Battling Iron- a specialised approach

Iron is an essential element to almost all organisms, particularly those containing iron-rich cytochromes and iron-sulfur proteins which are essential for photosynthesis and nitrogen fixation. Fe(III) has very low solubility resulting in very low iron bioavailability. Many organisms have developed iron reducing mechanisms to overcome iron limitation: alagal species such as Chlorella vulgaris, plants and fungi use ferric reductase proteins within cell membranes for FE(III) reduction before internalisation whereas, microorganisms including cyanobacteria have developed siderophores- FE(III) binding chelating agents which prevent oxidation and transport into the cell. There are however, exceptions: the cyanobacteria Synechocystis lacks the genes for siderophore production and has been observed reducing Fe(III) extracellularly before transportation of FE(II) into the plasma where Fe(II) and Fe(III) transport systems (FutABC and FeoB) have been identified.  

Relatively little is known about iron reduction involving ferricyanide and its limiting factors and much that is, e.g. effects of light, is up for dispute. Previous studies have investigated bio-applications which make use of the electrochemical cell as a fuel source e.g. interactions of Synechocystic sp. 6803 with ferricyanide to produce hydrogen however, these have been largely abandoned due to unsustainability. Despite this, ferricyanide is still an important probe for cell membrane activity due to the strong binding of its iron atom which is not released during reduction to ferrocyanide.

This study investigated the ability of Synechocystis sp. PCC 6803 to reduce Fe(III) in ferricyanide with varied concentrations of ferric cyanide and cells present. Using electrochemical techniques: rotating disk electrochemistry (RDE), a technique using varied electrode rotation speeds to pull the solution towards the planar disk electrode giving laminar flow across the electrode; and chronoamperometry at a static macromolecule.

Typical measurements using a static electrode showed light induced a small reduction in current compared to dark conditions for cultures with both cells and ferricyanide present. Control media and solutions with cells but no ferrocyanide had a very low amplitude while a control with no cells and 1mM ferrocyanide was slightly higher (~1 µA). To prevent discrepancies in the data, variable concentration experiments were carried out in the dark and control measurements were taken during all experiments and subtracted from the data. Increased rate of ferricyanide reduction per cell with ferricyanide increase and constant rate of ferricyanide reduction per cell with constant ferricyanide and cell concentration manipulations implies Synechocystis cells reduce ferricyanide at a constant rate while the mass transport of ferricyanide to the cells is the limiting factor of reduction.

I thought this paper was an interesting read finding light causes ~10-20% difference in current and Synechocystis Fe(III) reduction is mainly limited by Fe concentration, which will be of high importance to anyone investigating Fe(III) reduction or Synechocystis. Unfortunately, much of their static electrode experiment findings are difficult to accept as more than trends due to the chance cells settled on the electrode, it would be interesting to repeat these with the mentioned dialysis membrane over the electrode to prevent cell settling. Overall, I thought this was a well composed study further developing our understanding of Synechocystis membrane activity and electrochemical techniques. 

Reference:
Thorne, R. J., Schneider, K., Hu, H., Cameron, P. J. (2015) Iron reduction by the cyanobacterium Synechocystis sp. PCC 6803. Bioelectrochemistry. 105: 103-109.