Current Perspectives and Research on Lacustrine Whiting Events and Microbialites (Invited)

The role of bacterial carbonate precipitation has been demonstrated by light and electron microscopy, calcification experiments, and carbon isotopic studies in recent years. Most bacterial carbonate precipitation is thought to occur by either cyanobacteria or sulfate-reducing bacteria (SRB). However, it is likely that many other groups of bacteria can also cause the precipitation of carbonate minerals. Our work focuses on unicellular coccoid cyanobacteria and their role in form whiting events and microbialites in Fayetteville Green Lake, NY, predominantly. Electron microscopic data illustrates that the carbonate minerals precipitate on the cell surface and in the microenvironment surrounding the cell or microcolony of cells. Bacterial calcification experiments have shown the development of carbonate minerals is related to bacterial alkali production, which increases the pH of their microenvironment. In the case of unicellular cyanobacteria it is the production of OH- ions due to a HCO3-/OH- exchange across the cell envelope related to their inorganic carbon uptake for organic biosyntheses. In the case of Synechococcus sp. it occurs on a protein surface layer or S-layer. Carbon isotopic studies show that the carbonate minerals are highly enriched in 13C, by 3-4 ‰, relative to the δ13CDIC value of the aqueous milieu. The isotopic enrichment occurs in both whiting and microbialite carbonate minerals. In addition, our research in various other lakes have shown that modern thrombolites are dominated by unicellular cyanobacteria while stromatolites are dominated by filamentous cyanobacteria and that modern stromatolites are not as heavily calcified as the thrombolites. It may be the thrombolite calcification is caused by unicellular cyanobacterial calcification whereas; stromatolite calcification is mostly influenced by SRB that are consuming the primary organic matter produce by the filamentous cyanobacteria at depth in the stromatolite. All of our studies in many environments show that coccoid cyanobacteria tend to active calcify whereas filamentous cyanobacteria usually do not. It may be related to a surface-area to volume problem and their different microenvironments. Bacterial carbonate precipitation may result in the formation of vast carbonate mud (e.g. whitings) and large areas covered by microbialites (e.g. thrombolites and stromatolites) in marine and freshwater environments over the past 3.5 Ga.