A biophotoelectrochemical approach to unravelling the role of cyanobacterial cell structures in exoelectrogenesis

Engineering strategies in bio-photoelectrochemical cells for sustainable energy and environmental applications

Emerging technologies in sustainable energy platforms are gaining significant attention globally. Recently, research has increasingly focused on advanced systems known as bio-photoelectrochemical cells (BPECs) which harness the benefits of both biological processes and photoelectrochemical methods for energy and environmental applications. Researchers are concentrating on improving the performance of BPECs through the use of both photocatalysts and biocatalysts. Photocatalysts are being studied for their application in both anodic and cathodic processes, employing heterojunctions and nanomaterials to optimize solar energy utilization. Additionally, there is a strong emphasis on developing visible light-responsive catalysts through spectral and band gap engineering to enhance solar energy capture. Both photosynthetic and non-photosynthetic microorganisms play essential roles in BPECs by capturing photoelectrons/photoholes and producing bioelectrons, respectively. This review outlines the fundamental principles of BPECs and examines the latest advancements in the field, while also addressing the challenges and future prospects for improving the performance of these systems.

The diversity of physiology and metabolism in chlorophototrophic bacteria

Photosynthesis by (bacterio)chlorophyll-producing organisms ("chlorophototrophy") sustains virtually all life on Earth, providing the biosphere with food and energy. The oxygenic process carried out by plants, algae and cyanobacteria also generates the oxygen we breathe, and ancient cyanobacteria were responsible for oxygenating the atmosphere, creating the conditions that allowed the evolution of complex life. Cyanobacteria were also the endosymbiotic progenitors of chloroplasts, play major roles in biogeochemical cycles and as primary producers in aquatic ecosystems, and act as genetically tractable model organisms for studying oxygenic photosynthesis. In addition to the Cyanobacteriota, eight other bacterial phyla, namely Proteobacteria/Pseudomonadota, Chlorobiota, Chloroflexota, Bacillota, Acidobacteriota, Gemmatimonadota, Vulcanimicrobiota and Myxococcota contain at least one putative chlorophototrophic species, all of which perform a variant of anoxygenic photosynthesis, which does not yield oxygen as a by-product. These chlorophototrophic organisms display incredible diversity in the habitats that they colonise, and in their biochemistry, physiology and metabolism, with variation in the light-harvesting complexes and pigments they produce to utilise solar energy. Whilst some are very well understood, such as the proteobacterial 'purple bacteria', others have only been identified in the last few years and therefore relatively little is known about them - especially those that have not yet been isolated and cultured. In this chapter, we aim to summarise and compare the photosynthetic physiology and central metabolic processes of chlorophototrophic members from the nine phyla in which they are found, giving both a short historical perspective and highlighting gaps in our understanding.

Enhancing the Cellular Robustness of Cyanobacteria to Improve the Stability and Efficiency of Bio-Photovoltaics

Solar photovoltaic technology has consistently been regarded as a crucial direction for the development of clean energy systems in the future. Bio-photovoltaics (BPV), an emerging solar energy utilization technology, is mainly based on the photosynthesis process of photoautotrophic organisms to convert solar energy into electrical energy and output a photocurrent via extracellular electron transfer. As the fundamental unit of the bio-photovoltaic system, the stability of photosynthetic microorganisms under fluctuating and stressful light and heat conditions is likely to have a significant influence on the efficiency of bio-photovoltaic devices. However, this aspect has often been overlooked in previous bio-photovoltaics research. This study took an important cyanobacteria chassis strain, Synechococ elongatus PCC 7942, as the model organism and explored the impact of physiological robustness optimization on its performance as a bio-photovoltaic functional unit. In this work, two types of BPV systems, namely the suspension mode and the biofilm attachment mode, were assembled to evaluate the electricity-generating activity of Synechococcus cells. Overall, the latter demonstrated a remarkable photoelectric output performance. When its light and temperature tolerance was enhanced through FoF1-ATP synthase engineering, the optimized Synechococcus strain exhibited stronger photosynthetic physiology and photoelectric output activity. Under the condition of a light intensity of 2400 μmol photons/m2/s, the maximum photocurrent output of the Synechococcus-based BPV device was increased significantly by 41% over the system based on the wild-type control strain. The results of this study provided a new perspective for the future development and optimization of bio-photovoltaics.

Engineering of bespoke photosensitiser-microbe interfaces for enhanced semi-artificial photosynthesis

Biohybrid systems for solar fuel production integrate artificial light-harvesting materials with biological catalysts such as microbes. In this perspective, we discuss the rational design of the abiotic-biotic interface in biohybrid systems by reviewing microbes and synthetic light-harvesting materials, as well as presenting various approaches to coupling these two components together. To maximise performance and scalability of such semi-artificial systems, we emphasise that the interfacial design requires consideration of two important aspects: attachment and electron transfer. It is our perspective that rational design of this photosensitiser-microbe interface is required for scalable solar fuel production. The design and assembly of a biohybrid with a well-defined electron transfer pathway allows mechanistic characterisation and optimisation for maximum efficiency. Introduction of additional catalysts to the system can close the redox cycle, omitting the need for sacrificial electron donors. Studies that electronically couple light-harvesters to well-defined biological entities, such as emerging photosensitiser-enzyme hybrids, provide valuable knowledge for the strategic design of whole-cell biohybrids. Exploring the interactions between light-harvesters and redox proteins can guide coupling strategies when translated into larger, more complex microbial systems.

Enhancing biophotovoltaic efficiency: Study on a highly productive green algal strain Parachlorella kessleri MACC-38

Biophotovoltaic (BPV) devices are a potential decentralized and environmentally friendly energy source that harness solar energy through photosynthesis. BPV devices are self-regenerating, promising long-term usability. A practical strategy for enhancing BPV performance is to systematically screen for highly exoelectrogenic algal strains capable of generating large electric current density. In this study, a previously uncharacterized green algal strain - Parachlorella kessleri MACC-38 was found to generate over 340 µA mg-1 Chl cm-2. This output is approximately ten-fold higher than those of Chlamydomonas reinhardtii and Chlorella species. The current production of MACC-38 primarily originates from photosynthesis, and the strain maintains its physiological integrity throughout the process. MACC-38 exhibits unique traits such as low extracellular O2 and Fe(III) reduction, substantial copper (II) reduction, and significant extracellular acidification during current generation, contributing to its high productivity. The exoelectrogenic and growth characteristics of MACC-38 suggest that it could markedly boost BPV efficiency.

Life in biophotovoltaics systems

As the most suitable potential clean energy power generation technology, biophotovoltaics (BPV) not only inherits the advantages of traditional photovoltaics, such as safety, reliability and no noise, but also solves the disadvantages of high pollution and high energy consumption in the manufacturing process, providing new functions of self-repair and natural degradation. The basic idea of BPV is to collect light energy and generate electric energy by using photosynthetic autotrophs or their parts, and the core is how these biological materials can quickly and low-loss transfer electrons to the anode through mediators after absorbing light energy and generating electrons. In this mini-review, we summarized the biological materials widely used in BPV at present, mainly cyanobacteria, green algae, biological combinations (using multiple microorganisms in the same BPV system) and isolated products (purified thylakoids, chloroplasts, photosystem I, photosystem II), introduced how researchers overcome the shortcomings of low photocurrent output of BPV, pointed out the limitations that affected the development of BPV' biological materials, and put forward reasonable assumptions accordingly.

Biophotovoltaics: Recent advances and perspectives

Biophotovoltaics (BPV) is a clean power generation technology that uses self-renewing photosynthetic microorganisms to capture solar energy and generate electrical current. Although the internal quantum efficiency of charge separation in photosynthetic microorganisms is very high, the inefficient electron transfer from photosystems to the extracellular electrodes hampered the electrical outputs of BPV systems. This review summarizes the approaches that have been taken to increase the electrical outputs of BPV systems in recent years. These mainly include redirecting intracellular electron transfer, broadening available photosynthetic microorganisms, reinforcing interfacial electron transfer and design high-performance devices with different configurations. Furthermore, three strategies developed to extract photosynthetic electrons were discussed. Among them, the strategy of using synthetic microbial consortia could circumvent the weak exoelectrogenic activity of photosynthetic microorganisms and the cytotoxicity of exogenous electron mediators, thus show great potential in enhancing the power output and prolonging the lifetime of BPV systems. Lastly, we prospected how to facilitate electron extraction and further improve the performance of BPV systems.

Photosynthesis re-wired on the pico-second timescale

Photosystems II and I (PSII, PSI) are the reaction centre-containing complexes driving the light reactions of photosynthesis; PSII performs light-driven water oxidation and PSI further photo-energizes harvested electrons. The impressive efficiencies of the photosystems have motivated extensive biological, artificial and biohybrid approaches to 're-wire' photosynthesis for higher biomass-conversion efficiencies and new reaction pathways, such as H₂ evolution or CO₂ fixation^1,2. Previous approaches focused on charge extraction at terminal electron acceptors of the photosystems³. Electron extraction at earlier steps, perhaps immediately from photoexcited reaction centres, would enable greater thermodynamic gains; however, this was believed impossible with reaction centres buried at least 4 nm within the photosystems^4,5. Here, we demonstrate, using in vivo ultrafast transient absorption (TA) spectroscopy, extraction of electrons directly from photoexcited PSI and PSII at early points (several picoseconds post-photo-excitation) with live cyanobacterial cells or isolated photosystems, and exogenous electron mediators such as 2,6-dichloro-1,4-benzoquinone (DCBQ) and methyl viologen. We postulate that these mediators oxidize peripheral chlorophyll pigments participating in highly delocalized charge-transfer states after initial photo-excitation. Our results challenge previous models that the photoexcited reaction centres are insulated within the photosystem protein scaffold, opening new avenues to study and re-wire photosynthesis for biotechnologies and semi-artificial photosynthesis.

Living electronics: A catalogue of engineered living electronic components

Biology leverages a range of electrical phenomena to extract and store energy, control molecular reactions and enable multicellular communication. Microbes, in particular, have evolved genetically encoded machinery enabling them to utilize the abundant redox-active molecules and minerals available on Earth, which in turn drive global-scale biogeochemical cycles. Recently, the microbial machinery enabling these redox reactions have been leveraged for interfacing cells and biomolecules with electrical circuits for biotechnological applications. Synthetic biology is allowing for the use of these machinery as components of engineered living materials with tuneable electrical properties. Herein, we review the state of such living electronic components including wires, capacitors, transistors, diodes, optoelectronic components, spin filters, sensors, logic processors, bioactuators, information storage media and methods for assembling these components into living electronic circuits.

Utilizing Cyanobacteria in Biophotovoltaics: An Emerging Field in Bioelectrochemistry

Anthropogenic global warming is driven by the increasing energy demand and the still dominant use of fossil energy carriers to meet these needs. New carbon-neutral energy sources are urgently needed to solve this problem. Biophotovoltaics, a member of the so-called bioelectrochemical systems family, will provide an important piece of the energy puzzle. It aims to harvest the electrons from sunlight-driven water splitting using the natural oxygenic photosystem (e.g., of cyanobacteria) and utilize them in the form of, e.g., electricity or hydrogen. Several key aspects of biophotovoltaics have been intensively studied in recent years like physicochemical properties of electrodes or efficient wiring of microorganisms to electrodes. Yet, the exact mechanisms of electron transfer between the biocatalyst and the electrode remain unresolved today. Most research is conducted on microscale reactors generating small currents over short time-scales, but multiple experiments have shown biophotovoltaics great potential with lab-scale reactors producing currents over weeks to months. Although biophotovoltaics is still in its infancy with many open research questions to be addressed, new promising results from various labs around the world suggest an important opportunity for biophotovoltaics in the decades to come. In this chapter, we will introduce the concept of biophotovoltaics, summarize its recent key progress, and finally critically discuss the potentials and challenges for future rational development of biophotovoltaics.

Harnessing photosynthesis to produce electricity using cyanobacteria, green algae, seaweeds and plants

The conversion of solar energy into electrical current by photosynthetic organisms has the potential to produce clean energy. Life on earth depends on photosynthesis, the major mechanism for biological conversion of light energy into chemical energy. Indeed, billions of years of evolution and adaptation to extreme environmental habitats have resulted in highly efficient light-harvesting and photochemical systems in the photosynthetic organisms that can be found in almost every ecological habitat of our world. In harnessing photosynthesis to produce green energy, the native photosynthetic system is interfaced with electrodes and electron mediators to yield bio-photoelectrochemical cells (BPECs) that transform light energy into electrical power. BPECs utilizing plants, seaweeds, unicellular photosynthetic microorganisms, thylakoid membranes or purified complexes, have been studied in attempts to construct efficient and non-polluting BPECs to produce electricity or hydrogen for use as green energy. The high efficiency of photosynthetic light-harvesting and energy production in the mostly unpolluting processes that make use of water and CO2 and produce oxygen beckons us to develop this approach. On the other hand, the need to use physiological conditions, the sensitivity to photoinhibition as well as other abiotic stresses, and the requirement to extract electrons from the system are challenging. In this review, we describe the principles and methods of the different kinds of BPECs that use natural photosynthesis, with an emphasis on BPECs containing living oxygenic photosynthetic organisms. We start with a brief summary of BPECs that use purified photosynthetic complexes. This strategy has produced high-efficiency BPECs. However, the lifetimes of operation of these BPECs are limited, and the preparation is laborious and expensive. We then describe the use of thylakoid membranes in BPECs which requires less effort and usually produces high currents but still suffers from the lack of ability to self-repair damage caused by photoinhibition. This obstacle of the utilization of photosynthetic systems can be significantly reduced by using intact living organisms in the BPEC. We thus describe here progress in developing BPECs that make use of cyanobacteria, green algae, seaweeds and higher plants. Finally, we discuss the future challenges of producing high and longtime operating BPECs for practical use.

3D-printed hierarchical pillar array electrodes for high-performance semi-artificial photosynthesis

The rewiring of photosynthetic biomachineries to electrodes is a forward-looking semi-artificial route for sustainable bio-electricity and fuel generation. Currently, it is unclear how the electrode and biomaterial interface can be designed to meet the complex requirements for high biophotoelectrochemical performance. Here we developed an aerosol jet printing method for generating hierarchical electrode structures using indium tin oxide nanoparticles. We printed libraries of micropillar array electrodes varying in height and submicrometre surface features, and studied the energy/electron transfer processes across the bio-electrode interfaces. When wired to the cyanobacterium Synechocystis sp. PCC 6803, micropillar array electrodes with microbranches exhibited favourable biocatalyst loading, light utilization and electron flux output, ultimately almost doubling the photocurrent of state-of-the-art porous structures of the same height. When the micropillars' heights were increased to 600 µm, milestone mediated photocurrent densities of 245 µA cm-2 (the closest thus far to theoretical predictions) and external quantum efficiencies of up to 29% could be reached. This study demonstrates how bio-energy from photosynthesis could be more efficiently harnessed in the future and provide new tools for three-dimensional electrode design.