(Invited) Electrochemical Activity of Cyanobacteria: From Biophotovoltaics to Environmental Monitoring
Authors: Freguia Stefano , Darus Libertus , Vieira Lemos Rita , Ledezma Pablo 
Published: February 2020

Introduction
Phototrophic microbial consortia have revealed the unique capability to convert light energy into electrical energy via the process of extracellular electron transfer. When hit by light, oxygenic photosynthesis generates reducing equivalents from the oxidation of water to molecular oxygen. These are normally used by the organisms to reduce carbon dioxide to organic compounds. However, a fraction of the electrons can be diverted towards the extracellular environment, in search for extracellular electron acceptors (such as dissolved oxygen, mineral oxides or polarised electrodes), especially in situations of reductive stress, such as low carbon dioxide or high light intensity.
Intrigued by this interesting phenomenon, researchers have investigated the source of extracellular electron flow, discovering unequivocal links with the photosynthetic electron transfer chain (PTEC [1]). Technologists quickly proposed to harness the electrical currents generated this way through devices which have been named biophotovoltaics, which are envisaged as alternatives to the well established semiconductor-based photovoltaic systems. Both pure and mixed cultures of phototrophic organisms were shown to produce electrical currents, albeit at very low levels. A limitation is that, when grown as mixed culture biofilms on electrodes, these biological photovoltaic systems tend to produce anodic currents only during the night, while during illumination the current turns cathodic even at very high electrode potential (+0.6 V vs. SHE [2]). This was explained with the presence of eletroactive gamma-proteobacteria, which have been shown to catalyse cathodic oxygen reduction.
Our work aims have been to: (i) establish whether there exist particular species of either microalgae or cyanobacteria with high propensity for extracellular electron transfer; (ii) investigate redox polymer coatings as a way to enhance the harnessed current densities.
Materials and Methods
Glass-made electrochemical cells were used and hosted working electrodes made of either unmodified graphite felt or graphite rods (unmodified and modified with redox polymers), Pt counter electrodes separated from the working electrodes via a cation exchange membrane (Membranes International, USA) and Ag/AgCl reference electrodes. The light source was a white fluorescent lamp at 135 μmol m-2 s-1. The working electrode potential was controlled at +0.6 V vs. SHE using a CHI-1000B potentiostat (CH Instruments, USA). Mixed and pure phototrophic cultures were pre-grown in flasks using standard growth media and added to the electrochemical cells for testing.
Results and Discussion
Five freshwater cultures were grown from environmental inocula obtained from local lakes in Southeast Queensland (Australia); two seawater cultures were obtained from two local sea shores. Electrochemical activity on unmodified graphite felt was not detected for any of the freshwater cultures in the absence of added mediators, while the two seawater cultures quickly generated anodic currents. Microscopic analysis of the freshwater cultures revealed that all of them were dominated by eukaryotic microalgae. Instead, the seawater cultures were dominated by cyanobacteria Geitlerinema and Leptolyngbya. Adding riboflavin (1 mM) to a freshwater culture dominated by microalga Dictyosphaerium sp. established an anodic current, suggesting that microalgae do not possess the genetic gear to extrude electrons out of the cells. When pure freshwater cyanobacterial cultures including Anabaena and Microcystis were tested, they exhibited electrochemical activity, confirming that freshwater systems do deliver anodic currents when cyanobacteria are the main players. The conclusion of this work is that anodic electrochemical activity is ubiquitous in both freshwater and seawater environments, as long as they are dominated by cyanobacteria.
To enhance the current densities delivered by the above mixed cultures cultures, graphite rods were modified with a number of redox polymers, including osmium polymers and poly-methylene blue. The polymers enabled an increase in current density, with best performing [Os(2,2’-bipyridine)2(polyvinyl-imidazole)10Cl]Cl (E0’=+0.39 V vs. SHE) increasing the current output by a seawater mixed culture 64 fold to 34 μA cm-2. Interestingly, modifying the surface with the redox polymers removed the daytime current inversion in mixed culture systems, leading to a device which delivered anodic current 24 hours a day. Despite the promising results, these current densities are still too low for electricity generation applications. The next challenge will be to use these bioelectrochemical devices as cyanobacteria sensors for monitoring of toxic blooms in surface waters. Preliminary results indicate that cyanobacteria can be selectively detected with this tool, as shown in Figure 1.

Figure 1. Anodic current profiles at +0.6 V vs. SHE with Anabaena circinalis: blank rod (black), rod+cells (grey), rod+poly-methylene blue+cells (red and green).
References

Pisciotta, J.M., Y. Zou, Baskakov, I.V. Light-dependent electrogenic activity of cyanobacteria.
PLoS ONE, 2010, 5(5):e10821.

Darus, L., Yang, L., Ledezma, P., Keller, J., Freguia, S. Fully reversible current driven by a dual marine photosynthetic microbial community. Bioresource Technology, 2015, 195, 248-253.

Figure 1