Photosynthetic Microorganism-Mediated Synthesis of Akaganeite (β-FeOOH) Nanorods

Abstract

Common Anabaena and Calothrix cyanobacteria and Klebsormidium green algae are shown to form intracellularly akaganeite beta-FeOOH nanorods of well-controlled size and unusual morphology at room temperature. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy X-ray energy dispersive spectrometry (SEM-EDS) analyses are used to investigate particle structure, size, and morphology. A mechanism involving iron-siderophore complex formation is proposed and compared with iron biomineralization in magnetotactic bacteria.

Iron minerals within specific microfossil morphospecies of the 1.88 Ga Gunflint Formation

“…Hence, these microfossils either concentrated iron in vivo through intracellular biomineralization, or in their decaying cytoplasm. In recent microorganisms, in vivo internal Fe biomineralization can be demonstrated when cytoplasmic ultrastructures are preserved40, when minerals occur within intracytoplasmic vesicles464748 or when there is a persistence or increase of metabolism despite systematic Fe mineralization4749. Fe mineralization may alternatively be explained as post mortem in recent DNA-free cells5 that show extensive mineral replacement of cytoplasmic ultrastructures together with cell wall mineralization5051.…”

“…In vivo , microorganisms sometimes favour intracellular (in cytoplasm) over epicellular (on cell wall) or extracellular Fe biomineralization4047496667. Although extra- and epicellular Fe-silicate biomineralization is known68, intracellular (that is, in vivo ) greenalite precipitation is not.…”

“…Recrystallization through Fe 3+ reduction is consistent with the Fe isotope record of Gunflint stromatolites14 (Supplementary Discussion). Initial biominerals could have been Fe 3+ -oxyhydroxides or -phosphates (for example, cyanobacteria4049, euglena algae47), Fe 2+ –Fe 3+ -oxides or -sulfides (for example, magnetotactic bacteria46) or Fe 2+ –Fe 3+ -phosphates (Fe-reducing bacteria67). …”

“…The intracellular mineralization occurs after complete external encrustation in the nitrate-reducing bacteria5, contrasting with the absence of Fe minerals near the cell walls of the microfossils. The only microorganisms known to produce intracellular Fe minerals and possible counterparts for the observed microfossils are: magnetotactic bacteria46, dissimilatory iron-reducing bacteria67, Fe-polyphosphate-accumulating bacteria69 and oxygenic phototrophs including cyanobacteria4049 and eukaryotic algae4766. Akaganeite (Fe 3+ OOH) crystals formed intracellularly by living cyanobacteria have sizes and spatial distributions comparable to those of greenalite and siderite in Gunflint fossils, and may accordingly have been their precursors49.…”

“…In their shallow-water environment, the Gunflint microfossils we interpret as cyanobacteria could have performed oxygenic photosynthesis during the day. In these conditions, Fe oxidation by O 2 likely triggered iron mineralization, although the processes leading to intracellular biomineralization remain obscure4049.…”