Electron Paramagnetic Resonance Study of a Photosynthetic Microbial Mat and Comparison with Archean Cherts

Abstract

Organic radicals in artificially carbonized biomass dominated by oxygenic and non-oxygenic photosynthetic bacteria, Microcoleus chthonoplastes-like and Chloroflexus-like bacteria respectively, were studied by Electron Paramagnetic Resonance (EPR) spectroscopy. The two bacteria species were sampled in mats from a hypersaline lake. They underwent accelerated ageing by cumulative thermal treatments to induce progressive carbonization of the biological material, mimicking the natural maturation of carbonaceous material of Archean age. For thermal treatments at temperatures higher than 620 °C, a drastic increase in the EPR linewidth is observed in the carbonaceous matter from oxygenic photosynthetic bacteria and not anoxygenic photosynthetic bacteria. This selective EPR linewidth broadening reflects the presence of a catalytic element inducing formation of radical aggregates, without affecting the molecular structure or the microstructure of the organic matter, as shown by Raman spectroscopy and Transmission Electron Microscopy. For comparison, we carried out an EPR study of organic radicals in silicified carbonaceous rocks (cherts) from various localities, of different ages (0.42 to 3.5 Gyr) and having undergone various degrees of metamorphism, i.e. various degrees of natural carbonization. EPR linewidth dispersion for the most primitive samples was quite significant, pointing to a selective dipolar broadening similar to that observed for carbonized bacteria. This surprising result merits further evaluation in the light of its potential use as a marker of past bacterial metabolisms, in particular oxygenic photosynthesis, in Archean cherts.

The Raman-Derived Carbonization Continuum: A Tool to Select the Best Preserved Molecular Structures in Archean Kerogens

“…The Middle Marker chert (n°16; 3.4 Ga; Table 1 ) was sampled in the Barberton greenstone belt, South Africa. It consists of silicified detrital sediment comprising volcanic grain, fluid inclusions, and CM floccules (Bourbin et al , 2012b ).…”

Methane, water, quartz, siderite, hematite, and possibly greenalite and sodium hydroxide in a BARB3 drilled sample of the 3.4 Ga Buck ReefChert. Crustal anoxic alkaline high subcritical water at the origin of the observed ferric minerals, methane and disordered graphitic carbon, and of plausible molecules of life

“…Fig.15e). The Raman spectra of 1.88 Ga Gunflint carbon show a D band, three times wider than the 3.416 Ga Buck Reef (Bourbin et al 2012) which is characteristic of a low greenschist facies with peak temperatures of 200° and 250°C Lazzeri 2012, Fig.8, Buseck andBeyssac 2014 Fig.5).…”

EPR of Primitive Organic Matter: A Tool for Astrobiology

“…Statistical theory predicts that the peak-to-peak linewidth of a Lorentzian line in a magnetically diluted spin system is related to the spin concentration N by [35]: with N in cm −3 and ΔB in mT. As domain IV of the IOM evolution corresponds to a condensation of aromatic groups with a decrease of N, the fact that ΔB increases by a factor 5-7 shows that the surviving radicals concentrate in the form of shallow aggregates with local concentrations N loc > N. These aggregates of radicals are probably localized in the boundary between ordered aromatic domains [36].…”

“…In order to get further insight into this effect, accelerated ageing experiment were performed by cumulative step annealing treatment as described above (Fig. 15.5) on a 45 Myrold chert (Clarno Formation, Oregon, USA) containing a variety of fossil remains, and on cyanobacteria microcoleus chtonoplastes collected in the lake "La Salada de Chiprana" (Spain) [36]. The evolution of R 10 factor with temperature for these two samples is shown as black symbols in Fig.…”