Cyanobacterial viability during hydrothermal biomineralisation

The cyanobacterium Calothrix sp., an isolate from the Krisuvik hot spring, Iceland, was mineralised in both silica and iron-silica solutions. After I? days of incubation, many filaments in the silica solution developed extens ive mineral crusts up to 5 mu m thick. Mineralisation of filaments in the F e-Si solution was more rapid; in 12 days, the entire colony became totally encased within a mineralised matrix. Examination by transmission electron m icroscopy (TEM) revealed mineralisation of intact cells only occurred upon the extracellular sheath; no intracellular mineralisation was observed. Add itionally, mineralisation was predominantly restricted to the sheath's oute r surface. Analysis of the mineralised bacteria by autofluorescence reveale d the mineralised cells were intact and therefore likely viable. The viabil ity of these cells was confirmed by oxygen electrode analysis, which showed that the mineralised colonies were photosynthetically active. Moreover, th e mineralised colonies exhibited comparable rates of photosynthesis to the non-mineralised colonies, suggesting mineralisation was not notably detrime ntal. It is thus proposed that mineralisation can occur on living microorga nisms, providing it is restricted to extracellular material such as the she ath. We further suggest that the sheath may be necessary in enabling some m icroorganisms to survive mineralisation, by both acting as an alternative m ineral nucleation site (preventing cell wall and cytoplasmic mineralisation ) and by providing a filter against colloidal silica. (C) 2000 Elsevier Sci ence B.V. All rights reserved.