An early-branching microbialite cyanobacterium forms intracellular carbonates

Estelle Couradeau, institut de minéralogie et de physique des milieux condensés (IMPMC) et laboratoire écologie, systématique et évolution à Orsay

mardi 12 juin 2012 à 10 heures, salle à préciser

The evolution of Earth environments has been deeply linked with that of cyanobacteria for billions of years. Cyanobacteria have impacted major geochemical cycles (C, N & O) through Earth history. They have played a major role in the carbon cycle by converting CO₂ into organic carbon and carbonates and by enriching the atmosphere in oxygen. They have been looked for in the geological record in the form of fossil encrusted cells based on the assumption that cyanobacterial calcification is always an extracellular process. Here, we report the discovery of a cyanobacterium found in microbialites from Lake Alchichica (Mexico) [1,2] that forms intracellular carbonate phase inclusions, revealing an unexplored pathway for calcification. Electron diffraction shows that these phases are amorphous although XANES nanospectroscopy reveals local ordering consistent with the structure of benstonite, a Mg-Ca-Sr-Ba carbonate. Phylogenetic analyses place this cyanobacterium within the deeply divergent order Gloeobacterales. Accordingly, we tentatively name it Candidatus Gloeomargarita lithophora. This discovery opens the possibility that ancestral calcifying cyanobacteria may have biomineralized carbonates intracellularly, and thus are not prone to encrustation in extracellular precipitates. This lack of encrustation provides an alternative explanation for the absence of cyanobacterial microfossils in the oldest fossil stromatolites and opens questions about the evolution of calcification in cyanobacteria.

[1] Couradeau et al., (2011) PLoS ONE 6(12): e28767

[2] Couradeau et al.(2012) Science Vol. 336 no. 6080 pp. 459-462