Electron Mediation and Photocurrent Enhancement in Dunalliela salina Driven Bio-Photo Electrochemical Cells

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.

Roots Fuel Cell Produces and Stores Clean Energy

In recent years, extensive scientific efforts have been conducted to develop clean bioenergy technologies. A promising approach that has been under development for more than a hundred years is the microbial fuel cell (MFC) which utilizes exoelectrogenic bacteria as an electron source in a bioelectrochemical cell. The viability of bacteria in soil MFCs can be maintained by integrating plant roots, which release organic materials that feed the bacteria. In this work, we show that rather than organic compounds, roots also release redox species that can produce electricity in a biofuel cell. We first studied the reduction of the electron acceptor Cytochrome C by green onion roots. We integrate green onion roots into a biofuel cell to produce a continuous bias-free electric current for more than 24 h in the dark. This current is enhanced upon irradiation of the onion's leaves with light. We apply cyclic voltammetry and 2D-fluorescence measurements to show that NADH and NADPH act as major electron mediators between the roots and the anode, while their concentrations in the external root matrix are increased upon irradiation of the leaves. Finally, we show that roots can contribute to energy storage by charging a supercapacitor.

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.

Mammalian Fuel Cells Produce Electric Current

The increasing concern about climate change has led scientists around the world to develop clean energy technologies that may replace the traditional use of fossil fuels. A promising approach is the utilization of unicellular organisms as electron donors in bio-fuel cells. To date, this method has been limited to microorganisms such as bacteria, yeast, and microalgae. In this work, we show for the first time the concept of using mammalian cell cultures and organoids as electron donors in biofuel cells. We apply cyclic voltammetry to show that upon association of ARPE19 cells with the anode, they release reducing molecules to produce electricity. Furthermore, we apply 2D-fluorescence measurements and show that upon illumination, photosensitive stem cell-derived retinal organoids, which consist of rod photoreceptors and interneurons, secrete NADH and NADPH molecules that can donate electrons at the anode to produce photocurrent.

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.

Photocurrent Production from Cherries in a Bio-Electrochemical Cell

In recent years, clean energy technologies that meet ever-increasing energy demands without the risk of environmental contamination has been a major interest. One approach is the utilization of plant leaves, which release redox-active NADPH as a result of photosynthesis, to generate photocurrent. In this work, we show for the first time that photocurrent can be harvested directly from the fruit of a cherry tree when associated with a bio-electrochemical cell. Furthermore, we apply electrochemical and spectroscopic methods to show that NADH in the fruit plays a major role in electric current production.

Non-photosynthetic bacteria produce photocurrent mediated by NADH

In recent years, the concern from the global climate change has driven an urgent need to develop clean energy technologies that do not involve combustion process that emit carbon into the atmosphere. A promising concept is microbial fuel cells that utilize bacteria as electron donors in a bio-electrochemical cell performing a direct electron transfer via conductive protein complexes or by secretion of redox active metabolites such as quinone or phenazine derivatives. In the case of photosynthetic bacteria (cyanobacteria) electrons can also be extracted from the photosynthetic pathway mediated mostly by NADH and NADPH. In this work, we show for the first time that the intact non-photosynthetic bacteria Escherichia coli can produce photocurrent that is enhanced upon addition of an exogenous electron mediator. Furthermore, we apply 2D-fluorescence measurement to show that NADH is released from the bacterial cells, which may apply as a native electron mediator in microbial fuel cells.

Self-Enclosed Bio-Photoelectrochemical Cell in Succulent Plants

Harvesting an electrical current from biological photosynthetic systems (live cells or isolated complexes) is typically achieved by immersion of the system into an electrolyte solution. In this study, we show that the aqueous solution found in the tissues of succulent plants can be used directly as a natural bio-photo electrochemical cell. Here, the thick water-preserving outer cuticle of the succulent Corpuscularia lehmannii serves as the electrochemical container, the inner water content as the electrolyte into which an iron anode and platinum cathode are introduced. We produce up to 20 μA/cm² bias-free photocurrent. When 0.5 V bias is added to the iron anode, the current density increases ∼10-fold, and evolved hydrogen gas can be collected with a Faradaic efficiency of 2.1 and 3.5% in dark or light, respectively. The addition of the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibits the photocurrent, indicating that water oxidation is the primary source of electrons in the light. Two-dimensional fluorescence measurements show that NADH and NADPH serve as the major mediating electron transfer molecules, functionally connecting photosynthesis to metal electrodes. This work presents a method to simultaneously absorb CO₂ while producing an electrical current with minimal engineering requirements.

Direct Electricity Production from Nematostella and Arthemia's Eggs in a Bio-Electrochemical Cell

In recent years, extensive efforts have been made to develop clean energy technologies to replace fossil fuels to assist the struggle against climate change. One approach is to exploit the ability of bacteria and photosynthetic organisms to conduct external electron transport for electricity production in bio-electrochemical cells. In this work, we first show that the sea anemones Nematostella vectensis and eggs of Artemia (brine shrimp) secrete redox-active molecules that can reduce the electron acceptor Cytochrome C. We applied 2D fluorescence spectroscopy and identified NADH or NADPH as secreted species. Finally, we broaden the scope of living organisms that can be integrated with a bio-electrochemical cell to the sea anemones group, showing for the first time that Nematostella and eggs of Artemia can produce electrical current when integrated into a bio-electrochemical cell.

Trichodesmium erythraeum produces a higher photocurrent than other cyanobacterial species in bio-photo electrochemical cells

The increase in world energy consumption, and the worries from potential future disasters that may derive from climate change have stimulated the development of renewable energy technologies. One promising method is the utilization of whole photosynthetic cyanobacterial cells to produce photocurrent in a bio-photo electrochemical cell (BPEC). The photocurrent can be derived from either the respiratory or photosynthetic pathways, via the redox couple NADP⁺/NADPH mediating cyclic electron transport between photosystem I inside the cells, and the anode. In the past, most studies have utilized the fresh-water cyanobacterium Synechocystis sp. PCC 6803 (Syn). Here, we show that the globally important marine cyanobacterium Trichodesmium erythraeum flourishing in the subtropical oceans can provide improved currents as compared to Syn. We applied 2D-fluorescence measurements to detect the secretion of NADPH and show that the resulting photocurrent production is enhanced by increasing the electrolyte salinity, Further enhancement of the photocurrent can be obtained by the addition of electron mediators such as NAD⁺, NADP⁺, cytochrome C, vitamin B1, or potassium ferricyanide. Finally, we produce photocurrent from additional cyanobacterial species: Synechocystis sp. PCC6803, Synechococcus elongatus PCC7942, Acaryochloris marina MBIC 11017, and Spirulina, using their cultivation media as electrolytes for the BPEC.

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.

Identification of bacteria by poly-aromatic hydrocarbon biosensors

Human health is consistently threatened by different species of pathogenic bacteria. To fight the spread of diseases, it is important to develop rapid methods for bacterial identification. Over the years, different kinds of biosensors were developed for this cause. Another environmental risk is poly-aromatic hydrocarbons (PAHs) that may be emitted from industrial facilities and pollute environmental water and soil. One of the methods for their purification is conducted by the addition of bacteria that can degrade the PAHs, while the bacteria can be filtrated at the end of the process. Although many studies reported monitoring of the PAHs degradation by fluorescence, not much attention was dedicated to studying the influence of the PAHs on the intrinsic fluorescence of the degrading bacteria. In this work, we apply synchronous fluorescence (SF) measurements to study the ability of the 5 PAHs: 9-Antracene carboxylic acid (9ACA), Pyrene, Perylene, Pentacene, and Chrysene to interact with bacteria and change its fluorescence spectra. We show that upon incubation of each PAH with the bacterium E. coli, only the 2 PAHs 9ACA and Perylene cause an intensity decrease in the emission at λ = 300-375 nm, which derives from the emission of tyrosine and tryptophan (TT). Also, we show that upon incubation of 9ACA and Perylene with 5 different pathogenic bacteria, the intensity increase or decrease in the TT emission is unique to each bacterial species. Based on this observation, we suggest that the PAHs 9ACA and Perylene can be utilized as biosensors for bacterial identification.