Abstract

It is the intention of this study to elucidate the nested formation of calcium carbonate polymorphs or polyamorphs in the different nanosized compartments. With these observations, it can be concluded how the bacteria can survive in a harsh environment with high calcium carbonate supersaturation. The mechanisms of calcium carbonate precipitation at the surface membrane and at the underlying cell wall membrane of the thermophilic soil bacterium Geobacillus stearothermophilus DSM 13240 have been revealed by high-resolution transmission electron microscopy and atomic force microscopy. In this Gram-positive bacterium, nanopores in the surface layer (S-layer) and in the supporting cell wall polymers are nucleation sites for metastable calcium carbonate polymorphs and polyamorphs. In order to observe the different metastable forms, various reaction times and a low reaction temperature (4 °C) have been chosen. Calcium carbonate polymorphs nucleate in the confinement of nanosized pores (⌀ 3–5 nm) of the S-layer. The hydrous crystalline calcium carbonate (ikaite) is formed initially with [110] as the favored growth direction. It transforms into the anhydrous metastable vaterite by a solid-state transition. In a following reaction step, calcite is precipitated, caused by dissolution of vaterite in the aqueous solution. In the larger pores of the cell wall (⌀ 20–50 nm), hydrated amorphous calcium carbonate is grown, which transforms into metastable monohydrocalcite, aragonite, or calcite. Due to the sequence of reaction steps via various metastable phases, the bacteria gain time for chipping the partially mineralized S-layer, and forming a fresh S-layer (characteristic growth time about 20 min). Thus, the bacteria can survive in solutions with high calcium carbonate supersaturation under the conditions of forced biomineralization.

Highlights

  • The thermophilic soil Gram-positive bacterium Geobacillus stearothermophilus DSM 13240 (G. stearothermophilus) can be classified into the archaebacteria, as one of the first forms of life on earth

  • The cell wall of Gram-positive bacteria composed of a thicker peptidoglycan layer and often covered with a two-dimensional protein arrangement, the socalled surface layer (S-layer), can act as an effective place for nucleation and growth of minerals known as biologically induced mineralization.[2−4] positively charged metal ions can be precipitated at such structures.[3,5,6]

  • The nonstained cell (Figure 2a,b) displays regions where the bacterial membrane (S-layer linked to the underlying PG-layer of the cell wall) is partially exfoliated from the cell body

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Summary

Introduction

The thermophilic soil Gram-positive bacterium Geobacillus stearothermophilus DSM 13240 (G. stearothermophilus) can be classified into the archaebacteria, as one of the first forms of life on earth. The growth of calcium carbonate polymorphs at the cell membrane of G. stearothermophilus provides the option to study processes of forced biomineralization.[1] The cell wall of Gram-positive bacteria composed of a thicker peptidoglycan layer and often covered with a two-dimensional protein arrangement, the socalled S-layer, can act as an effective place for nucleation and growth of minerals known as biologically induced mineralization.[2−4] positively charged metal ions can be precipitated at such structures.[3,5,6] Recently, special processes of forced biomineralization have been studied in more detail, when a high concentration of metallic ions leads to the development of diverse biomineralized structures contributing to the survival of extremophiles.[1] Forced biomineralization is focused on the following phenomena:

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