NADPH performs mediated electron transfer in cyanobacterial-driven bio-photoelectrochemical cells

Abstract

Summary Previous studies have shown that live cyanobacteria can produce photocurrent in bio-photoelectrochemical cells (BPECs) that can be exploited for clean renewable energy production. Electron transfer from cyanobacteria to the electrochemical cell was proposed to be facilitated by small molecule(s) mediator(s) whose identity (or identities) remain unknown. Here, we elucidate the mechanism of electron transfer in the BPEC by identifying the major electron mediator as NADPH in three cyanobacterial species. We show that an increase in the concentration of NADPH secreted into the external cell medium (ECM) is obtained by both illumination and activation of the BPEC. Elimination of NADPH in the ECM abrogates the photocurrent while addition of exogenous NADP + significantly increases and prolongs the photocurrent production. NADP + is thus the first non-toxic, water soluble electron mediator that can functionally link photosynthetic cells to an energy conversion system and may serve to improve the performance of future BPECs.

Bioelectricity generation from live marine photosynthetic macroalgae

“…We have recently reported that under illumination, NADPH is the major endogenous mediator which transfer electrons between cyanobacterial cells and the anode (40). There, 2Dfluorescence maps (2D-FM) measured the accumulation of NAD(P)H in the external cellular media (ECM).…”

“…Higher photocurrents of up to 500 µA/cm 2 were obtained by utilization of isolated thylakoid membranes from spinach with the addition ferricyanide ions (FeCN) for MET. In most cases, addition of the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to photosynthetic organisms or isolated membranes inhibits harvested photocurrent, indicating that the major electron source is PSII (30,38,39) However, in some cases electrons from the respiratory system can be collected by the photosynthetic membrane, bypassing PSII (37,38,40). As isolated membranes are disconnected from photodamage cellular repair mechanisms, a decrease in current levels is apparent after 10 minutes of illumination (30).…”

“…As the membranes are disconnected from internal photodamage cellular repair mechanisms, a decrease in current levels is apparent after 10 minutes of illumination 28 . Recently, we have discovered that the main native electron mediator in cyanobacteria is NADPH 37 , and that addition of exogenous NADP + can significantly enhance the photocurrent of intact cells from 5 to 30 µA / cm 2 * mg chl. In contrast to FeCN, NADPH is not toxic and, therefore, has the potential to be integrated in algae cultivation pools without harming the cells.…”

A system-oriented strategy to enhance electron production of Synechocystis sp. PCC6803 in bio-photovoltaic devices: experimental and modeling insights

“…Saper et al 28 reported that Synechocystis could secrete some undefined small molecules to shuttle electrons to the anode. Similarly, Shlosberg et al 29 showed that Synechocystis employed endogenous NADPH as an electron mediator to generate bioelectricity in bio-photoelectrochemical cells.…”

A 35-fold enhancement in photocurrent generation of Synechocystis sp. PCC 6803 by outer membrane deprivation

“…Moreover, they are easy to culture, amenable to genetic manipulation 8 , grow fast, and possess a simple cell structure compared with eukaryotes. Many studies on the mechanism of EET have been conducted using cyanobacteria [9][10][11][12][13][14][15] , and so far, direct EET via conductive nanowires 16 and indirect, mediated EET by endogenous mediators 14,15,17 have been put forward as possible EET pathways; some suggest that the latter is more likely than the former to occur in the case of cyanobacteria 2,18 .…”