NUCLEATION OF CELESTITE AND STRONTIANITE ON A CYANOBACTERIAL S-LAYER

Synechococcus strain GL24 is a unicellular cyanobacterium that was iso lated from Fayetteville Green Lake, New York, a meromictic lake which has high Ca2+ and SO42- concentrations. Epicellular mineralization of Synechococcus cells in the lake is the mechanism by which extensive ca lcitic bioherms (or microbial reefs) have been formed on the lake's sh ore and a marl sediment has been built on the lake bottom. Previous st udies have shown that calcium carbonate (calcite) formation on the Syn echococcus surface is dependent upon an alkaline pH, which is produced in the cellular microenvironment by the cells as their activity incre ases with seasonal warming of the lake water. At the circumneutral pH of bulk lake water, calcium sulfate (gypsum) is formed. In this study, we show that Synechococcus mediates a similar sulfate-to-carbonate tr ansformation when Sr2+ is the major divalent cation present, forming c elestite and strontianite. In experimental systems to which equimolar amounts of Ca2+ and Sr2+, Ca2+ or Sr2+ and Mg2+, or all three ions tog ether were added to artificial lake water, Ca2+ and Sr2+ were incorpor ated equally into mineral formation to form CaSr(CO3)(2). No Mg2+-cont aining carbonates were formed when either or both of the other two ion s were present. Mineral formation takes place on a hexagonally arrange d proteinaceous template (an S-layer) which forms the outermost surfac e of the Synechococcus cell. Our results provide evidence that the S-l ayer exhibits selectivity with respect to the ions bound and subsequen tly incorporated into carbonate minerals and that celestite and stront ianite, previously thought to be purely evaporitic minerals, can be bi ogenically formed.