AUTHOR=Ionescu Danny , Buchmann Bettina , Heim Christine , Häusler Stefan , de Beer Dirk , Polerecky Lubos TITLE=Oxygenic photosynthesis as a protection mechanism for cyanobacteria against iron-encrustation in environments with high Fe2+ concentrations JOURNAL=Frontiers in Microbiology VOLUME=5 YEAR=2014 URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2014.00459 DOI=10.3389/fmicb.2014.00459 ISSN=1664-302X ABSTRACT=

If O₂ is available at circumneutral pH, Fe²⁺ is rapidly oxidized to Fe³⁺, which precipitates as FeO(OH). Neutrophilic iron oxidizing bacteria have evolved mechanisms to prevent self-encrustation in iron. Hitherto, no mechanism has been proposed for cyanobacteria from Fe²⁺-rich environments; these produce O₂ but are seldom found encrusted in iron. We used two sets of illuminated reactors connected to two groundwater aquifers with different Fe²⁺ concentrations (0.9 μM vs. 26 μM) in the Äspö Hard Rock Laboratory (HRL), Sweden. Cyanobacterial biofilms developed in all reactors and were phylogenetically different between the reactors. Unexpectedly, cyanobacteria growing in the Fe²⁺-poor reactors were encrusted in iron, whereas those in the Fe²⁺-rich reactors were not. In-situ microsensor measurements showed that O₂ concentrations and pH near the surface of the cyanobacterial biofilms from the Fe²⁺-rich reactors were much higher than in the overlying water. This was not the case for the biofilms growing at low Fe²⁺ concentrations. Measurements with enrichment cultures showed that cyanobacteria from the Fe²⁺-rich environment increased their photosynthesis with increasing Fe²⁺ concentrations, whereas those from the low Fe²⁺ environment were inhibited at Fe²⁺ > 5 μM. Modeling based on in-situ O₂ and pH profiles showed that cyanobacteria from the Fe²⁺-rich reactor were not exposed to significant Fe²⁺ concentrations. We propose that, due to limited mass transfer, high photosynthetic activity in Fe²⁺-rich environments forms a protective zone where Fe²⁺ precipitates abiotically at a non-lethal distance from the cyanobacteria. This mechanism sheds new light on the possible role of cyanobacteria in precipitation of banded iron formations.