Carbonate Mineral Formation by Microalgae: Precipitation Potential and Morphological Analysis

This study evaluated the ability of microalgae to produce carbonate minerals through CO2 uptake, in comparison with abiotic, direct chemical synthesis through CO2 absorption. A freshwater microalga (Synechococcus elongatus) isolated from garden soil in East Anglia, UK, was cultivated under laboratory conditions with CO2 injection to generate a bicarbonate-rich aqueous solution, in which Fe2+, Mg2+, and Ca2+ ions were added to facilitate carbonate formation. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analyses revealed distinct morphologies and mineral types. The algae-based process precipitated calcite, siderite, magnesite, and dolomite, whereas the abiotic process yielded, respectively, calcite, siderite, high-Mg calcite and nesquehonite. Biogenic minerals were finer and more morphologically diverse than their abiotically formed counterparts. The results indicated that microalgae produced 0.21 mol/L of calcium carbonate, compared to 0.51 mol/L obtained through abiotic CO2 sequestration, and a comparable yield of about 0.25 mol/L reported for Sporosarcina pasteurii-induced precipitation. Acid resistance tests showed that algae-induced minerals had similar or improved resistance to acidic conditions compared to minerals formed through abiotic CO2 consumption. Overall, despite slower kinetics, algae-induced carbonate precipitation offers advantages for soil stabilization by biocementation in the context of environmental sustainability, climate change mitigation and circular economy.