A mineral that is usually unusable and rarely found in nature appears to coat the leaves of plants living in Arctic and alpine environments.
Known as vaterite, and a polymorphism of calcium carbonate, it was previously thought that it couldn’t exist naturally in any meaningful amount. But it turns out that some plants have the ability to make it themselves, and cover their leaves in the substance.
Once exposed to water – which is not exactly unusual in Earth’s humid atmosphere – vaterite will rapidly convert to either calcite if the temperatures are low, or to aragonite if the temperatures are high. This, therefore, limits vaterite's abundance, although it is found in small amounts in the shells of some crustaceans, the ear bones of salmon, and even in some meteorites.
But this is the first time that the mineral has been discovered in such large quantities, as well as the first time it has ever been found to be produced by plants.
“The microscope analysis of the plant material came up with the exciting discovery that some plants were exuding vaterite from 'chalk glands' (hydathodes) on the margins of their leaves,” explained Simon Wallis, who co-authored the paper published in Flora. “We then noticed a pattern emerging.”
The plants that were producing the mineral belong to a group known as Saxifrages, which typically live in Arctic and alpine environments, and are rarely found outside of the Northern Hemisphere. What's more, it wasn’t just any Saxifrage making the vaterite, only those in the subsection of plants known as Porphyrion, some species of which are popular cultivars.
The discovery was only made possible by the now widespread use of the cryo-electron microscopes, the development of which won the inventors the 2017 Nobel Prize in Chemistry. This allowed the researchers to image the leaves of the alpine plants in their native fully-hydrated state by freezing them rapidly before putting them in a vacuum. Other attempts to look at the leaves probably destroyed the mineral due to its instability.
They found that many Porphyrion plants were coated in both vaterite and calcite, but that at least one species, Saxifraga sempervivum, was producing pure crystals of vaterite. How the mineral is helping the plants is still not understood, nor why some have a mix of the two substances on their leaves. The researchers suggest, however, that many more plants might be making vaterite, and that as the leaves are exposed to wind and rain, it could simply be being converted into more stable forms.
“We expect that there may be other plants that also produce vaterite and have special leaf anatomies that have evolved in harsh environments like alpine regions,” said co-author Paul Aston.
Interestingly, the mineral could be used in the world of drug delivery, as it has some properties that make it suitable for delivering a sustained and targeted release of medicine to the exact place it is needed.