A mineral that doesn’t stay put
Calcium carbonate exists in several forms. Some, called crystalline phases (such as calcite), are stable and common in nature. Others, such as amorphous calcium carbonate (ACC), are metastable: as they have no defined internal structure, they rapidly reorganize into a more stable crystalline form. As a result, ACC is rarely observed in nature, as it transforms almost immediately after formation.Yet cyanobacteria - micro-organisms capable of photosynthesis like plants, naturally present in aquatic environments and sometimes referred to as "blue-green algae" - are capable of producing and storing ACC inside their cells and, even more surprisingly, of maintaining it in an amorphous state for long periods. The mechanisms behind this remarkable stability remain poorly understood. How do cyanobacteria manage to stabilize a metastable phase - that’s the question addressed in this new study.
How cyanobacteria stabilize CCA
Using solid-state NMR spectroscopy, the team analyzed the internal structure of intracellular ACC granules present in cyanobacterial cells. They found that they are composed exclusively of hydrated ACC , with no trace of crystalline calcium carbonate.The researchers also discovered that each granule is surrounded by a thin protein envelope, while water molecules act as "bridges" between the ACC and this protein layer. "This specific organization limits the mobility of ions and water, slows down dehydration and prevents crystallization", explains Neha Mehta - Biogeochemistry and Modelling of the Earth System (BGeoSys), Faculty of Science -- lead author of the study.
In other words, the bacteria "lock" the mineral in its amorphous state, in much the same way as a sheet of paper is held flat by tape to prevent it from curling. The study thus provides the first molecular mechanism explaining how cyanobacteria stabilize ACC.
Understanding how living organisms form minerals opens up new perspectives in materials science and environmental research. These results could contribute to the development of bio-inspired mineral materials through more sustainable processes, or to the improvement of methods using bacteria to trap alkaline-earth elements or radionuclides in a stable mineral phase.