The current studies show that cyanobacteria exhibit light-dependent electrogenic activity and that this activity is conserved among diverse genera. Previously, only chemotrophic bacteria were found to display intrinsic electrogenic activity 
. In electrogenic chemotrophs such as Geobacter sulfurreductens
, electrons are derived from oxidation of organic compounds and transported via a respiratory electron transfer chain (R-ETC) to extracellular terminal electron acceptors 
. In Geobacter
, the electron transfer to the environment was shown to be mediated by a diverse group of c-type cytochromes and possibly involved electrically-conductive microbial nanowires 
. In previous studies, exogenous electron mediators such as 2-hydroxy-1, 4-naphtoquinone were employed for recording electrogenic activity from cyanobacterial cultures 
. Because this synthetic quinone is capable of intercepting electrons at numerous sites along the R-ETC and the P-ETC, it was difficult to determine whether cyanobacteria possess natural mechanism(s) for electron discharge to the extracellular environment and, if they do, whether R-ETC or P-ETC is the source of electrons. The present work expands our basic understanding of bacterial electrogenesis and demonstrates that diverse cyanobacteria exhibit light-dependent electrogenic activity. In contrast to electrogenic chemotrophs, cyanobacterial electrogenic activity was observed in the absence of any exogenous organic fuel and was driven entirely by the energy of light.
We show that the P-ETC is the source of electrons and that PQ plays a central role in electron flow from cyanobacteria to the extracellular environment; in this case the MFC anode. In cyanobacteria PQ is known to be present in substantial amounts in both cellular and thylakoid membranes 
. Inhibition of PS-II by DCMU or CCCP rapidly reduced the light-dependent electrogenic activity in Nostoc
, providing support that electrons originate from PS-II-mediated photolysis of water (). CCCP inhibits electron flow through PS-II but can also uncouple the electron-transport-dependent ATP synthesis by inactivating ATPase 
. DCMU, however, is known to be a specific PS-II inhibitor that blocks binding of PQ to PS-II 
. Blocking cytochrome b6f activity with DBMIB resulted in a significant increase in electrogenic activity. DBMIB is a PQ analogue that binds cytochrome b6f and prevents electron transfer from reduced PQ to cytochrome b6f 
. The opposing effects of DCMU and DBMIB indicate electrons probably exit P-ETC downstream of PS-II and upstream of cytochrome b6f. PQ is the only P-ETC component located between these two P-ETC complexes. Furthermore, the rescue of DCMU-induced inhibition by duroquinone, which has a very high capacity as a PS-II electron acceptor and can shuttle water-derived electrons from PS-II 
, strongly support the mechanism that electrons originate from PS-II-mediated photolysis of water. Inhibition of electrogenic activity by PMA was likely due to interference of PMA with the Q-cycle of electron transfer 
. In the Q-cycle, electrons are transported from PS-I to ferredoxin, and then fed back to PS-I through PQ and cytochrome b6f complex (). P-ETC inhibitors acted in a similar fashion in both Nostoc
implying that the mechanism responsible for electrogenic activity is conserved. Although the biological function of electrogenic activity in cyanobacteria is not yet clear, it is tempting to speculate that it could help regulate the redox state of PQ by shedding excess energy to the environment when PQ becomes over reduced in high light. Future studies will examine this possibility.
An independent approach that employed lights of different color strongly supported a direct link between electrogenic activity and the cyanobacterial photosystem. The fact that electrogenic response was observed only under red or blue light is consistent with the notion that the electrons are donated to the extracellular environment via the P-ETC (). Green light, which is not absorbed by light-harvesting pigments, was unable to induce a positive light response. The electrogenic response under red light was found to be almost as high as that under the reference white light. This result is consistent with the fact that red light absorbing phycobilisomes play a major role in light harvesting in cyanobacteria. The differences in the ratio of photosynthetic pigments in individual genera could explain the different amplitude of the electrogenic response between Nostoc and Lyngbya under the blue light ().
While it is tempting to speculate electrogenic activity may provide a means of shedding excess energy under intense light, other functions, such as a role in carbon fixation, might also be possible. For efficient carbon fixation, cyanobacteria actively import HCO3−
using membrane-spanning transporters 
. Interestingly, some cyanobacteria that lack systems for CO2
uptake still possess HCO3−
uptake systems, indicating HCO3−
is the preferred inorganic source for carbon fixation 
. One of the reason for preferential HCO3−
uptake is that the HCO3−
anion is much less membrane permeable than CO2
, which readily diffuse out of cells. Once inside the cell, cytosol accumulated HCO3−
is converted to CO2
for carbon fixation by carboxysomal carbonic anhydrase (HCO3−
O). In the present study, pH and dissolved oxygen rose to high levels after illumination of diverse genera (Figure S3
) and a voltage spike was observed whenever the light turned on. One possibility, to be followed up on by future studies, is that electrogenic activity could relate to the carbon concentrating mechanism and intracellular filling of the inorganic carbon reserves upon illumination. Indeed, previous researchers have suggested CO2
entering cells can become trapped intracellularly by conversion to HCO3−
through a P-ETC associated pathway 
. When pure CO2
was administered to MFCs electrogenic activity dropped sharply before rebounding within a few minutes indicating electron donation to the extracellular environment had been temporarily interrupted (data not shown). In the future, MFCs can serve as tools to study electrogenic activity as it may relate to the carbon concentrating mechanism.
In anaerobic chemotrophs, electrogenic activity is a constituent part of bacterial respiration, whereas in cyanobacteria we speculate electrogenic activity might help cells adapt to unfavorable light conditions. To cope with the adverse effects of intense sunlight cyanobacteria have evolved several protective mechanisms 
. To prevent ultraviolet damage mat-building cyanobacteria synthesize sunscreens such as scytonemin which accumulates in trichome sheaths 
. Inducible non-photochemical quenching can limit the solar energy conveyed to PS-II by increasing the amount of light dissipated as heat 
. Negative phototaxis triggered by increasing ROS concentration allows some motile cyanobacteria to shield themselves from intense sunlight and gain exposure to optimal intensity light by burrowing downward 
. Despite this broad diversity, the currently known adaptive responses may not be effective against rapid fluctuations in light intensity, suggesting other protective mechanisms could also exist. Considering that light-dependent electrogenic activity is conserved among diverse genera of cyanobacteria, one possibly is that electrogenic activity might serve as a protective mechanism to adverse environmental conditions, such as rapid fluctuation or high intensity of sunlight.
Our study found that a cyanobacteria-containing biofilm consortium exhibited electrogenic activity in a manner similar to that of individual cultures (). DGGE analysis of 16S and 23S rRNA genes indicated the presence of several mat building cyanobacteria. Gene sequence analysis of the electrogenic pond biofilm indicated these cyanobacteria were phylogenetically most similar to Phormidium, Leptolyngba and Pseudanabaena. A green algae and chemotrophic bacteria were also indicated. Minor microbial constituents of the biofilm may not have been detected by DGGE analysis. Nevertheless, each of these three cyanobacterial genera showed light-dependent electrogenic activity when grown as individual cultures ().
Cyanobacteria are the most successful mat-building organisms. They form the topmost, aerobic layer of microbial mats where access to light, atmospheric CO2
is greatest 
. A layer of oxidized iron may separate the cyanobacterial oxygenic layer from a lower anoxygenic layer composed of purple sulfur and green sulfur bacteria. In marine mats, anaerobic sulfate-reducing bacteria can be found throughout the mat below the top layer of cyanobacteria. Sulfate-reducing bacteria play a major role in decomposing organic materials produced by cyanobacteria. The joint metabolic activity of microorganisms in mats results in steep gradients of light, oxygen, carbon dioxide, pH, and redox potential 
. One untested possibility is that rather than merely shedding excess solar energy to the abiotic environment, cyanobacteria might donate excess water-derived electrons to biofilm symbionts.
The ability of cyanobacteria to donate electrons directly to the extracellular environment (i.e. anode) was illustrated by the decreasing anode potential observed when a Nostoc containing half MFC was exposed to light (). An anode without cyanobacteria displayed no such response, indicating this phenomenon is mediated by cyanobacterial cells. The relatively low yield of electron harvesting by extracellular acceptors observed in our experiments was in part due to high amounts of dissolved oxygen present at concentrations substantially exceeding those found in natural cyanobacterial mats. However, the yield of electron harvesting could be improved by as much as 4.5-fold simply by changing the nanostructure of the anode surface. In natural cyanobacterial mats, the yield of electron discharge is likely to be variable, depending on the chemical environment, the physical properties of electron acceptors, the intensity of solar radiation, the concentration of dissolved oxygen and other factors. Even if a very modest yield is used for estimating the average rate of electron discharge, the transfer of solar energy to the environment via cyanobacterial electrogenic pathway could proceed at the rate of ~9 TW on a global scale. Therefore, the electrogenic pathway appears to be an important microbiological conduit of solar energy into the biosphere and could have significant impact on a global scale. Anticipated applications of the electrogenic activity of cyanobacteria described here might be the biological conversion of solar energy to electrical energy or self-sustainable light sensors. Although at present the conversion yield is quite low, future studies on improvements in anode design, genetic manipulations of P-ETC or strain selection will answer the question of whether self-sustainable, CO2 free technologies based on the light-dependent electrogenic activity of cyanobacteria are feasible.