[S]ometimes garnets are marred with intricate traceries of microscopic tunnels. When Magnus Ivarsson, a geobiologist at the Swedish Museum of Natural History, first saw these tunnels, he wondered what could be making them... In a paper in PLOS One, the researchers are floating a new hypothesis: Perhaps what’s making the tunnels is alive.That was from the Abstract; here are a couple excerpts from elsewhere in the manuscript -
The origin of such tunneling has previously been attributed to abiotic processes. Here we present physical and chemical remains of endolithic microorganisms within the tunnels and discuss a probable biological origin of the tunnels. Extensive investigations with synchrotron-radiation X-ray tomographic microscopy (SRXTM) reveal morphological indications of biogenicity that further support a euendolithic interpretation. We suggest that the production of the tunnels was initiated by a combination of abiotic and biological processes, and that at later stages biological processes came to dominate. In environments such as river sediments and oxidized soils garnets are among the few remaining sources of bio-available Fe2+, thus it is likely that microbially mediated boring of the garnets has trophic reasons. Whatever the reason for garnet boring, the tunnel system represents a new endolithic habitat in a hard silicate mineral otherwise known to be resistant to abrasion and chemical attack.
Endoliths are microorganisms living inside substrates, mostly rocks and minerals, but also shells, corals or wood [1,2]. Endolithic lineages have been developed among bacteria, fungi, algae, and several animal phyla, and they can either be chemolithoautotrophs (which utilize inorganically stored energy and carbon from inorganic sources like minerals), heterotrophs, or even photoautotrophs (like cyanobacteria) [2,3]. 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. However, heterotrophs and chemolithoautotrophs may bore a substrate for trophic reasons as well. Saprophytic fungi, for instance, frequently bore into wood and bone , and mycorrhizal fungi are known to bore into soil minerals to mobilize nutrients for symbiotic plants [4,5]. Prokaryotic microborers are believed to bore in volcanic glass to oxidize reduced iron and manganese species for their metabolism [6,7]...Many interesting images in the paper, q.v. -
A plethora of microorganisms including bacteria, fungi and algae are known to chemically etch minerals by excreting organic acids or chelators, such as siderophores, that act corrosively to certain minerals or elements..
The organic content of the garnet interior detected by ToF-SIMS and the complex nature of these organic molecules indicate microbial presence within the tunnel system of the garnets... Complex tunnel structures, as in the current study, are not likely to be formed exclusively by chemical dissolution but need the involvement of an agent that controls the direction .
The complexity of the networks with anastomoses between branches further rules out AITs. Anastomosis is in fact exclusively a biological feature but anastomosing tunnels produced by endolithic microorganisms have not yet been reported. Thus, even though the tunnels, at least partly, might look non-biogenic at first glance there is no conceivable non-biological mechanism that can explain the formation of them...
The transition from polygonal entrance pits at the mineral surfaces to more circular and tapering tunnels further into the minerals suggests that the tunnels were initiated by abiotic processes or a combination of abiotic and biological processes, which further into the mineral shift to predominantly biological processes.
Explained at the link.