We describe an efficient and environment-friendly approach to synthesizing porous calcium carbonate (CaCO3) microspheres via carbon dioxide (CO2) mineralization using naturally occurring microalgae as a bio-template. The CaCO3 microspheres showed a surface area of 39.1 m(2) g(-1) with pore diameters ranging between 12 and 30 nm. Exploiting its highly porous trait, this bio-mimetically manufactured particle was used as an inert support for silver nanoparticles (nAg). The characteristics of the composite Ag-embedded CaCO3 microspheres were examined by FE-SEM, TEM, XRD, BET surface area analysis, XPS and TG-DTA, confirming that the nAg particles were compatible with and uniformly distributed on the CaCO3 microspheres. This novel composite, when added in some commercial paints, showed a rather expected yet potent antimicrobial activity against model bacteria such as Escherichia coli, Psychrobacter alimenterius and Staphylococcus euroum. The minimum bactericidal concentration to kill 99% of bacteria was found to be 0.1 mg mL(-1). Moreover, this potency was not accompanied by any quality degradation of the paint color. In view of all this, it is believed that the proposed "green" way of making CaCO3 microspheres has great potential for commercialization.