N. Quéric

Assessing the utility of trace and rare earth elements as biosignatures in microbial iron oxyhydroxides

Microbial iron oxyhydroxides are common deposits in natural waters, recent sediments and mine drainage systems and often contain significant accumulations of trace and rare earth elements (TREE). TREE patterns are widely used to characterize minerals and rocks, and to elucidate their evolution and origin. Whether and which characteristic TREE signatures distinguish between a biological and an abiological origin of iron minerals is still not well understood. Long-term flow reactor studies were performed in the Aspo Hard Rock Laboratory to investigate the development of microbial mats dominated by iron-oxidizing bacteria, namely Mariprofundus sp. and Gallionella sp. The experiments investigated the accumulation and fractionation of TREE under controlled conditions and enabled us to assess potential biosignatures evolving within the microbial iron oxyhydroxides. Concentrations of Be, Y, Zn, Zr, Hf, W, Th, Pb, and U in the microbial mats were 1e3- to 1e5-fold higher than in the feeder fluids whereas the rare earth elements and Y (REE+Y) contents were 1e4 and 1e6 fold enriched. Except for a hydrothermally induced Eu anomaly, the normalized REE+Y patterns of the microbial iron oxyhydroxides were very similar to published REE+Y distributions of Archaean Banded Iron Formations. The microbial iron oxyhydroxides from the flow reactors were compared to iron oxyhydroxides that were artificially precipitated from the same feeder fluid. These abiotic and inorganic iron oxyhydroxides show the same REE+Y distribution patterns. Our results indicate that the REE+Y mirror quite exactly the water chemistry, but they do not allow to distinguish microbially mediated from inorganic iron precipitates. All TREE studied showed an overall similar fractionation behavior in biogenic, abiotic and inorganic iron oxyhydroxides. Exceptions are Ni and Tl, which were only accumulated in the microbial iron oxyhydroxides and may point to a potential usage of these elements as microbial biosignatures.

Organic Compounds and Conditioning Films Within Deep Rock Fractures of the Äspö Hard Rock Laboratory, Sweden

Palaeoproterozoic grano-dioritic rocks of the island of Äspö exhibit several mineralized fracture generations mainly filled by quartz, calcite, fluorite and/or epidote. Manganese-rich calcite fractures of probably Palaeozoic age are related to younger, possibly Pleistocene/Holocene cracks formed during the last ice age and successive crustal uplift, in contact to the host rock, which are sometimes associated with organic matter. Signals of organic molecules could be gained on the corresponding phase boundaries with Raman spectroscopy, likewise HPLC and HPAE-PAD reveal the presence of carbohydrates and amino acids in bulk rock samples. It is supposed that most of the preserved organic matter is related with thin conditioning films. Extracted bacterial and fungal DNA from the grano-dioritic rocks indicates still active microbial activity in fracture micro-niches.

Scanning Hard X-ray Microscopy Imaging Modalities for Geobiological Samples

Modern scanning X-ray microscopy can help to unravel the spatial context between biotic and abiotic compounds of geobiological assemblies with the aim to finally link chemical pathways to biological activities at the nanometre scale. This work presents some multi-modal imaging techniques provided by hard X-ray microscopes at synchrotron radiation sources to address analytical needs in geobiological research. Using the examples of 1) a calcified basal skeleton of the demosponge Astrosclera willeyana, 2) an anaerobic methane-oxidizing microbial mat and 3) a bacterial sulfur-oxidizing consortium, we illustrate the potential of scanning X-ray fluorescence and scanning transmission X-ray microscopy, and a novel quantitative approach of ptychographic imaging at single cell level.

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

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.