The cyanobacterium Calothrix sp., an isolate from the Krisuvik hot spring, Iceland, was mineralised in both silica and iron-silica solutions. After 12 days of incubation, many filaments in the silica solution developed extensive mineral crusts up to 5 μm thick. Mineralisation of filaments in the Fe-Si solution was more rapid; in 12 days, the entire colony became totally encased within a mineralised matrix. Examination by transmission electron microscopy (TEM) revealed mineralisation of intact cells only occurred upon the extracellular sheath; no intracellular mineralisation was observed. Additionally, mineralisation was predominantly restricted to the sheath's outer surface. Analysis of the mineralised bacteria by autofluorescence revealed the mineralised cells were intact and therefore likely viable. The viability of these cells was confirmed by oxygen electrode analysis, which showed that the mineralised colonies were photosynthetically active. Moreover, the mineralised colonies exhibited comparable rates of photosynthesis to the non-mineralised colonies, suggesting mineralisation was not notably detrimental. It is thus proposed that mineralisation can occur on living microorganisms, providing it is restricted to extracellular material such as the sheath. We further suggest that the sheath may be necessary in enabling some microorganisms to survive mineralisation, by both acting as an alternative mineral nucleation site (preventing cell wall and cytoplasmic mineralisation) and by providing a filter against colloidal silica. (C) 2000 Elsevier Science B.V. All rights reserved.