Achromatium is a genus of unicellular sulfur bacteria, which appears in freshwater and brackish water sediments all over the world.
With a length of 15 µm to more than 100 µm and a width of up to 40 µm (Head et al. 1996) it belongs to the giant bacteria, like Beggiatoa or Thiomargarita namibiensis.
In 1903, the first species of Achromatium was described. Until now it is not possible to culture these bacteria in the laboratory.
Left: Achromatium cells collected manually by pipette in a small bowl (4 cm) from a water sample. Right: Manually collected Achromatium cells (40x)
The bacterium gains energy from oxidising sulfide to sulfate. In addition to that, it fixes inorganic carbon. Because of this metabolic pathway, the intracellular structure of Achromatium oxaliferum consists of elemental sulfur droplets. Another characteristic is the occurrence of large calcium carbonate crystals within the cell. Their function is under discussion till now. Maybe the carbonate crystals maximise the ratio of the cell surface to the volume of the cytoplasma, so the substrate uptake and the transport limitation can be reduced (Head et al. 2000). Another possibility is that they are responsible for the buffering of pH or to support a high pressure of CO2 to enable the inorganic carbon fixation (Head et al. 1996).
Left: Image stack of Achromatium cell in light microscopy. Right: Image stack of the same cell in fluorescence microscopy, stained with Sybr Green I.
By staining with DAPI or Sybr Green, epibionts on the surface of the Achromatium cell can be visualized. Also the DNA inside the cells can be shown. It is conspicuous, that the DNA is not distributed consistent in the whole cell, but concentrated in one part of it.
Current Objectives
Although the genus Achromatium was discovered more than one century ago, there are still a lot of questions to be answered. Our current work focuses on the DNA distribution inside the cell. First visualizations with Sybr Green I suggest specific regions and not a random distribution of DNA inside the cell. Furthermore, we try get an overview about the epibionts, before and after washing, by sequencing. This is a necessary step on the way to completely sequence achromatium.
Team
Heribert Cypionka
Hans-Peter Grossart
Sina Schorn
Links
Mikrobiologischer Garten
Literature
Head, I.M., Gray, N.D., Clarke, K.J., Pickup, R.W., Jones, J.G., The phylogenetic po-sition and ultrastructure of the uncultured Bacterium Achromatium oxaliferum. Micro-biology (1996) 142, 2341-2354
Head, IM., Gray, ND., Babenzien, H-D., Glöckner, FO., Uncultured giant sulfur bacteria of the genus Achromatium. FEMS Microbiology Ecology 33 (2000) 171-180
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