The impact of cyanobacteria Gloeocapsa sp. on calcium carbonate precipitation has been examined by combining physico-chemical macroscopic and in-situ microscopic techniques. For this, Ca adsorption and assimilation and kinetic experiments were used to assess the existence of the metabolic process responsible for CaCO3 mineralization by Gloeocapsa sp. Experimental products were characterized by Scanning and Transmission Electron Microscopy (SEM and TEM) imaging, XRD analyses, coupled with Confocal Laser Scanning Microscopy (CLSM) and Raman micro-spectroscopy. Ca carbonate precipitation experiments were performed at an initial pH of 7.8 to 9.4 and 25 °C in supersaturated solutions (Ωcalcite = 1.5 to 150) in the presence of active cyanobacterial cells. During cyanobacterial photosynthesis, the solution pH increased up to 9.5-10.8 after the first 5-10 days of growth, the Ca concentration decreased and the supersaturation index attained a maximum followed by a gradual decrease due to progressive CaCO3 precipitation. Ca adsorption at the surface of live and inactivated Gloeocapsa sp. cells and Ca intracellular assimilation during cell growth were measured as a function of pH and Ca concentration in solution. The contribution of surface adsorption and intracellular uptake to total Ca removal from solution due to biocalcification does not exceed 10%. The presence of calcium carbonate, identified as calcite using Raman spectroscopy, on active Gloeocapsa sp. surfaces and in the vicinity of bacterial cell surfaces was evidenced using SEM. TEM and CLSM demonstrated cyanobacterial cell encrustation by CaCO3 precipitated in the form of nano-spheres adjacent to the cell surface. In contrast to other previously investigated calcifying bacteria, no cellular protection mechanism against Ca2 + adsorption and subsequent carbonate precipitation has been demonstrated for Gloeocapsa sp. This is most likely linked to the specific cellular organization of this species, which involves several cells in one single capsule. As such, planktonic cultures of Gloeocapsa sp. exhibit significant calcifying potential, making them important CO2-fixing microorganisms for both paleo-environmental reconstructions and technological applications.