Frutexites-like structures formed by iron oxidizing biofilms in the continental subsurface (Äspö Hard Rock Laboratory, Sweden)

Abstract

Stromatolitic iron-rich structures have been reported from many ancient environments and are often described as Frutexites, a cryptic microfossil. Although microbial formation of such structures is likely, a clear relation to a microbial precursor is lacking so far. Here we report recent iron oxidizing biofilms which resemble the ancient Frutexites structures. The living Frutexites-like biofilms were sampled at 160 m depth in the Äspö Hard Rock Laboratory in Sweden. Investigations using microscopy, 454 pyrosequencing, FISH, Raman spectroscopy, biomarker and trace element analysis allowed a detailed view of the structural components of the mineralized biofilm. The most abundant bacterial groups were involved in nitrogen and iron cycling. Furthermore, Archaea are widely distributed in the Frutexites-like biofilm, even though their functional role remains unclear. Biomarker analysis revealed abundant sterols in the biofilm most likely from algal and fungal origins. Our results indicate that the Frutexites-like biofilm was built up by a complex microbial community. The functional role of each community member in the formation of the dendritic structures, as well as their potential relation to fossil Frutexites remains under investigation.

Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia

“…Similar modern environments with microbially derived UCMs include sediments from Lizard Island on the Great Barrier Reef, Australia and St Croix, US Virgin Islands (Reitner et al. ), and iron‐rich biofilms in Sweden (Heim, Quéric, Ionescu, Schäfer, & Reitner, ). A GCxGC TOFMS study on a UCM extracted from the Mesoproterozoic Xiamaling shale in China proposed two causes of the UCM: microbial activity and enhanced isomerisation.…”

On the biogenicity of Fe‐oxyhydroxide filaments in silicified low‐temperature hydrothermal deposits: Implications for the identification of Fe‐oxidizing bacteria in the rock record

“…While some Fe‐oxidizing species are known to form dendrite‐like extracellular structures (Chiu, Kato, McAllister, Field, & Chan, ; Edwards, Rogers, Wirsen, & McCollom, ), dendritic growth of stalks produced by Mariprofundus ferrooxydans or other benthic Zetaproteobacteria has not previously been reported. Twisted stalks have been discovered in association with Frutexites ‐like iron shrubs in biofilms with active Fe and N‐cycling, but were not found to constitute the main framework of these dendrites (Heim, Quéric, Ionescu, Schäfer, & Reitner, ). We hypothesize that dendritic and radial filament textures in the JMVF mound deposit form as a response to rapidly shifting redox configurations.…”

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

“…A complex form characterized by frequent branching is referred to as Frutexites , a structure found on cavity walls and, less commonly, encrusting the filaments themselves. Biogenic Frutexites structures have been observed growing in situ at the present day embedded in iron‐oxidizing bacterial biofilms at 160 m depth in Sweden . However, stromatolite‐like, laminated, and branching microstructures are difficult to interpret since they can result from accretionary or diffusive mineral growth and are not by themselves indicative of biological processes …”

“…Biogenic Frutexites structures have been observed growing in situ at the present day embedded in iron-oxidizing bacterial biofilms at 160 m depth in Sweden. [78] However, stromatolite-like, laminated, and branching microstructures are difficult to interpret since they can result from accretionary or diffusive mineral growth and are not by themselves indicative of biological processes. [79][80][81]…”