Enhanced photo‐bioelectrochemical energy conversion by genetically engineered cyanobacteria

Abstract

Photosynthetic energy conversion using natural systems is increasingly being investigated in the recent years. Photosynthetic microorganisms, such as cyanobacteria, exhibit light-dependent electrogenic characteristics in photo-bioelectrochemical cells (PBEC) that generate substantial photocurrents, yet the current densities are lower than their photovoltaic counterparts. Recently, we demonstrated that a cyanobacterium named Nostoc sp. employed in PBEC could generate up to 35 mW m(-2) even in a non-engineered PBEC. With the insights obtained from our previous research, a novel and successful attempt has been made in the current study to genetically engineer the cyanobacteria to further enhance its extracellular electron transfer. The cyanobacterium Synechococcus elongatus PCC 7942 was genetically engineered to express a non-native redox protein called outer membrane cytochrome S (OmcS). OmcS is predominantly responsible for metal reducing abilities of exoelectrogens such as Geobacter sp. The engineered S. elongatus exhibited higher extracellular electron transfer ability resulting in approximately ninefold higher photocurrent generation on the anode of a PBEC than the corresponding wild-type cyanobacterium. This work highlights the scope for enhancing photocurrent generation in cyanobacteria, thereby benefiting faster advancement of the photosynthetic microbial fuel cell technology.

Tapping into cyanobacteria electron transfer for higher exoelectrogenic activity by imposing iron limited growth

“…48 Moreover, PCC7942 was transformed with the outer membrane c-type cytochrome S (OmcS) from Geobacter sulfurreducens, improving PCC7942 exoelectrogenic capacity with increased ferricyanide reduction rates as well as higher mediatorless electricity generation. In order to develop platforms for direct current generation from cyanobacteria it is imperative to understand their exoelectrogenic mechanisms and to nd conditions favouring higher rates of exoelectrogenesis.…”

“…In the PCC7942 strain harbouring the OmcS, FeCN-R rates were improved to 35 pmol ([Fe(CN) 6 ] 3À ) nmol Chl À1 min À1 , only 2 times higher than the rates for the wild-type. 48 The latter indicates that in addition to the introduced outer membrane c-type cytochrome, there is a constitutional mechanism redirecting electrons to the outer membrane, which was available for the c-cytochrome to react. Under normal conditions, this activity is low, but under stress conditions, such as neutral pH and especially in iron starvation, this reductive mechanism is enhanced.…”

Photoelectrochemistry of Photosystem II in Vitro vs in Vivo

“…3,4 Currently, there are also growing efforts to immobilize photosynthetic microorganisms onto electrodes to perform useful photoelectrochemistry. 5−7 This is because such organisms are robust, abundant, inexpensive to culture, and capable of performing energy storage/conversions, including photobioelectrogenesis (i.e., light-driven electricity production by living organisms). 8,9 …”

“…7,11 This low performance may be due to the micro-sized “footprint” of the cells compared to the nanosized enzymes, resulting in substantially lower photocatalyst loading densities on commonly employed flat electrodes. Furthermore, the ET mechanism at the biofilm–electrode interface is poorly understood, though three ET pathways have been put forward (Figure 1).…”

A synthetic biology approach to engineering living photovoltaics

“…Whereas multiple foreign, heme-containing proteins had been expressed in E. coli years before MtrA expression was first attempted 90,91 , biologists have only recently begun to develop a comprehensive understanding of the proteins involved in the Ccm pathway in cyanobacteria 57,92 . Though cyanobacteria may benefit from the inherent ability to mature c-type cytochromes under aerobic growth conditions 93 , to the best of our knowledge, there is only one successful demonstration of a foreign heme-containing cytochrome expressed in cyanobacteria 94 . Therefore, challenges in extending this platform to cyanobacteria may include developing a system for the expression of specialized Ccm maturation pathways in addition to the heterologous cytochromes and iteratively optimizing expression conditions as done over the past decade with E. coli , albeit now in a host with a growth rate that can be over 30 times slower.…”