Microbially-induced mineralization is considered as one of the main natural processes controlling CO2 levels in the atmosphere and a major structural and ecological player, in the modern and in the past ecosystems. In this study are presented the data of laboratory experimental work on CaCO3 precipitation with pure cultures of two anoxygenic phototrophs bacteria (APB): haloalcaliphilic Rhodovulum steppens A-20s and neutrophilic halophilic Rhodovulum sp. S-17-65; and cyanobacteria Gloeocapsa sp.. These bacteria represent two important groups of photosynthetic organisms in the past and at present time. APB is the oldest microorganism which could be dominant during the anoxygenic period of Earth's life (approximately 4 billon years ago) whereas the origin of oxygen evolving microorganisms (cyanobacteria) is placed at about 3.5 billion years ago as based on oxidation records of the Earth's crust. In modern ecosystems, cyanobacteria are the dominant primary producers. Nonetheless, the potential of APB are abundant in the modern microbial mats and stromatolites and thus may represent a considerable fraction of the standing biomass. However, biomineralization induce by these bacteria has not been thoroughly studied up to now. In this context, the aim of this thesis is to characterize the process of biological CaCO3 precipitation and to assess the existence of metabolic processes protecting studied bacteria against carbonate mineralization on their surfaces. For this, kinetic experiments, SEM and TEM imaging, EDX and XRD analyses, zeta-potential measurements and Ca adsorption into bacterial surface were carried out. The result of this study demonstrates the participation of studied bacteria in CaCO3 precipitation. Zeta-potential measurements suggest the existence of a cells protection mechanism for studied APB, based on the metabolic maintenance of a positive surface charge at alkaline pH, preserving active bacteria against Ca2+ adsorption and subsequent carbonate precipitation on their surfaces. The existence of the same mechanism for Gloeocapsa sp. was not confirmed. Overall, the results of this study show two different mechanisms of CaCO3-nucleation: an unspecific supersaturation by APB and a specific nucleation at the cell wall by cyanobacteria Gloeocapsa sp..
