Light-dependent electrogenic activity of cyanobacteria

Abstract

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 approximately 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 fundamentally different from the one exploited in previously discovered electrogenic bacteria, such as Geobacter, where electrons are derived from oxidation of organic compounds and transported via a respiratory electron transfer chain (R-ETC) [3], [4]. The electrogenic pathway of cyanobacteria might be exploited to develop light-sensitive devices or future technologies that convert solar energy into limited amounts of electricity in a self-sustainable, CO(2)-free manner.

Figure 1. Light-dependent electrogenic activity of diverse cyanobacteria genera.

Individual cyanobacterial genera were cultured under photoautotrophic conditions and 24 h illumination cycles (12 h of light/12 h of dark). Electrogenic activity was monitored by recording MFC voltage at 1 KΩ fixed external resistance. Rapid increase in voltage was observed at the beginning of each 12 h light cycle, and rapid drop in voltage was seen at the beginning of each 12 h dark cycle. Electrogenic activity monitored from duplicate MFCs (black and gray lines) revealed reproducible electrogenic profiles for individual genera. The negative controls, where cell voltage was monitored in MFCs loaded only with corresponding media in the absence of cyanobacterial cultures (dotted line), are shown for each experiment. 12 h dark-phases are indicated by black bars along x-axis.

Figure 2. Long-term electrogenic activity.

Light-dependent 24 hour oscillations in electrogenic activity could be observed for many days with little variation within each individual light dark cycle, as shown for Lyngbya. 12 h dark-phases are indicated by black bars along x-axis.

Figure 3. Light dependent oscillations in anode potential.

Anode potential in a Nostoc containing half MFC (black line) was found to oscillate relative to an Ag/AgCl reference electrode as a function of illumination. An identical anode devoid of biofilm (gray line) was unresponsive to illumination. 12 h dark-phases are indicated by black bars along x-axis.

Figure 4. Electrogenic activity of phototrophic biofilm consortium.

Scanning electron microscopy (A) and intrinsic fluorescence microscopy (B) images of mixed phototrophic biofilm consortium. Microalgae included individual cells with spherical morphology (blue arrows) and sets of four cells (red arrows). Filamentous and non-filamentous (green arrows) cyanobacteria were apparent in biofilm. (C) Electrogenic activity of duplicate MFCs containing pond biofilm consortia cultivated under photoautotrophic conditions and 24 h illumination cycles (black and gray lines). The negative control represents cell voltage in MFC loaded only with BG11 media in the absence of biofilm (dotted line). 12 h dark-phases are indicated by black bars along x-axis.

Figure 5. Effect of site-specific inhibitors on electrogenic activity.

The site-specific P-ETC inhibitors were added to Lyngbya (A) or Nostoc (B) at 2 hours intervals at the following cumulatively increasing concentrations: 1, 5, 10, 25, 50, 75 and 100 µM for DCMU (thin black lines), DBMIB (thick gray lines) and PMA (thick black lines); or 5, 10, 25, 50, 100, 150 and 200 µM for CCCP (thin gray lines) as indicated by arrows. The electrogenic activity was monitored under constant illumination and at 1 KΩ fixed external resistance. The effect of DBMIB on electrogenic activity of Nostoc is shown in the inset. The experiments were repeated three times giving consistent results. (C) Schematic diagram showing P-ETC and sites targeted by inhibitors. Cyclic electron transfer (Q-cycle) is indicated by dashed line.

Figure 6. Electrogenic activity under light of different colors.

Electrogenic activity was recorded from Lyngbya (left panel) or Nostoc (right panel) cultured under 24 h illumination cycles (12 h of light/12 h of dark) under red light (red lines), blue light (blue lines), or green light (green lines) of equal intensities. Electrogenic activity under standard white light source of the intensity equal to that of individual colors is provided as a reference (black lines). These experiments were performed using improved anodes that were constructed using nanostructured polypyrrole as previously described . Because of improvements in anode properties, the amplitude of the light response was substantially higher that those reported for the same cultures in Fig. 1. Because nanostructured polypyrrole has properties of a capacitor, the shape of light response curves changed (more gradual increase in response to light). 12 h dark-phases are indicated by black bars along x-axis.

Figure 7. Effect of anode material on electron harvesting.

Power density curves (normalized by the cathode surface area  = 9.6 cm²) measured for MFC with mixed photosynthetic biofilm consortium formed on anode coated with polypyrrole (□) or nanostructured polypyrrole (○). Error bars represent standard deviation based on three independent experiments.