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According to new research led by the University of Southern Denmark, tubular structures found inside garnets (pyrope and almandine) from Thailand are most likely the result of endoliths making their homes inside these crystals.
Photograph of a garnet crystal with distinct tubular structures. Image credit: Ivarsson et al, doi: 10.1371/journal.pone.0200351.
Endoliths are microorganisms living inside substrates, mostly rocks and minerals, but also shells, corals or wood. They are known among bacteria, fungi, algae, and several animal groups.
The usual advantage of entertaining an endolithic lifestyle is to obtain residence space — a hard or soft substrate provides a stable and protected environment compared to the outside.
Some endoliths move into pre-existing cavities while others dig their own way in — as the result of either physical force but more likely through chemical dissolution.
In the new study, University of Southern Denmark researcher Magnus Ivarsson and co-authors examined the structure and content of intricately branching tunnels inside garnet crystals from river sediments and soils in Thailand to determine whether they were formed by abiotic or biological processes.
Chemical analysis of the tunnels found lingering organic compounds and filament-like structures reminiscent of bacteria and fungi, strongly suggesting that microbes once lived inside. Whether or not these organisms excavated the tunnels is less clear.
Microphotograph of network of tubular structures originating at the mineral surface and stretching into the garnet relatively localized to the margin of the garnet. Ms – mineral surface. Image credit: Ivarsson et al, doi: 10.1371/journal.pone.0200351.
“These tunnels all originate from the grain surface and extend into the mineral,” Dr. Ivarsson and colleagues said.
“They are typically funnel-shaped, with hexagonal or rectangular cross sections in the coarser portions, attaining more rounded shape towards the tip. The diameter at the opening varies considerably; from about 5 μm to about 100 μm.”
“The tunnels are usually straight near their opening to the mineral surface but tend to change direction or branch toward the tips. The direction may sometimes change sharply with a kink, but commonly the tunnels follow a smooth and sinuous curvature, suggesting that the direction is not primarily governed by crystallography.”
“Some tunnels are arch-shaped with both ends at the same surface close to each other. Others reach from one side of a grain to the other, stretching right through the mineral grain.”
“Some contain filamentous structures with a diameter of 5–15 μm and lengths of at least a few hundred μm.”
The shape of the tunnels, examined under microscopy, doesn’t completely rule out an abiotic origin, but certain features characteristic of endolithic lairs, such as anastomoses (connecting passages between adjacent tunnels) suggest the tunnels were at least partially formed by endolithic microbes.
“These tunnels were originally noted because they significantly decrease the quality and value of the garnets as gems, but this study has shown that they also represent a previously unrecognized habitat for endolithic organisms,” the scientists said.
“In iron-poor sediments like those we studied, garnets represent a rare source of iron for iron-oxidizing microbes, but confirming the identity of the tunnel-borers will require observations of live organisms in a laboratory setting.”
The study was published in the journal PLoS ONE.
M. Ivarsson et al. 2018. Intricate tunnels in garnets from soils and river sediments in Thailand — Possible endolithic microborings. PLoS ONE 13 (8): e0200351; doi: 10.1371/journal.pone.0200351