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Abstract

A mechanism by which Cyanobacteria can avoid encrustation by iron oxides is subject of this study. Such a crust can lead to cell death, by hampering the uptake of nutrients, the excretion of waste products and shielding phototrophic bacteria from light. Cyanobacteria biofllms that have been growing in illuminated bioreactors at the Aspo Hard Rock Laboratory (Sweden), fed with either iron-rich or iron-poor ground water, were studied. Biofilms growing at low F2e concentrations (1 pM) were highly encrusted in Fe-oxides and low in biomass. Remarkably, biofilms growing at high F2e concentrations (30 pM) were not encrusted and attained high biomass. Oxygen profiles were measured in-situ. The cyanobacteria from the high F2e environment showed much higher photosynthesis rates than those from the low F2e environment. Measurements under controlled laboratory conditions showed increasing photosynthesis rates with increasing F2e concentrations (up to 50 pM) for the biofilms originating from a high F2e reactor. The biofilms from the low F2e reactor decreased their photosynthesis under increasing F2e levels. This suggests that the biofllms growing under high F2e levels developed a protective mechanism avoiding encrustation: In response to high F2e the cells enhance their photosynthesis rates to create a microenvironment conducive for Fe-oxidation (high pH and 02). It is well possible that a high photosynthetic rate is a selective force for phototrophs living in high F2e environments. These cyanobacteria can modulate their photosynthesis rates to produce an 02 and/or pH barrier against iron precipitation on the cell. In this case the F2e oxidizes in some distance from the biofilm and can no longer adsorb onto the cell surface.