Endolithic microbial fossils in sea-floor-weathered serpentinite
Serpentinized peridotites of the Iberia Abyssal Plain contain an assemblage of mineralized filamentous structures encased in fracture-filling marine calcite of late Cretaceous age. 3D light microscopy reveals that the filaments are attached to fracture surfaces and manifest diverse forms, including radiating clusters, branching splays, and spiraling loops. SEM microscopy and EDS analysis performed after acid etching of the calcite reveals that filaments are composed of iron oxide, iron silicate (nontronite?), and magnesium silicate (palygorskite?). High magnification inspection of the acid-exposed filaments with the SEM reveals a rich morphological complexity, extending into the submicron range, that is unexpected for strictly inorganic precipitates. Iron-oxide filaments coated with spiky crystals are convincingly similar to structures created by modern iron-oxidizing bacteria. In the silicate filaments, segmentation, telescoping filament diameters, branching, thread-wrapped rods, hair-like structures extending from rod tips, and threads (100 nm) tipped by balls (300-400 nm) are all suggestive of a microbial origin. Taxonomic affinities for the silicate filaments are uncertain and could possibly include prosthecate and budding bacteria and fungi