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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.