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

Federico Andreetto,Luis Gibert,Simona Ferrando,Marcello Natalicchio,Francesco Dela Pierre

doi:10.3389/fmicb.2019.01031

Federico Andreetto, Luis Gibert + Show 3 more

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https://doi.org/10.3389/fmicb.2019.01031

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Abstract

The sulfur-bearing limestones interbedded in the upper Miocene diatomaceous sediments (Tripoli Formation) of the Lorca Basin (SE Spain) are typified, as other Mediterranean coeval carbonate and gypsum deposits, by filamentous, circular and rod-shaped microstructures of controversial origin. These features have been interpreted both as fecal pellets of brine shrimps and/or of copepods, remains of algae or cyanobacteria and fossilized sulfide-oxidizing bacteria. To shed light on their origin, a multidisciplinary study including optical, UV and scanning electron microscopy, Raman microspectroscopy, and geochemical (carbon and oxygen stable isotopes) analyses has been carried out on three carbonate beds exposed along the La Serrata ridge. The different composition of the filamentous and circular objects with respect to the rod-shaped microstructures suggest that the former represent remains of bacteria, while the latter fecal pellets of deposit- or suspension-feeder organisms. Size and shape of the filamentous and circular microfossils are consistent with their assignment to colorless sulfide-oxidizing bacteria like Beggiatoa (or Thioploca) and Thiomargarita, which is further supported by the presence, only within the microfossil body, of tiny pyrite grains. These grains possibly result from early diagenetic transformation of original sulfur globules stored by the bacteria, which are a diagnostic feature of this group of prokaryotes. The development of microbial communities dominated by putative sulfide-oxidizing bacteria at Lorca was favored by hydrogen sulfide flows generated through degradation of organic matter by sulfate-reducing bacteria thriving in underlying organic-rich sediments.

Highlights

The upper Miocene sedimentary record of the Mediterranean region comprises peculiar carbonate rocks typified by molds of evaporitic minerals and by elemental sulfur nodules
Scanning electron microscopy (SEM) analyses were performed on six stubs for morphological analyses and on twelve polished carbon-coated thin sections for semiquantitative elemental analyses and backscattered electron imagery using an environmental scanning electron microscope (ESEM FEI-Quanta 2000) coupled with a backscattered electron detector (BSED) (Centres Científics i Tecnològics, Universitat de Barcelona) and a JSM-IT300LV SEM equipped with an energy-dispersive EDS Oxford Instrument Link System microprobe (Department of Earth Sciences, University of Torino)
Considering that microbe biomass decomposes very quickly after deposition and early diagenesis, the hollowness in putative microbial microfossils is regarded as a key indicator to prove their biogenicity (Buick, 1990; Cady, 2001; Schopf et al, 2010); we suggest that rod-shaped microstructures represent fecal pellets of deposit-feeder organisms (Oliveri et al, 2010) or, alternatively, of planktonic organisms such copepods (Guido et al, 2007)

Summary

Introduction

The upper Miocene sedimentary record of the Mediterranean region comprises peculiar carbonate rocks typified by molds of evaporitic minerals (mainly gypsum) and by elemental sulfur nodules. The oxidation of hydrogen sulfide produced by microbial sulfate reduction is considered as responsible for the formation of native sulfur during early (syngenetic) or late (epigenetic) diagenetic processes (Ruckmick et al, 1979; Wessel, 1994; Ziegenbalg et al, 2010) Another common feature of the upper Miocene limestones is the almost completely absence of macrofossils, which is believed to result from harsh environmental conditions (e.g., hypersalinity, anoxia) lethal for most eukaryotes (e.g., Bellanca et al, 2001; BlancValleron et al, 2002). Their investigation with a multidisciplinary approach, which combines petrographic (optical, electronic, and fluorescence microscopy), spectroscopic (micro-Raman) and geochemical (C and O stable isotopes) techniques, allowed to shed light on the precursor organisms, and on the environmental conditions during deposition and early diagenesis

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