Imaging Organic-Mineral Aggregates Formed by Fe(II)-Oxidizing Bacteria Using Helium Ion Microscopy

Helium ion microscopy (HIM) has been used to image the development of mineralized twisted stalks produced by a neutrophilic, microaerophilic Fe(II)-oxidizing bacteria of the class Zetaproteobacteria. HIM is a relatively new type of microscopy which has several advantages over conventional scanning electron microscopy (SEM) due to its higher spatial resolution and the fact that samples can be imaged without coating (e.g., with Pt/Au). Here, we use HIM to show the development of nanometer- and micrometer-sized twisted stalk features consisting of organic material and Fe(III) minerals which appear to be loosely bound to the bacterial cells. These appendages are thought to be essential for eliminating Fe(III) waste produced during Fe(II) oxidation by sorbing Fe(III) and transporting it away from the cell. The results show the initial formation of long, precipitate-free stalks. After just 2 days, these became encrusted with spiky lepidocrocite crystals, and by 1 month, the characteristic twisted shape was almost indiscernible. These results demonstrate the high quality of images which can be obtained with helium ion microscopy from organic and mineral structures produced by bacteria without the requirement to sputter coat samples with conductive metals and can thus be considered to be more representive of how these structures would exist in the environment.