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

Abstract

The deep biosphere of the subseafloor crust is believed to contain a significant part of Earth's biomass, but because of the difficulties of directly observing the living organisms, its composition and ecology are poorly known. We report here a consortium of fossilized prokaryotic and eukaryotic micro-organisms, occupying cavities in deep-drilled vesicular basalt from the Emperor Seamounts, Pacific Ocean, 67.5 m below seafloor (mbsf). Fungal hyphae provide the framework on which prokaryote-like organisms are suspended like cobwebs and iron-oxidizing bacteria form microstromatolites (Frutexites). The spatial inter-relationships show that the organisms were living at the same time in an integrated fashion, suggesting symbiotic interdependence. The community is contemporaneous with secondary mineralizations of calcite partly filling the cavities. The fungal hyphae frequently extend into the calcite, indicating that they were able to bore into the substrate through mineral dissolution. A symbiotic relationship with chemoautotrophs, as inferred for the observed consortium, may be a pre-requisite for the eukaryotic colonization of crustal rocks. Fossils thus open a window to the extant as well as the ancient deep biosphere.

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 .…”

“…The presence of a mineralized central strand has been shown as a common feature among fossilized fungal hyphae 33–36 . Lining of mineral surfaces by a basal biofilm from which further hyphal growth emanates to form a mycelium is also typical for endolithic fungi 15, 19, 33, 34 . Except for fungi, actinobacteria and the stramenopile oomycetes are the only microorganisms forming mycelium-like networks of branching filaments.…”

“…Usually, subsurface fungi are completely fossilized to clay minerals and Fe-oxides with rare carbonaceous elements and no biomarkers at all 33, 34 , except rare chitin observations 16 . Our findings give insights into the fossilization process of fungi through a transition from maturation of the organic matter to a carbonaceous material, before being finally mineralized by clays and Fe-oxides.…”

The roles of endolithic fungi in bioerosion and disease in marine ecosystems. I. General concepts

“…Various types of endoliths, including fungi, have been found in marine shells in the Late Ordovician and Middle Devonian volcanic rocks, while microborings have also been found in early Cambrian phosphatic and phosphatised fossils (Taylor et al 2015). Fossilised microorganisms have also been observed in drilled cores and dredged samples from the ocean floor, with a majority of these findings representing fungi (Schumann et al 2004; Bengtson et al 2014). These fungi existed in a symbiotic-like relationship with two types of chemolithotrophic prokaryotes, which appeared to use the structural framework of the mycelium for their growth (Bengtson et al 2014).…”

“…Fossilised microorganisms have also been observed in drilled cores and dredged samples from the ocean floor, with a majority of these findings representing fungi (Schumann et al 2004; Bengtson et al 2014). These fungi existed in a symbiotic-like relationship with two types of chemolithotrophic prokaryotes, which appeared to use the structural framework of the mycelium for their growth (Bengtson et al 2014). Early fossil records of eukaryotes, including fungi and primitive plants in terrestrial ecosystems, appear to have come from the Ordivician period (Heckman et al 2001).…”