Bartholomeus E.M., Schaub and Lucas J. Stal.
Laboratory for Microbiology, University of Amsterdam, Nieuwe Achtergracht 127, NL-1018 WS Amsterdam, The Netherlands
The possibility of ferric iron reduction of the mat building cyanobacterium Microcoleus chthonoplastes was investigated under dark anoxic conditions. Under these conditions the organism switches to fermentative metabolism during which endogenous storage carbohydrate is degraded to CO2, H2, formate, acetate and ethanol as major fermentation products. Fermentation in the presence of elemental sulfur (S°) resulted in the cessation of H2 and a lower production of formate. The amount of H2 and formate produced was less than expected on the basis of redox-balance calculations. It was found that iron was reduced in cultures and therefore we assumed that this element could serve as electron acceptor during dark anoxic conditions. At physiological pH (7.5) formate or H2 were not capable of reducing ferric iron abiotically. However iron reduction took place when cells were present. Iron reduction was enhanced when the cells were broken by French press treatment. This rate increased when formate (2 mM) was added to the suspension. On basis of our experiments it was concluded that ferric iron is able to serve as an electron acceptor during fermentation in cyanobacteria. This reaction may be important in the biogeochemical cycles of iron and sulfur in microbial mats. This work was supported by the European Union under contract EV5V-CT94-0411.
