Experimental Modeling of Bacterially-Induced Ca Carbonate Precipitation: New Insights on Possible Mechanisms

Abstract

The contribution of microorganisms, particularly bacteria, in carbonate mineral formation, the main natural processes controlling CO2 level in the atmosphere, has played an important role since the Archean Eon. In this study we review our recent experimental work on CaCO3 precipitation induced by two anoxygenic phototrophic bacteria (APB), Rhodovulum steppense A-20sT and Rhodovulum sp. S-17-65, and by cyanobacteria Gloeocapsa sp. f-6gl. These bacteria are representatives of two important groups of photosynthetic organisms present at the Earth surface both in the past and at the present times. The mechanisms of organomineralization deriving from APB and cyanobacteria activities are drastically different and relate to the main physical and chemical processes controlling CaCO3 precipitation from aqueous solution, essentially local supersaturation with respect to carbonates induced by photosynthesis outside the living cells and Ca adsorption onto cell surface and the associated extracellular polymeric substances (EPS).The APB can physiologically control their surface potential to electrostatically attract nutrients at alkaline pH, while rejecting Ca ions to prevent Ca adsorption and subsequent CaCO3 precipitation in the vicinity of cell surface and thus, cell incrustation. In contrast to other previously-investigated calcifying bacteria, no cellular protection mechanism against Ca2+ adsorption and subsequent carbonate precipitation has been evidenced for cyanobacteria Gloeocapsa sp. f-6gl. This is most likely linked to the peculiar cellular organization of this species that involves several cells clustered in one single capsule. In this regard, colony-forming, EPS-rich, capsular cyanobacteria may be among the most efficient calcifying microorganisms all along the Earth history and can be of interest for various technological applications involving carbonation.