Paolo Bombelli

Photosynthetic biofilms in pure culture harness solar energy in a mediatorless bio-photovoltaic cell (BPV) system

Microbial fuel cells are an emerging technology for converting organic substrates into electrical power. Recent research has shown that biofilms of some bacterial species are capable of self-mediated extracellular electron transfer. The prospect of exploiting this trait in photoautotrophic microbes that do not require an organic substrate has important implications for the future development of renewable solar energy technologies. Here we report on light-driven electrical power generated with biofilms grown from photosynthetic fresh water or marine species without the addition of an artificial electron-shuttling mediator. Green alga (Chlorella vulgaris, Dunaliella tertiolecta) or cyanobacteria (Synechocystis sp. PCC 6803, Synechococcus sp. WH 5701) strains were grown directly on a transparent, conductive anode (indium tin oxide-coated polyethylene terephthalate) and power generation under light and dark conditions was evaluated using a single-chamber bio-photovoltaic cell (BPV) system. Increased power outputs were observed for all strains upon illumination, with the largest light effect observed for Synechococcus (maximum 10.3 mW m−2 total power output recorded under 10 W m−2 white light). Further experiments conducted with Synechococcus and C. vulgaris showed that photosynthetic oxygen evolution rates were consistent with BPV power outputs under different light regimes (red, green and blue light), indicating a direct link between power output and photosynthetic activity. Biofilm power generation in these BPV devices was self-sustained for several weeks and was highly sensitive to ambient light levels. When connected in series, four BPVs (each 0.011 m2) generated enough power to run a commercial digital clock.

Biophotovoltaics: oxygenic photosynthetic organisms in the world of bioelectrochemical systems

The field of bioelectrochemical system (BES) research includes a wide range of emerging technologies that utilise microbes to catalyze anodic and/or cathodic reactions within a fuel cell setup, and has developed greatly in the last 2–3 years. Although the vast majority of BESs utilise organic substrates as a fuel source (e.g. microbial fuel cells), several systems have been developed that are fuelled by light energy. In this review we focus on and contextualise a specific subset of light-harvesting BESs, which we have called biophotovoltaic systems (BPVs). BPVs utilise oxygenic photosynthetic organisms, such as microalgal and cyanobacterial species, to harvest light energy to generate current, critically, in the absence of an organic feedstock. Here we discuss the state-of-the-art for all light-harvesting BESs and present a novel classification system to illustrate how BPVs integrate into the broad fields of BES and photovoltaic research. We compare and contrast the present understanding of electron transfer pathways in systems that use heterotrophic microbes with those in cyanobacteria-based BPVs. Finally we present, for the first time, an estimate of the achievable power outputs of this emerging technology.

In Vivo Changes of the Oxidation-Reduction State of NADP and of the ATP/ADP Cellular Ratio Linked to the Photosynthetic Activity in Chlamydomonas reinhardtii

The ATP/ADP and NADP/NADPH ratios have been measured in whole-cell extract of the green alga Chlamydomonas reinhardtii, to understand their availability for CO2 assimilation by the Calvin cycle in vivo. Measurements were performed during the dark-light transition of both aerobic and anaerobic cells, under illumination with saturating or low light intensity. Two different patterns of behavior were observed: (a) In anaerobic cells, during the lag preceding O2 evolution, ATP was synthesized without changes in the NADP/NADPH ratio, consistently with the operation of cyclic electron flow. (b) In aerobiosis, illumination increased the ATP/ADP ratio independently of the intensity used, whereas the amount of NADPH was decreased at limiting photon flux and regained the dark-adapted level under saturating photon flux. Moreover, under these conditions, the addition of low concentrations of uncouplers stimulated photosynthetic O2 evolution. These observations suggest that the photosynthetic generation of reducing equivalents rather than the rate of ATP formation limits the photosynthetic assimilation of CO2 in C. reinhardtii cells. This situation is peculiar to C. reinhardtii, because neither NADPH nor ATP limited this process in plant leaves, as shown by their increase upon illumination in barley (Hordeum vulgare) leaves, independent of light intensity. Experiments are presented and were designed to evaluate the contribution of different physiological processes that might increase the photosynthetic ATP/NADPH ratio—the Mehler reaction, respiratory ATP supply following the transfer of reducing equivalents via the malate/oxaloacetate shuttle, and cyclic electron flow around PSI—to this metabolic situation.

A High Power-Density, Mediator-Free, Microfluidic Biophotovoltaic Device for Cyanobacterial Cells.

Biophotovoltaics has emerged as a promising technology for generating renewable energy because it relies on living organisms as inexpensive, self-repairing, and readily available catalysts to produce electricity from an abundant resource: sunlight. The efficiency of biophotovoltaic cells, however, has remained significantly lower than that achievable through synthetic materials. Here, a platform is devised to harness the large power densities afforded by miniaturized geometries. To this effect, a soft-lithography approach is developed for the fabrication of microfluidic biophotovoltaic devices that do not require membranes or mediators. Synechocystis sp. PCC 6803 cells are injected and allowed to settle on the anode, permitting the physical proximity between cells and electrode required for mediator-free operation. Power densities of above 100 mW m-2 are demonstrated for a chlorophyll concentration of 100 μM under white light, which is a high value for biophotovoltaic devices without extrinsic supply of additional energy.

Photosynthetic, respiratory and extracellular electron transport pathways in cyanobacteria☆

Cyanobacteria have evolved elaborate electron transport pathways to carry out photosynthesis and respiration, and to dissipate excess energy in order to limit cellular damage. Our understanding of the complexity of these systems and their role in allowing cyanobacteria to cope with varying environmental conditions is rapidly improving, but many questions remain. We summarize current knowledge of cyanobacterial electron transport pathways, including the possible roles of alternative pathways in photoprotection. We describe extracellular electron transport, which is as yet poorly understood. Biological photovoltaic devices, which measure electron output from cells, and which have been proposed as possible means of renewable energy generation, may be valuable tools in understanding cyanobacterial electron transfer pathways, and enhanced understanding of electron transfer may allow improvements in the efficiency of power output. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Conrad Mullineaux.

Surface morphology and surface energy of anode materials influence power outputs in a multi-channel mediatorless bio-photovoltaic (BPV) system

Bio-photovoltaic cells (BPVs) are a new photo-bio-electrochemical technology for harnessing solar energy using the photosynthetic activity of autotrophic organisms. Currently power outputs from BPVs are generally low and suffer from low efficiencies. However, a better understanding of the electrochemical interactions between the microbes and conductive materials will be likely to lead to increased power yields. In the current study, the fresh-water, filamentous cyanobacterium Pseudanabaena limnetica (also known as Oscillatoria limnetica) was investigated for exoelectrogenic activity. Biofilms of P. limnetica showed a significant photo response during light-dark cycling in BPVs under mediatorless conditions. A multi-channel BPV device was developed to compare quantitatively the performance of photosynthetic biofilms of this species using a variety of different anodic conductive materials: indium tin oxide-coated polyethylene terephthalate (ITO), stainless steel (SS), glass coated with a conductive polymer (PANI), and carbon paper (CP). Although biofilm growth rates were generally comparable on all materials tested, the amplitude of the photo response and achievable maximum power outputs were significantly different. ITO and SS demonstrated the largest photo responses, whereas CP showed the lowest power outputs under both light and dark conditions. Furthermore, differences in the ratios of light : dark power outputs indicated that the electrochemical interactions between photosynthetic microbes and the anode may differ under light and dark conditions depending on the anodic material used. Comparisons between BPV performances and material characteristics revealed that surface roughness and surface energy, particularly the ratio of non-polar to polar interactions (the CQ ratio), may be more important than available surface area in determining biocompatibility and maximum power outputs in microbial electrochemical systems. Notably, CP was readily outperformed by all other conductive materials tested, indicating that carbon may not be an optimal substrate for microbial fuel cell operation.

Phycobilisome-Deficient Strains of Synechocystis sp. PCC 6803 Have Reduced Size and Require Carbon-Limiting Conditions to Exhibit Enhanced Productivity

Gradual reduction of the light-harvesting complex in Synechocystis, the phycobilisome, results in decreased cell size and chlorophyll levels, lower photosynthesis, photoinhibition, and respiration, with increased productivity and biomass accumulation under carbon-limited conditions. Reducing excessive light harvesting in photosynthetic organisms may increase biomass yields by limiting photoinhibition and increasing light penetration in dense cultures. The cyanobacterium Synechocystis sp. PCC 6803 harvests light via the phycobilisome, which consists of an allophycocyanin core and six radiating rods, each with three phycocyanin (PC) discs. Via targeted gene disruption and alterations to the promoter region, three mutants with two (pcpcT→C) and one (ΔCpcC1C2:pcpcT→C) PC discs per rod or lacking PC (olive) were generated. Photoinhibition and chlorophyll levels decreased upon phycobilisome reduction, although greater penetration of white light was observed only in the PC-deficient mutant. In all strains cultured at high cell densities, most light was absorbed by the first 2 cm of the culture. Photosynthesis and respiration rates were also reduced in the ΔCpcC1C2:pcpcT→C and olive mutants. Cell size was smaller in the pcpcT→C and olive strains. Growth and biomass accumulation were similar between the wild-type and pcpcT→C under a variety of conditions. Growth and biomass accumulation of the olive mutant were poorer in carbon-saturated cultures but improved in carbon-limited cultures at higher light intensities, as they did in the ΔCpcC1C2:pcpcT→C mutant. This study shows that one PC disc per rod is sufficient for maximal light harvesting and biomass accumulation, except under conditions of high light and carbon limitation, and two or more are sufficient for maximal oxygen evolution. To our knowledge, this study is the first to measure light penetration in bulk cultures of cyanobacteria and offers important insights into photobioreactor design.

Porous ceramic anode materials for photo-microbial fuel cells

This study focuses on porous ceramics as a promising new type of anode material for photo-microbial fuel cells (p-MFCs). The anodes were made from titanium dioxide and chemical vapour deposition was used to coat them with a layer of fluorine doped tin oxide (FTO) to make them conducting. Chlorella vulgaris biofilms were grown in the millimetre sized pores of the ceramic electrodes, producing an extensive extra cellular matrix that was anchored directly to the electrode surface. In contrast algal cells grown on carbon felt appeared misshapen and lacked a continuous extra cellular matrix. A preliminary comparison of different anodes in p-MFCs showed that the power density was ∼16 times higher on a ceramic anode compared to the best performing carbon anode. Good power densities were also found for algae grown directly onto FTO coated glass, but in contrast to the ceramic anodes the biofilm did not adhere strongly to the planar surface and was easily removed or damaged.

Polyethylene bio-degradation by caterpillars of the wax moth Galleria mellonella

Plastics are synthetic polymers derived from fossil oil and largely resistant to biodegradation. Polyethylene (PE) and polypropylene (PP) represent ∼92% of total plastic production. PE is largely utilized in packaging, representing ∼40% of total demand for plastic products (www.plasticseurope.org) with over a trillion plastic bags used every year [1]. Plastic production has increased exponentially in the past 50 years (Figure S1A in Supplemental Information, published with this article online). In the 27 EU countries plus Norway and Switzerland up to 38% of plastic is discarded in landfills, with the rest utilized for recycling (26%) and energy recovery (36%) via combustion (www.plasticseurope.org), carrying a heavy environmental impact. Therefore, new solutions for plastic degradation are urgently needed. We report the fast bio-degradation of PE by larvae of the wax moth Galleria mellonella, producing ethylene glycol.

Exploiting algal NADPH oxidase for biophotovoltaic energy

Summary Photosynthetic microbes exhibit light‐dependent electron export across the cell membrane, which can generate electricity in biological photovoltaic (BPV) devices. How electrons are exported remains to be determined; the identification of mechanisms would help selection or generation of photosynthetic microbes capable of enhanced electrical output. We show that plasma membrane NADPH oxidase activity is a significant component of light‐dependent generation of electricity by the unicellular green alga Chlamydomonas reinhardtii. NADPH oxidases export electrons across the plasma membrane to form superoxide anion from oxygen. The C. reinhardtii mutant lacking the NADPH oxidase encoded by RBO1 is impaired in both extracellular superoxide anion production and current generation in a BPV device. Complementation with the wild‐type gene restores both capacities, demonstrating the role of the enzyme in electron export. Monitoring light‐dependent extracellular superoxide production with a colorimetric assay is shown to be an effective way of screening for electrogenic potential of candidate algal strains. The results show that algal NADPH oxidases are important for superoxide anion production and open avenues for optimizing the biological component of these devices.