| Summary: | Background: Cyanobacteria account for 20–30 % of Earth’s primary photosynthetic productivity and convert solar energy into biomass-stored chemical energy at the rate of,450 TW [1]. These single-cell microorganisms are resilient predecessors of all higher oxygenic phototrophs and can be found in self-sustaining, nitrogen-fixing communities the world over, from Antarctic glaciers to the Sahara desert [2]. Methodology/Principal Findings: Here we show that diverse genera of cyanobacteria including biofilm-forming and pelagic strains have a conserved light-dependent electrogenic activity, i.e. the ability to transfer electrons to their surroundings in response to illumination. Naturally-growing biofilm-forming photosynthetic consortia also displayed light-dependent electrogenic activity, demonstrating that this phenomenon is not limited to individual cultures. Treatment with site-specific inhibitors revealed the electrons originate at the photosynthetic electron transfer chain (P-ETC). Moreover, electrogenic activity was observed upon illumination only with blue or red but not green light confirming that P-ETC is the source of electrons. The yield of electrons harvested by extracellular electron acceptor to photons available for photosynthesis ranged from 0.05 % to 0.3%, although the efficiency of electron harvesting likely varies depending on terminal electron acceptor. Conclusions/Significance: The current study illustrates that cyanobacterial electrogenic activity is an important microbiological conduit of solar energy into the biosphere. The mechanism responsible for electrogenic activity in cyanobacteria appears to be
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