Biologists Find Rare and Unstable Mineral Vaterite in Alpine Plants

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.

Saxifraga sempervivum, a species of alpine plant in Cambridge University Botanic Gardens, ‘produces pure vaterite.’ Image credit: Paul Aston.

Saxifraga sempervivum, a species of alpine plant in Cambridge University Botanic Gardens, ‘produces pure vaterite.’ Image credit: Paul Aston.

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.

“We suspect that vaterite may be present on more plant species, but that the unstable mineral is being converted to calcite when exposed to wind and rain. This may also be the reason why some plants have both vaterite and calcite present at the same time,” they said.

The microscopy research also turned up some novel cell structures.

“As well as producing vaterite, Saxifraga scardica has a special tissue surrounding the leaf edge that appears to deflect light from the edge into the leaf. The cells appear to be producing novel cell wall structures to achieve this deflection. This may be to help the plant to collect more light, particularly if it is growing in partly shaded environments,” Aston said.

The researchers believe the novel cell wall structures of Saxifrages could one day help inform the manufacture of new bio-inspired optical devices and photonic structures for industry such as communication cables and fiber optics.

“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,” Aston said.

“The next species we will be looking to study is Saxifraga lolaensis, which has super tiny leaves with an organization of cell types not seen in a leaf before, and which we think will reveal more fascinating secrets about the complexity of plants.”

_____

Raymond Wightman et al. 2018. Leaf margin organisation and the existence of vaterite-producing hydathodes in the alpine plant Saxifraga scardica. Flora 241: 27-34; doi: 10.1016/j.flora.2018.02.006

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