Biological photovoltaics: intra- and extra-cellular electron transport by cyanobacteria

Abstract

A large variety of new energy-generating technologies are being developed in an effort to reduce global dependence on fossil fuels, and to reduce the carbon footprint of energy generation. The term 'biological photovoltaic system' encompasses a broad range of technologies which all employ biological material that can harness light energy to split water, and then transfer the resulting electrons to an anode for power generation or electrosynthesis. The use of whole cyanobacterial cells is a good compromise between the requirements of the biological material to be simply organized and transfer electrons efficiently to the anode, and also to be robust and able to self-assemble and self-repair. The principle that photosynthetic bacteria can generate and transfer electrons directly or indirectly to an anode has been demonstrated by a number of groups, although the power output obtained from these devices is too low for biological photovoltaic devices to be useful outside the laboratory. Understanding how photosynthetically generated electrons are transferred through and out of the organism is key to improving power output, and investigations on this aspect of the technology are the main focus of the present review.

A micro-sized microbial solar cell

“…4. This is consistent with a mechanism postulating that the photosynthetic electron-transfer reaction is the source of the electrons harvested at the anode [9]. The OCV peaked at around 0.45V (Fig.…”

Tapping into cyanobacteria electron transfer for higher exoelectrogenic activity by imposing iron limited growth

“…12, 13 Our results show that by exposing PCC7942 cultures to iron limited growth, exoelectrogenesis can favourably compete for reducing power, suggesting that there is an active mechanism redirecting electrons to the cell surface, possibly involving electron transfer by IrpB (oxidoreductase), which is overexpressed under iron starvation together with IrpA (plasma membrane protein), r1 (ferrous uptake) and SomB1 (outer membrane porin), all being part of a strategy to increase iron uptake capacity. 47 Reported value of electrons from photosynthesis redirected extracellularly to ferricyanide is approximately 0.3% in the cyanobacterium Synechocysistis sp.…”

“…12 The mechanisms and metabolic pathways involved in the redirection of electrons to the extracellular membrane are a matter of study and yet to be fully understood. 13,14 Good exoelectrogenic bacteria are those coupling respiration to an extracellular nal electron acceptor, and examples of exoelectrogens such as Shewanella oneidensis and Geobacter sulfurreducens have been reported extensively in the literature. In both cases, outer membrane c-cytochromes associated to extracellular respiratory pathways are responsible of direct electron transfer.…”

“…NADPH and NADH are the intracellular metabolites known to be the substrate for the reaction. 13 Ferrocyanide is then available in the extracellular space to react with an electrode or alternatively, the reaction can be used to quantify exoelectrogenesis by measuring ferricyanide concentration spectrophotometrically. Triplicates of iron sufficient and iron limited cultures were started from a common culture, which was divided into two, centrifuged at 4,000 Â g for 10 minutes, with two washing steps using fresh media.…”

Photosynthetic Energy Conversion: Recent Advances and Future Perspective

“…A robust approach to efficiently collect more electrons from the P-ETC without using the redox mediators would be a welcome strategy to improve the performance of PMFC on par with that of MFC. 1,39 The photosynthetic microorganisms such as cyanobacteria have been performing photosynthesis for over 3.5 billion years. However they have not evolved for extracellular electron transport.…”