Cell Architecture of the Giant Sulfur Bacterium Achromatium oxaliferum: Extra-cytoplasmic Localization of Calcium Carbonate Bodies.

Sina Schorn,Danny Ionescu,Heribert Cypionka,Verena Salman-Carvalho,Hans-Peter Grossart,Sten Littmann

doi:10.1093/femsec/fiz200

Abstract

ABSTRACT Achromatium oxaliferum is a large sulfur bacterium easily recognized by large intracellular calcium carbonate bodies. Although these bodies often fill major parts of the cells’ volume, their role and specific intracellular location are unclear. In this study, we used various microscopy and staining techniques to identify the cell compartment harboring the calcium carbonate bodies. We observed that Achromatium cells often lost their calcium carbonate bodies, either naturally or induced by treatments with diluted acids, ethanol, sodium bicarbonate and UV radiation which did not visibly affect the overall shape and motility of the cells (except for UV radiation). The water-soluble fluorescent dye fluorescein easily diffused into empty cavities remaining after calcium carbonate loss. Membranes (stained with Nile Red) formed a network stretching throughout the cell and surrounding empty or filled calcium carbonate cavities. The cytoplasm (stained with FITC and SYBR Green for nucleic acids) appeared highly condensed and showed spots of dissolved Ca2+ (stained with Fura-2). From our observations, we conclude that the calcium carbonate bodies are located in the periplasm, in extra-cytoplasmic pockets of the cytoplasmic membrane and are thus kept separate from the cell's cytoplasm. This periplasmic localization of the carbonate bodies might explain their dynamic formation and release upon environmental changes.

Highlights

IntroductionBacteria of the genus Achromatium are found in oxic-anoxic transition zones of sediments worldwide, including freshwater (Schewiakoff 1893; Babenzien 1991; Head et al 1996; Glockner et al 1999; Gray et al 1999, Schorn and Cypionka 2018), brackish (Mansor et al 2015) and marine environments (Salman et al 2015)
While the occurrence of intracellular sulfur globules is common among large sulfur bacteria (Larkin and Henk 1996; Schulz et al 1999) the calcium carbonate bodies are a unique feature of the genus Achromatium
The slime could be washed off with NaHCO3 (50 mmol l−1), which removed the calcium carbonate bodies, but not the sulfur globules from the cells interior

Summary

Introduction

Bacteria of the genus Achromatium are found in oxic-anoxic transition zones of sediments worldwide, including freshwater (Schewiakoff 1893; Babenzien 1991; Head et al 1996; Glockner et al 1999; Gray et al 1999, Schorn and Cypionka 2018), brackish (Mansor et al 2015) and marine environments (Salman et al 2015) They belong to the large sulfur-oxidizing bacteria, and are recognized microscopically due to the calcium carbonate bodies deposited inside their cells, and smaller sulfur globules (Babenzien 1991; Head et al 2000b). This suggests that the cells have the ability to quickly adjust their cellular calcium carbonate content

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