The Unique Role of Intracellular Calcification in the Genus Achromatium

Abstract

Achromatium are found in freshwater and brackish sediments, where, as giant sulfur oxidisers, they play a key role in the carbon and sulfur cycles of the sediments they inhabit. The most striking feature of this genus is its enigmatic precipitation of intracellular calcite. Past explanations for this process have included the dissolution of stored calcite to regulate acidity generated by H2S oxidation, the use of calcite as a buoyancy-regulating mechanism, the use of calcite as an electron acceptor source in “carbonate respiration”, and the use of calcification to generate CO2 for carbon fixation. However, more recent in situ physiological studies and detailed characterisation of the environments inhabited by these organisms have indicated a possible role for intracellular calcification in the dissolution of sulfide minerals. It is proposed that this unique adaptation of Achromatium is a means of overcoming a challenge not faced by other giant sulfur bacteria, namely inherently low levels of free sulfide in their sedimentary environment.