A team of biologists from the University of Cambridge, UK, has found that a very rare and unstable mineral called vaterite is a dominant component of the protective silvery-white crust that forms on the leaves of a number of alpine plants. The findings are published in the journal Flora.
Naturally occurring vaterite — a form (polymorph) of calcium carbonate — is rarely found on Earth.
Small amounts of vaterite crystals have been found in some sea and freshwater crustaceans, bird eggs, the inner ears of salmon, meteorites and rocks.
This is the first time that this mineral has been found in such a large quantity and the first time it has been found to be associated with plants.
“Vaterite was often associated with outer space and had been detected in planetary objects in the Solar System and meteorites,” said Dr. Raymond Wightman, lead author on the study.
“”Vaterite is not very stable in the Earth’s humid atmosphere as it often reverts to more common forms of calcium carbonate, such as calcite. This makes it even more remarkable that we have found vaterite in such large quantities on the surface of plant leaves.”
“Vaterite was of interest to the pharmaceutical industry. Biochemists are working to synthetically manufacture vaterite as it has potential for use in drug delivery, but it is not easy to make,” he added.
“Vaterite has special properties that make it a potentially superior carrier for medications due to its high loading capacity, high uptake by cells and its solubility properties that enable it to deliver a sustained and targeted release of therapeutic medicines to patients.”
“For instance, vaterite nanoparticles loaded with anti-cancer drugs appear to offload the drug slowly only at sites of cancers and therefore limit the negative side-effects of the drug.”
Dr. Wightman and his colleagues, Paul Aston and Simon Wallis, started by sampling as wide a range of species within the genus Saxifraga as possible.
The microscope analysis of the plant material came up with the discovery that some plants were exuding vaterite from ‘chalk glands’ (hydathodes) on the margins of their leaves.
“We then noticed a pattern emerging. The plants producing vaterite were from the section of Saxifraga called Porphyrion. Further to this, it appears that although many species in this section produced vaterite along with calcite, there was at least one species, Saxifraga sempervivum, that was producing pure vaterite,” Wallis said.
So why do these species produce a calcium carbonate crystal crust and why are some crusts calcite and others vaterite?
The study authors are hoping to answer this question through further analysis of the leaf anatomy of the Saxifraga group.
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