Abstract
The deep biosphere is one of the least understood ecosystems on Earth. Although most microbiological studies in this system have focused on prokaryotes and neglected microeukaryotes, recent discoveries have revealed existence of fossil and active fungi in marine sediments and sub-seafloor basalts, with proposed importance for the subsurface energy cycle. However, studies of fungi in deep continental crystalline rocks are surprisingly few. Consequently, the characteristics and processes of fungi and fungus-prokaryote interactions in this vast environment remain enigmatic. Here we report the first findings of partly organically preserved and partly mineralized fungi at great depth in fractured crystalline rock (−740 m). Based on environmental parameters and mineralogy the fungi are interpreted as anaerobic. Synchrotron-based techniques and stable isotope microanalysis confirm a coupling between the fungi and sulfate reducing bacteria. The cryptoendolithic fungi have significantly weathered neighboring zeolite crystals and thus have implications for storage of toxic wastes using zeolite barriers.
Highlights
The deep biosphere is one of the least understood ecosystems on Earth
Even though the presence of fungi in the continental crystalline basement is confirmed by a few reports, there is a huge gap in knowledge compared to what is known about the prokaryotes, and there is an urgent need to investigate the abundance, diversity and ecological role of fungi in these deep environments
Mycelial-like networks consisting of long filamentous structures were discovered at 740 m depth in a drill core (KLX09) at the Laxemar site, Sweden, where the Swedish Nuclear Fuel and Waste Management Co. (SKB) recently carried out site investigations for a deep nuclear-waste repository
Summary
Most microbiological studies in this system have focused on prokaryotes and neglected microeukaryotes, recent discoveries have revealed existence of fossil and active fungi in marine sediments and sub-seafloor basalts, with proposed importance for the subsurface energy cycle. We present an extensive study of previously unseen organically preserved and partly mineralized fungal hyphae, inferred as anaerobic fungi, from deep fractured continental crystalline rocks (740 m depth at the Laxemar site in Sweden). Utilization of state-of-the-art methodology including secondary ion mass spectrometry (SIMS), synchrotron radiation X-ray tomographic microscopy (SRXTM), and Time-of-Flight (ToF-) SIMS enables comprehensive new characterisation of the fungi, the cryptoendolithic behaviour of fungi, and the prokaryote-fungus interaction in this environment This increases the understanding of the role fungi play in the energy cycling of the deep biosphere, and has societal implication for long-term storage of toxic wastes
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