Direct Fe(III) Reduction from Synthetic Ferrihydrite by Haloalkaliphilic Lithotrophic Sulfidogens

Ability to reduce insoluble Fe(III) compounds has not been shown for alkaliphilic lithotrophic sulfate and sulfur reducers. Detection of this metabolic process in sulfidogenic prokaryotes could significantly expand the present knowledge on physicochemical range of their growth and physiological activity, which is now limited by low negative ambient redox potential. Capacity for direct reduction of Fe(III) from chemically synthesized ferrihydrite was tested for eight species of hydrogenotrophic haloalkaliphilic sulfidogens grown with formate or H2 as electron donors in the absence of sulfur compounds in the medium. Out of eight tested species, six reduced iron with formate and five, with hydrogen as the electron donor. Iron reduction correlated with stimulation of growth on formate or hydrogen only in two sulfidogenic species. Analysis of available genomes of five tested species revealed that only Dethiobacter alkaliphilus and Desulfuribacillus alkaliarsenatis possess the gene sets of multiheme cytochromes c required for typical dissimilatory iron reduction. The presence of these genes in two strains with high iron-reducing activity indicates the capacity of some haloalkaliphilic sulfidogenic bacteria for carrying out direct dissimilatory reduction of insoluble Fe(III) forms in the absence of sulfur-containing electron acceptors, i.e., without using sulfide as a soluble mediator of iron reduction. In other studied microorganisms, the ability to reduce iron is probably caused by nonspecific metabolic activity and is not directly linked to energy generation for growth, although the rates of Fe(III) reduction determined in our experiments make it possible to suggest significant role of sulfidogenic microorganisms (normally reducing sulfur and sulfate) in the iron cycle in haloalkaline ecosystems upon decreased content of sulfur compounds.