Nanometer‐scale characterization of exceptionally preserved bacterial fossils in Paleocene phosphorites from Ouled Abdoun (Morocco)

Abstract

Micrometer-sized spherical and rod-shaped forms have been reported in many phosphorites and often interpreted as microbes fossilized by apatite, based on their morphologic resemblance with modern bacteria inferred by scanning electron microscopy (SEM) observations. This interpretation supports models involving bacteria in the formation of phosphorites. Here, we studied a phosphatic coprolite of Paleocene age originating from the Ouled Abdoun phosphate basin (Morocco) down to the nanometer-scale using focused ion beam milling, transmission electron microscopy (TEM), and scanning transmission x-ray microscopy (STXM) coupled with x-ray absorption near-edge structure spectroscopy (XANES). The coprolite, exclusively composed of francolite (a carbonate-fluroapatite), is formed by the accumulation of spherical objects, delimited by a thin envelope, and whose apparent diameters are between 0.5 and 3 μm. The envelope of the spheres is composed of a continuous crown dense to electrons, which measures 20-40 nm in thickness. It is surrounded by two thinner layers that are more porous and transparent to electrons and enriched in organic carbon. The observed spherical objects are very similar with bacteria encrusting in hydroxyapatite as observed in laboratory experiments. We suggest that they are Gram-negative bacteria fossilized by francolite, the precipitation of which started within the periplasm of the cells. We discuss the role of bacteria in the fossilization mechanism and propose that they could have played an active role in the formation of francolite. This study shows that ancient phosphorites can contain fossil biological subcellular structures as fine as a bacterial periplasm. Moreover, we demonstrate that while morphological information provided by SEM analyses is valuable, the use of additional nanoscale analyses is a powerful approach to help inferring the biogenicity of biomorphs found in phosphorites. A more systematic use of this approach could considerably improve our knowledge and understanding of the microfossils present in the geological record.

Facula, Faculae

“…For example, a 40-nm-thick cell wall of Gram-negative bacteria can be fossilized by calcium phosphates (e.g., Benzerara et al 2004a) or by iron minerals (e.g., Miot et al 2009). Such a preservation of the 40-nm-thick cell wall of Gram-negative bacteria has also been evidenced recently in phosphorite samples from Morocco dating from the Paleocene (Cosmidis et al 2013). Embryos of eukaryotes could be preserved as well in the laboratory at different development stages by phosphatization, simulating what likely occurred during the fossilization of the famous~630 Ma old Doushantuo embryos (e.g., Yin et al 2007).…”

Potential Role of Nitrite for Abiotic Fe(II) Oxidation and Cell Encrustation during Nitrate Reduction by Denitrifying Bacteria

“…Because nitrate reduction is a very widespread microbial metabolism, the observed Fe(II) oxidation by normal denitrifiers has implications for cultivation-based studies focusing on quantification and isolation of nitrate-reducing Fe(II) oxidizers from the environment, since normal denitrifiers will also be recognized and counted as Fe(II) oxidizers in such experiments when additional organic electron donors are added. Furthermore, the encrustation and preservation of denitrifying cells by Fe(III) minerals is of importance, since there are many studies focusing on the preservation of microbial cells as biosignatures or microfossils to draw conclusions about the presence of microbial activity in modern and ancient environments (51,(75)(76)(77). And finally, the formation of biogenic iron minerals as a microbial by-product of denitrification also affects other geochemical cycles in nature.…”

New Insights in the Ontogeny and Taphonomy of the Devonian Acanthodian Triazeugacanthus affinis From the Miguasha Fossil-Lagerstätte, Eastern Canada

“…In contrast, carbonate-fluorapatite is less soluble than bioapatite [26,54] and often represents an ultimate stage of recrystallization of biomineralized anatomical structures in geological environments [26,28,52,54,56]. Carbonate-fluorapatite also occurs in sedimentary environments as an authigenic mineral, likely produced from the bacterial decay of organic matter [57][58][59][60].…”