show Abstracthide AbstractAchromatium is an ecologically-important bacterium with no cultivated representatives. Their large size (10-100 um) allows them to dominate in microenvironments between the sediment-water interface. >50% of the cytoplasm may be filled with calcite inclusions with yet unknown purposes. Genomic sequencing can reveal their metabolic capacities, giving clues for laboratory culturing and may yield insights into the genetic machinery behind intracellular calcite precipitation in prokaryotes.