Fossilized anaerobic and possibly methanogenesis-fueling fungi identified deep within the Siljan impact structure, Sweden

Recent discoveries of extant and fossilized communities indicate that eukaryotes, including fungi, inhabit energy-poor and anoxic environments deep within the fractured igneous crust. This subterranean biosphere may constitute the largest fungal habitat on our planet, but knowledge of abyssal fungi and their syntrophic interactions with prokaryotes and their concomitant metabolisms is scarce. Here we report findings of fossilized, chitin-bearing fungal hyphae at similar to 540m depth in fractured bedrock of the Siljan impact structure, the largest crater in Europe. Strong C-13-enrichment of calcite precipitated with and on the fungi suggests formation following methanogenesis, and that the anaerobic fungi decomposed dispersed organic matter producing for example H-2 that may have fueled autotrophic methanogens. An Eocene age determined for the calcite infers the first timing constraint of fossilized fungi in the continental igneous crust. Fungi may be widespread decomposers of organic matter and overlooked providers of H-2 to autotrophs in the vast rock-hosted deep biosphere.

Automated summary

Recent discoveries have found fossilized fungi in deep underground volcanic rock fractures, playing a role in methanogenesis by producing H-2 for autotrophic methanogens. This suggests that fungi may be widespread decomposers of organic matter on Earth and overlooked providers of fuel to autotrophs in the vast rock-hosted deep biosphere.