Fungal colonies in open fractures of subseafloor basalt

Abstract

The deep subseafloor crust is one of the few great frontiers of unknown biology on Earth and, still today, the notion of the deep biosphere is commonly based on the fossil record. Interpretation of palaeobiological information is thus central in the exploration of this hidden biosphere and, for each new discovery, criteria used to establish biogenicity are challenged and need careful consideration. In this paper networks of fossilized filamentous structures are for the first time described in open fractures of subseafloor basalts collected at the Emperor Seamounts, Pacific Ocean. These structures have been investigated with optical microscopy, environmental scanning electron microscope, energy dispersive spectrometer, X-ray powder diffraction as well as synchrotron-radiation X-ray tomographic microscopy, and interpreted as fossilized fungal mycelia. Morphological features such as hyphae, yeastlike growth and sclerotia were observed. The fossilized fungi are mineralized by montmorillonite, a process that probably began while the fungi were alive. It seems plausible that the fungi produced mucilaginous polysaccharides and/or extracellular polymeric substances that attracted minerals or clay particles, resulting in complete fossilization by montmorillonite. The findings are in agreement with previous observations of fossilized fungi in subseafloor basalts and establish fungi as regular inhabitants of such settings. They further show that fossilized microorganisms are not restricted to pore spaces filled by secondary mineralizations but can be found in open pore spaces as well. This challenges standard protocols for establishing biogenicity and calls for extra care in data interpretation.

Anaerobic consortia of fungi and sulfate reducing bacteria in deep granite fractures

“…Regarding the first mechanism, the extensive weathering of zeolites seen in the granite fracture in the present study, as well as similar observations in sub-seafloor basalts elsewhere 33, 34 , calls for consideration when planning to use this group of minerals as geochemical barriers in subsurface storages. Zeolites have been planned to function as an ion-exchange retention barrier for the storage of high-level nuclear waste in the US 68, 69 .…”

Fungal colonization of an Ordovician impact-induced hydrothermal system

“…The investigated filaments are characterized by microbial-like morphologies and correspond to other filamentous microfossils from subsurface and subseafloor settings293334. The dimensions and morphologies, including septa, anastomoses between branches, and mycelium-like networks, are further characteristic of fungi rather than filamentous prokaryotes3536.…”

Deep‐biosphere consortium of fungi and prokaryotes in Eocene subseafloor basalts

“…Previous work has established thin mycelial structures from these rocks as fungal hyphae based on characteristic fungal morphologies like repetitive septa, anastomoses between branches, a central strand, and the presence of chitin, which together suggest affinity to the Dikarya (Ivarsson et al ., , ). Because the hyphae seem to emanate from the basal film and become wider with increasing distance from the film, we tentatively interpret the film and its coarser protrusions to belong to the same organism as the hyphae.…”

“…Fossilized mycelial micro‐organisms in subseafloor basalts have been interpreted as remnants of fungi (Schumann et al ., ; Reitner et al ., ; Peckmann et al ., ; Eickmann et al ., ; Ivarsson, ; Ivarsson et al ., , ). The ability of fungi to live in such extreme environments is corroborated by discoveries of viable yeast cells deep in continental crust (Ekendahl et al ., ) and of active fungal genes in deep‐sea sediments at various depths below the seafloor (Orsi et al ., ,b).…”