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

Abstract

The exoelectrogenic capacity of the cyanobacterium Synechococcus elongatus PCC7942 was studied in iron limited growth in order to establish conditions favouring extracellular electron transfer in cyanobacteria for photo-bioelectricity generation. Investigation into extracellular reduction of ferricyanide by Synechococcus elongatus PCC7942 demonstrated enhanced capability for the iron limited conditions in comparison to the iron sufficient conditions. Furtheremore, the significance of pH showed that higher rates of ferricyanide reduction occurred at pH 7, with a 2.7-fold increase with respect to pH 9.5 for iron sufficient cultures and 24-fold increase for iron limited cultures. The strategy presented induced exoelectrogenesis driven mainly by photosynthesis and an estimated redirection of the 28% of electrons from photosynthetic activity was achieved by the iron limited conditions. In addition, ferricyanide reduction in the dark by iron limited cultures also presented a significant improvement, with a 6-fold increase in comparison to iron sufficient cultures. Synechococcus elongatus PCC7942 ferricyanide reduction rates are unprecedented for cyanobacteria and they are comparable to those of microalgae. The redox activity of biofilms directly on ITO-coated glass, in the absence of any artificial mediator, was also enhanced under the iron limited conditions, implying that iron limitation increased exoelectrogenesis at the outer membrane level. Cyclic voltammetry of Synechococcus elongatus PCC7942 biofilms on ITO-coated glass showed a midpoint potential around 0.22 V vs. Ag/ AgCl and iron limited biofilms had the capability to sustain currents in a saturated-like fashion. The present work proposes an iron related exoelectrogenic capacity of Synechococcus elongatus PCC7942 and sets a starting point for the study of this strain in order to improve photo-bioelectricity and darkbioelectricity generation by cyanobacteria, including more sustainable mediatorless systems.

The Development of Biophotovoltaic Systems for Power Generation and Biological Analysis

“…Growth conditions can influence exoelectrogenic activity. For example, iron limitation was reported to lead to an increase in exoelectrogenic activity in Synechococcus elongatus PCC7942 . Cells can be grown in suspension (sometimes called planktonic mode) or in layers on a surface such as an electrode.…”

“…For example, iron limitation was reported to lead to an increase in exoelectrogenic activity in Synechococcus elongatus PCC7942. [43] Cells can be grown in suspension (sometimes called planktonic mode) or in layers on a surface such as an electrode. Although these are often referred to as 'biofilms', they do not necessarily show the complex structure of many microbial biofilms, such as those of pathogenic Pseudomonas strains.…”

Bioelectrochemical detection of cyanobacteria for harmful bloom management

“…EET has been correlated with the production of organic exudates that contain reducing moieties for Fe(III) by Microcystis aeruginosa, possibly to facilitate iron uptake (Garg, Wang and Waite, 2017;Wang, Garg and Waite, 2017b). In fact, cyanobacteria Synechococcus elongatus PCC942 shows enhanced capability to reduce ferricyanide, a widely-used soluble redox 26 | mediator, when grown under iron-limiting conditions in comparison to iron sufficient conditions (Gonzalez-Aravena et al, 2018). Ferricyanide is reduced by ferric reductases in the plasma membrane which are overexpressed in iron-limited growth and operate optimally at an acidic pH (Nodop et al, 2008).…”

“…As mentioned above, the core idea of developing a filter-electrode was to concentrate the The use of soluble mediators such as ferricyanide is adequate for facilitating electrogenisis in poor exoelectrogens such as cyanobacteria (Gonzalez-Aravena et al, 2018), and therefore a reasonable choice for proof of concept of the filter-electrode principle. However, the use of soluble mediators would be unsustainable for an in situ detection system as these would be washed out with each measurement.…”

“…Ferricyanide is reduced by ferric reductases in the plasma membrane which are overexpressed in iron-limited growth and operate optimally at an acidic pH (Nodop et al, 2008). Therefore, ferricyanide reduction and consequent current generation decrease at higher pH (Gonzalez-Aravena et al, 2018).…”