October's Paper of the Month

 Partly fossilized mycelium of fungal hyphae on a zeolite crystal from 740 m depth in fractured granite. Back-scattered Environmental Scanning Electron Microscopy image. Width of view 600 µm. Photo credit: Henrik Drake, Magnus Ivarsson.

Partly fossilized mycelium of fungal hyphae on a zeolite crystal from 740 m depth in fractured granite. Back-scattered Environmental Scanning Electron Microscopy image. Width of view 600 µm. Photo credit: Henrik Drake, Magnus Ivarsson.

Despite being considered vital for energy cycling of the earth; the deep biosphere is one of the least understood ecosystems. It is thought to have approximately 19% of the earth’s biomass yet samples are hard to come by, making their study difficult. Microorganisms from the deep biosphere that have been studied are generally prokaryotes, with microeukaryotes being largely ignored.

Recently samples were taken from a 740m deep drill core sample in Sweden after the site was investigated for its suitability for deep nuclear waste repositories. Findings have shown the presence of fossil and active fungi in these deep ecosystems, but little work has gone into understanding them.

Drake et al., studied the microorganisms in these deep crystalline fractured rock samples. Their aim was to gain a better understanding of the microbial processes in the continental crust. The knowledge of this vast realm is very scarce and tells us more about life forms and processes under extreme conditions which may also have important implications for nuclear waste storage. 

Their analyses found the microorganisms belonged to the Kingdom Fungi and were found to be anaerobic. The closest systems studied were that of anaerobic fungi in the rumina of ruminant animals. It was proposed that the fossilised fungi also shared a symbiotic relationship with bacteria in the deep biosphere. 

The group used a THMS600 to help indicate the approximate age of the fungi. Dr Drake said, “The THM600 was used to investigate fluid inclusions in calcite crystals that were spatially related to the fungi. The fluid inclusion signatures gave us information about past conditions (e.g. salinity) in the fracture void. Because no radiometric dating could be made of the fungi, the fluid inclusion signatures (when put in a paleohydrogeological context) serve as an important temporal indicator for when the fungi were active.” 

Their work highlighted an intimate relationship between the fungi and sulphate reducing bacteria, further drawing attention to the richness of the deep oligotrophic biosphere which is often neglected. These fungi were found to provide significant amounts of H2 to autotrophic microorganisms in the crystalline continental crust.

The group also looked at the biochemistry of these fungi and found they may pose a threat to repositories of toxic waste.  This is through either directly breaking down the barriers holding the waste, or by facilitating the bacterial community into doing so. 

Their work highlights the importance of studying these neglected geological microorganisms. With fossil fuels running out, nuclear energy may be the way forward. But to safely store away waste products, understanding their chemical and geological environment is of utmost importance as illustrated by Drake et al., As such it becomes vital to study ecosystems, such as the deep biosphere, in its entirety. 

By Tabassum Mujtaba

Drake et al., Anaerobic consortia of fungi and sulfate reducing bacteria in deep granite fractures. Nat. Comms 8, Article number: 55 doi:10.1038/s41467-017-00094-6 (2017)