizing acetyl-CoA which is a vital intermediate. As acetyl-CoA is synthesized, it can be the raw material of the second pathway, which is the pathway for isoleucine biosynthesis in G. sulfurreducens, to give the final product isoleucine [8]. There are three main reasons for us to choose this combination. Firstly, these two pathways are found in G. sulfurreducens. Secondly, carbon dioxide is a kind of environmentally friendly carbon source. Thirdly, comparing to carbon dioxide, isoleucine is a high value-added chemical that will bring us a high level of economic efficiency. Additionally, the second pathway can be replaced by other pathways to synthesize other value-added chemicals, such as butanol.

Collectively, we envision that these three parallel systems should build a powerful solar Hunter system to push the boundary of current artificial photosynthesis.

Reference

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[2] Strycharz S M, Glaven R H, Coppi M V, et al. Gene expression and deletion analysis of mechanisms for electron transfer from electrodes to Geobacter sulfurreducens[J]. Bioelectrochemistry, 2011, 80(2):142-150.

[3] Brown K A, Harris D F, Wilker M B, et al. Light-driven dinitrogen reduction catalyzed by a CdS:nitrogenase MoFe protein biohybrid.[J]. Science, 2016, 352(6284):448-450.

[4] Zakeri B, Fierer J O, Celik E, et al. Peptide tag forming a rapid covalent bond to a protein, through engineering a bacterial adhesin.[J]. Proceedings of the National Academy of Sciences of the United States of America, 2012, 109(12):690-7.

[5] Rotaru A E, Shrestha P M, Liu F, et al. A new model for electron flow during anaerobic digestion: direct interspecies electron transfer to Methanosaeta for the reduction of carbon dioxide to methane[J]. Journal of Virology, 2013, 18(1):324-31.

[6] Risso C, Van Dien S J A, Lovley D R. Elucidation of an Alternate Isoleucine Biosynthesis Pathway in Geobacter sulfurreducens[J]. Infection Control & Hospital Epidemiology, 2008, 190(7):277-81.

[7] Methé B A, Nelson K E, Eisen J A, et al. Genome of Geobacter sulfurreducens: metal reduction in subsurface environments.[J]. Science, 2003, 302(5652):1967-9.

[8] Mahadevan R, Palsson B Ø, Lovley D R. In situ to in silico and back: elucidating the physiology and ecology of Geobacter spp. using genome-scale modelling.[J]. Nature Reviews Microbiology, 2011, 9(1):39-50.