Biomineralization by Gallionella

Abstract

A new environmental scanning electron microscopic (ESEM) technique at low vacuum (5 torr) and 99% humidity, where the sample never has been exposed to high vacuum and coating of carbon or gold, has revealed a new insight into the nature of iron mineralization that develops in association with the stalked bacteria Gallionella. The stalk fibers contain minute flaky iron precipitates. The size of the crystallites is 0.1–0.5 micron and some of them exhibit a hexagonal feature. EDAX analyses on individual crystallites give an atomic ratio between Fe and O very close to 0.67. The stoichiometric formula would thus be Fe₂O₃. Stoichiometry and crystallinity are in accordance with the mineral hematite. The mineralization seems to take place inside the fibers of the stalk. With time the Gallionella stalk is covered with iron oxihydroxides of different kinds that probably are controlled by inorganic processes more than by the organic chemistry of the stalk. From a thermodynamic point of view, oxygen as well as carbon dioxide are required to explain the formation of hematite inside the fibers. The precipitation takes probably place at a pH close to 5.

Keywords:

• biomineralization
• ESEM
• ferrihydrate
• gallionella
• hematite

Lotta Hallbeck provided valuable advice on the taxonomy of Gallionella. We thank Olle Wahlberg for valuable contribution to the discussion of thermodynamics. Karsten Pedersen supplied the samples from Äspö, and the ESEM microscope was funded by the Alice and Knut Wallenberg foundation.