Exceptional Preservation of Fungi as H2-Bearing Fluid Inclusions in an Early Quaternary Paleo-Hydrothermal System at Cape Vani, Milos, Greece

Abstract

The production of H2 in hydrothermal systems and subsurface settings is almost exclusively assumed a result of abiotic processes, particularly serpentinization of ultramafic rocks. The origin of H2 in environments not hosted in ultramafic rocks is, as a rule, unjustifiably linked to abiotic processes. Additionally, multiple microbiological processes among both prokaryotes and eukaryotes are known to involve H2-production, of which anaerobic fungi have been put forward as a potential source of H2 in subsurface environments, which is still unconfirmed. Here, we report fungal remains exceptionally preserved as fluid inclusions in hydrothermal quartz from feeder quartz-barite veins from the Cape Vani Fe-Ba-Mn ore on the Greek island of Milos. The inclusions possess filamentous or near-spheroidal morphologies interpreted as remains of fungal hyphae and spores, respectively. They were characterized by microthermometry, Raman spectroscopy, and staining of exposed inclusions with WGA-FITC under fluorescence microscopy. The spheroidal aqueous inclusions interpreted as fungal spores are unique by their coating of Mn-oxide birnessite, and gas phase H2. A biological origin of the H2 resulting from anaerobic fungal respiration is suggested. We propose that biologically produced H2 by micro-eukaryotes is an unrecognized source of H2 in hydrothermal systems that may support communities of H2-dependent prokaryotes.

Precipitation of Mn Oxides in Quaternary Microbially Induced Sedimentary Structures (MISS), Cape Vani Paleo-Hydrothermal Vent Field, Milos, Greece

“…Powder XRD (PXRD) data (Table S1) confirm previous mineralogical investigations [41][42][43]46,48]. The presence of micro-and nano-crystalline todorokite cement has been revealed for the first time by means of PXRD, BSE-EDS, FEG-SEM and HR-TEM investigations.…”

“…Field relationships, combined with fluid inclusion data from hydrothermal barite and quartz, suggest that the MnMISS developed in a very shallow (<50 m) steaming marine paleo-environment, in close association with white smoker-type seafloor activity that probably acted as Mn(II) supplier. The temperatures were~125 • C at depth (feeder veins) cooling to <100 • C near and at the paleo-seafloor in response to boiling, mixing with seawater and conductive heat loss [43,62]. Sulphur (δ 34 S) and oxygen (δ 18 O) isotope evidence has shown that the investigated MnMISS are younger than 1.5 Ma, and possibly formed about 0.5 Ma ago [43]; moreover, the MnMISS associated barite was derived from seawater sulphate that has undergone some alteration through bacterial sulphate reduction (BSR) [43].…”

“…The temperatures were~125 • C at depth (feeder veins) cooling to <100 • C near and at the paleo-seafloor in response to boiling, mixing with seawater and conductive heat loss [43,62]. Sulphur (δ 34 S) and oxygen (δ 18 O) isotope evidence has shown that the investigated MnMISS are younger than 1.5 Ma, and possibly formed about 0.5 Ma ago [43]; moreover, the MnMISS associated barite was derived from seawater sulphate that has undergone some alteration through bacterial sulphate reduction (BSR) [43]. Furthermore, fungal remains, exceptionally preserved as fluid inclusions, have been identified in hydrothermal quartz from feeder veins [43].…”

The fossil record of igneous rock

“…However, the characterization of biomarkers has not yet been successful enough to use as a discriminating factor to distinguish between groups like eukaryotes or prokaryotes. An exception is chitin, which was detected by staining with WGA-FITC under fluorescence microscopy and thus could be used as an indicator for the presence of fungi (Ivarsson et al, 2012(Ivarsson et al, , 2019bBonneville et al, 2020).…”

Geochronology and Stable Isotope Analysis of Fracture-Fill and Karst Mineralization Reveal Sub-Surface Paleo-Fluid Flow and Microbial Activity of the COSC-1 Borehole, Scandinavian Caledonides

“…Fluid circulation and mixing in fractures and during vein formation can lead to metal leaching and accumulation of ores, such as those in the Harz mountains, Germany [1]. Microbial activity in fractured rock volumes can also be of importance for ore-forming processes [2,3] and may involve potentially significant microbial natural gas accumulations [4,5]. Furthermore, microbial activity and fluid flow may involve important redox processes and fluctuations that are relevant for safety [30,38].…”