Achromatium is the genus with the largest freshwater bacteria known to date. Single cells with a length of up to 125 μm  are visible even with the naked eye. The volume of an Achromatium cell exceeds that of a “normal” bacterium by a factor of 104–105 . Like other large sulfur-oxidising bacteria, such as Beggiatoa and Thiomargarita [3, 4], Achromatium cells contain small sulfur globules. In a recent study, we have shown that single Achromatium cells harbour multiple DNA spots showing a community-like genome diversity . Phenotypically most conspicuous and unique to Achromatium are numerous intracellular calcite bodies (CaCO3), which fill major parts of the cell volume . The biological role of these calcite bodies is under debate [7, 8].
Achromatium can be found within the oxic-anoxic transition zone in freshwater [2, 6, 9,10,11,12], brackish , and marine  sediments and may reach cell counts of 103–105 cells per cubic centimetre accounting for 90% of the bacterial biovolume in these layers [6, 14, 15]. The high abundance of Achromatium implies that the cells are either rapidly growing or not prone to predation. As Achromatium is uncultivated exact growth rates are unknown. Mortality factors, such as predation and parasitism, which might reduce natural population sizes of Achromatium, have neither been reported. However, for grazers Achromatium cells might be unattractive as food source due to the massive amounts of calcite. Thus, we assumed that the cells might escape predation by their unusual size and composition.
During a series of physiological experiments Achromatium cells were collected from sediment storages in glass jars and microscopically examined. In doing so, we repeatedly detected grazers that contained ingested Achromatium cells and took microphotographs of them. We present here a qualitative description that sheds new light on the ecological relationships of Achromatium with the sediment community.
For our study, sediment samples were taken from Lake Stechlin, an oligotrophic freshwater lake near Neuglobsow, Brandenburg, Germany (53° 9′ 5.59″ N; 13° 1′ 34.22″ E). The sediment samples were either immediately analysed or stored in glass jars at 15 °C with a diurnal 12 h/12 h light/dark cycle. Under these conditions, Achromatium cells stayed active over several months. Sediment material was collected from the upper layers (< 1 cm) of the glass jars and studied under an inverted microscope (Zeiss Diavert). Achromatium cells appear white in front of a black background (Fig. 1a) due to light reflection by the calcite bodies and sulfur globules (Fig. 1b), which allows to detect them in bulk sediment and even inside of grazers.
To estimate the natural abundance of the grazers in the sediment of Lake Stechlin, we took cores of fresh sediment. The cores were subsequently divided into oxic surface layer (0–3 mm), oxic-anoxic transition zone (6–10 mm) and anoxic layer (10–15 mm). The sediment samples were filtered through an 80-μm mesh to wash out small organisms (e.g. ciliates), the unsieved sediment was analysed for the presence of larger grazers.
Epifluorescence microscopy (Olympus BX51) and Sybr Green I were used to visualize mucous-associated prokaryotes. To study fungal infections, Achromatium cells were stained with Calcofluor White, a fluorescent dye used to stain fungal chitin. Pictures were taken with a Canon EOS 600D camera from live samples and further processed with PICOLAY .