Iron Oxidation and Deposition in the Biofilm Covering Montacuta ferruginosa (Mollusca,Bivalvia)

The shell of the bivalve Montacuta ferruginosa is covered with a rust-colored biofilm. This biofilm includes filamentous bacteria and protozoa encrusted with a mineral, rich in ferric ion and phosphate. The aim of this research was to study two possible microbial iron precipitation pathways in the biofilm, namely, microbial iron oxidation and microbial degradation of organic Fe(III) complexes. The iron-oxidizing activity was assayed spectrophotometrically by monitoring the formation of the dye Wurster blue in biofilm extracts. Iron-oxidizing activity was effectively detected in extracts obtained by oxalic acid treatment of biofilm fragments. Extracts obtained without oxalic acid treatment, heated extracts, or extracts supplemented with HgCl 2 did not show any activity. This suggests that an iron-oxidizing factor (IOF), possibly an enzyme, coprecipitated with the mineral. Additional information gathered by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel-filtration chromatography, and UV spectrophotometry indicate that the IOF would be a small peptide or glycopeptide (1,350 Da). Microbial degradation of organic Fe(III) complexes was assayed with biofilm fragments incubated in a medium containing ferric citrate. Analysis of the supernatants after various intervals revealed that the complex was degraded by living microorganisms much faster than in the heat-killed negative controls. We conclude that ferric iron precipitation in the biofilm may proceed by way of microbial Fe(II) oxidation as well as microbial degradation of organic Fe(III) complexes.