Are the large filamentous microfossils preserved in Messinian gypsum colorless sulfide-oxidizing bacteria?

Abstract

The thick gypsum deposits formed in the Mediterranean Basin during the Messinian salinity crisis incorporate dense mazes of filamentous fossils, which were interpreted as algae or cyanobacteria, thus pointing to a shallow-marine subtidal or intertidal environment. The data presented here reveal that these filaments represent remains of colorless, vacuolated sulfide-oxidizing bacteria. This interpretation is supported by the presence of small crystal aggregates of iron sulfide (pyrite) and associated polysulfide within the filamentous fossils. Pyrite and polysulfide are considered to result from early diagenetic transformation of original zero-valent sulfur globules stored within the cells, which is a clade-diagnostic feature of living and degraded sulfur bacteria. In addition to filamentous fossils, the studied gypsum crystals contain remains of euryhaline and stenohaline diatoms and clay-rich aggregates interpreted as alteration products of marine snow floccules. This peculiar fossil assemblage reflects conditions of increased productivity in the water column, triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and freshwater discharge. This study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells, shedding light on the paleoecology of ancient hypersaline environments.

Potential Fossilized Sulfide-Oxidizing Bacteria in the Upper Miocene Sulfur-Bearing Limestones From the Lorca Basin (SE Spain): Paleoenvironmental Implications

“…Filamentous and rod-shaped morphologies similar to those found in Lorca are a common feature of carbonate (Schreiber, 1978; Guido et al, 2007; Oliveri et al, 2010; Ziegenbalg et al, 2010; Dela Pierre et al, 2012, 2014; Natalicchio et al, 2013; Caruso et al, 2015; García-Veigas et al, 2015; Perri et al, 2017) and gypsum (Vai and Ricci Lucchi, 1977; Panieri et al, 2010; Schopf et al, 2012; Dela Pierre et al, 2015) layers deposited in the Mediterranean region during the late Miocene. Their origin is, however, controversial, since they have been attributed to both fecal pellets (Schreiber, 1978; Guido et al, 2007; Natalicchio et al, 2013; García-Veigas et al, 2015), algae (Vai and Ricci Lucchi, 1977) and remains of prokaryotes such as cyanobacteria (Rouchy and Monty, 1981, 2000; Martin et al, 1984; Panieri et al, 2010) and sulfide-oxidizing bacteria (Oliveri et al, 2010; Schopf et al, 2012; Dela Pierre et al, 2012, 2014, 2015; Perri et al, 2017). To shed light on the origin of these enigmatic microstructures, and especially on the affiliation of the putative microbial fossils, two requirements must be preliminary discussed: (1) the distinction between fecal pellets and the supposed prokaryotic microfossils; (2) the validation of the criteria proving the biogenicity of microbial fossils (Schopf and Walter, 1983; Buick, 1990; Cady, 2001; Cady et al, 2003; Schopf et al, 2005; Westall, 2008; Summons et al, 2011).…”

Mineralization and Preservation of an extremotolerant Bacterium Isolated from an Early Mars Analog Environment

“…Other studies previously reported the difficulty to detect fossilized biomolecules with RAMAN, like the restricted detection of minerals in samples containing microorganisms 22 , 23 , 54 or fluorescence emission inhibiting the detection of biomolecules 21 , highlighting thus the importance of resonance pigments 59 .…”

“…The preservation of microorganisms in gypsum has been described in natural environments 52 , either in recent evaporites reporting “fresh” eukaryotic and bacterial cells or on older deposits (5 to 6 My) 53 . It showed that communities embedded in evaporites can impact their environment by imprinting their past activity in minerals 54 , 55 . On the contrary, the mineralization of Yersinia in our liquid gypsum-saturated solution was very slow.…”

Efficient recycling of nutrients in modern and past hypersaline environments

“…Stratigraphic and geochemical investigations suggest that such an efficient carbon sequestration may actually have been a common feature of the MSC. For example, organic-rich shales, marls, and dolomites have been found widespread and repeatedly throughout the MSC sequence 43,44 , and there are descriptions of rapidly growing gypsum crystals effectively trapping filamentous organic material 45,46 . It is noteworthy that about 50% of the world’s largest oilfields are sealed by evaporites, although they constitute less than 2% of the world’s sedimentary rocks 47 .…”