The seashells you pick up at the beach might not seem extraordinary, but they are a source of inspiration for researchers searching for efficient ways to store extra atmospheric carbon. Through a process called biomineralization, organisms like mollusks, clams, and corals crystallize excess carbon in their environment into hard calcium carbonate shells. Understanding on a molecular level the way that inorganic minerals interact with a framework of biological macromolecules is a critical step toward mimicking the process in artificial systems—and one that has proven challenging.
Now, an international team of materials researchers has demonstrated that these organic scaffolds influence the crystallization process by binding clusters of positively charged calcium ions, inducing mineral formation in specific locations. The results, published recently in Nature Materials, challenge previous assumptions about the molecular-level mechanisms responsible for biomineralization.
“This work is of great value in the realm of fundamental materials science—in particular in the world of living systems, where soft matter controls hard matter,” said Jim De Yoreo of Pacific Northwest National Laboratory.
De Yoreo and his colleagues used liquid-phase transmission electron microscopy (TEM)—a relatively new imaging technology that visualizes atomic-level activity in liquid samples—to monitor the crystallization process in real time at nanoscale resolution.
They first observed vaterite and a little bit of calcite, two different calcium carbonate crystal structures, forming in a solution in the TEM.
Then, they repeated the experiment with added polystyrene sulfonate (PSS), an organic polymer with negatively charged side chains that is structurally similar to the macromolecules that guide biomineralization in natural systems.
This time, the mineralization process looked different: Amorphous calcium carbonate (ACC) formed first, then later transformed into vaterite. ACC is a precursor to many biologically based minerals.
To understand how the PSS scaffold interfered with vaterite formation, the researchers mixed the calcium with the macromolecules without the carbonate. The macromolecules clumped together, absorbing the calcium ions to form globules. Once the carbonate was added, ACC crystals only formed within the calcium-PSS globules, and stopped growing once the calcium ran out. The finding suggested to the team that calcium binding to the organic macromolecules mediated how and where the ACC formed.
Large charged molecules (red) lure in calcium ions (blue), which position carbonate ions (yellow and red) and form amorphous calcium carbonate (white ball).
Having monitored the crystallization process step by step, they concluded that the negatively charged polymer side chains act as a sponge for the positively charged calcium ions (or counterions), concentrating them in specific regions. Biomineralization then readily occurs where the calcium is clustered.
“The sponge makes it possible to locally increase the ion concentration, which makes nucleation easier,” said Nico Sommerdijk, a collaborator in the study from Eindhoven University of Technology in Eindhoven, The Netherlands.
Previous work had suggested an alternate route to crystallization: the scaffold might guide biomineralization by providing a low-energy surface upon which the crystals could deposit themselves in an ordered fashion. The crystals would form wherever it was most energetically favorable to do so.
These results add to this picture, suggesting that ion binding can sometimes overpower energetic favorability.
“Our experiment shows how important these counterions are to the hard system,” said De Yoreo, referring to the calcium ions that the organic scaffold soaks up. “Even when you make ordered structures out of proteins, the counterions play a huge role.”
Although the idea that ion binding might play a role in the biomineralization process was first suggested by a group of Israeli scientists 30 years ago, the technology to thoroughly test it was only recently developed.
“Liquid-phase TEM makes it possible to view the whole process—from the binding of calcium ions by the matrix, to the formation of amorphous calcium carbonate and its subsequent transformation to crystalline vaterite—at high resolution, as it proceeds in the reaction solution,” said Fiona Meldrum, a materials chemist from the University of Leeds who was not involved in the research. “In combination with other experimental methods, the work provides new insight into the mechanisms by which organic molecules control crystallization.”
The finding comes at an ideal moment, as concern grows about the impact of excess atmospheric carbon dioxide on the health of the planet. Biomineralization is a natural way to sequester this carbon, removing it from the atmosphere and depositing it elsewhere. Although such technology is a long way from implementation, understanding the process could help researchers recreate it artificially to deliberately remove carbon dioxide from the atmosphere. “Carbon dioxide begs for sequestration,” said De Yoreo. “What we’re looking at here is a way of promoting this process.”
