Zygomycetes in Vesicular Basanites from Vesteris Seamount, Greenland Basin – A New Type of Cryptoendolithic Fungi

Abstract

Fungi have been recognized as a frequent colonizer of subseafloor basalt but a substantial understanding of their abundance, diversity and ecological role in this environment is still lacking. Here we report fossilized cryptoendolithic fungal communities represented by mainly Zygomycetes and minor Ascomycetes in vesicles of dredged volcanic rocks (basanites) from the Vesteris Seamount in the Greenland Basin. Zygomycetes had not been reported from subseafloor basalt previously. Different stages in zygospore formation are documented in the studied samples, representing a reproduction cycle. Spore structures of both Zygomycetes and Ascomycetes are mineralized by romanechite-like Mn oxide phases, indicating an involvement in Mn(II) oxidation to form Mn(III,VI) oxides. Zygospores still exhibit a core of carbonaceous matter due to their resistance to degradation. The fungi are closely associated with fossiliferous marine sediments that have been introduced into the vesicles. At the contact to sediment infillings, fungi produced haustoria that penetrated and scavenged on the remains of fragmented marine organisms. It is most likely that such marine debris is the main carbon source for fungi in shallow volcanic rocks, which favored the establishment of vital colonies.

A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

“…Septation is evident in a few examples . Branching is common but not ubiquitous; anastomosis is less common, but occasionally results in complex mycelium‐like networks interconnected by anastomosis . Discrete swellings may rarely be found along the length of the filaments or, more typically, at their tips; otherwise, filaments are usually of constant thickness along their length and tapering is rare.…”

“…Where transverse sections have been taken at the appropriate scale, they typically reveal broadly circular cross‐sections with crystallites arrayed radially from the filament axis . A significant minority of filaments also contain carbonaceous material, probably organic in composition . In some instances, this carbon is very abundant and conspicuous; in others its presence can be detected from elemental analyses, yielding atomic ratios of C to Mg + Fe + Ca greater than unity, which suggests that carbon additional to that bound in carbonates is present .…”

“…In some instances, this carbon is very abundant and conspicuous; in others its presence can be detected from elemental analyses, yielding atomic ratios of C to Mg + Fe + Ca greater than unity, which suggests that carbon additional to that bound in carbonates is present . Raman spectroscopic analysis of filaments in subseafloor basalts has further shown the presence of disordered carbonaceous matter distinguished by characteristic peaks at 1350 cm −1 and 1600 cm −1 . The organic compounds have been further characterized by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), which has shown the presence of phosphates, hydrocarbons, and lipids .…”

“…Thus, early reports assumed that the mineralized filaments in rocks were autofossilized iron‐oxidizing bacteria; more recent work has tended to reserve this interpretation for unbranched examples . Branching/anastomosing morphologies and the presence of terminal swellings resembling fruiting bodies (sporophores) have led recent studies to favor a fungal origin of the filaments . Coccoidal structures clustering closely around the filaments have likewise been interpreted as yeast …”

“…The past decade has been especially productive, yielding new estimates of global deep biomass, new metagenomic surveys revealing extraordinary bacterial diversity, new indications that fungi are active and widespread in subseafloor habitats, and the first discovery of an animal indigenous to multikilometer‐deep fracture waters (a bacteriophagous nematode). Despite this flurry of discoveries in the modern deep biosphere, only a few workers have considered its antiquity, evolution, and interactions with the lithosphere, atmosphere, and oceans over geological time, leaving fundamental questions unanswered. How, for example, have the size and activity of the deep biosphere varied over geological time?…”