Biosignatures of Mineralizing Microbial Mats in a Deep Biosphere Environment

INTRODUCTION

The 450 m deep Tunnel of Äspö (Äspö Hard Rock Laboratory) in SE Sweden offers a unique window into the deep continental biosphere, where the growth of different microbial consortia strongly depends on ground water flow and chemical composition (Pedersen, 1997).

This contribution gives an overview of the geobiological setting and current experiments running in the Äspö Hard Rock Laboratory. Emphasis was placed on (i) long-term flow reactor experiments exploring the subsurface biofilm and biomineral formation under controlled conditions, and (ii) high resolution geochemical analyses of biosignatures in mineralized fracture fillings of the Äspö Diorite host rock.

MATERIALS & METHODS

(i) Sets of flow reactors were connected to three aquifers differing in chemical composition and fluid age. Over three to four years, the aquifers and flow reactors were continuously monitored for physiochemical fluctuations, microbial mat development, biomineralization, and potential organic and inorganic biosignatures using conventional geochemical methods. Trace and rare earth element (REE) concentrations were analyzed using laser ablation inductively coupled mass spectrometry (LA- ICP-MS).

(ii) In a second experiment, organic biosignature analyses were performed on fracture fillings in a drill core (KJ0052F01, SKB core library) taken from the surrounding host rock. Molecular analyses were performed and the signals were laterally resolved using time-of-flight secondary ion mass spectrometry (ToF-SIMS).

RESULTS

(i) In all flow reactors, microbially mediated iron oxidation was found to be a process of major importance, mostly due to the metabolism of the chemolithotrophic bacterium Gallionella ferruginea (Hallbeck et al., 1993). Along with the formation of iron minerals, trace and REE accumulated within the mineralizing microbial mats. After two, respectively nine months, the REE content in the microbial iron oxyhydroxides was found to be 10⁴- and 10⁶-fold enriched compared to the feeder fluids. Concentrations of Be, Y, Zn, Zr, Hf, W, Th, Pb, and U were 10³- to 10⁵-fold higher than those in the feeder fluids. The normalized REE patterns of the microbial iron oxyhydroxides clearly differed from published REE patterns of chemically precipitated iron oxyhydroxides.

(ii) Fracture fillings within the Äspö diorite frequently consist of a sequence of high-temperature fluorite and low-temperature calcite phases. ToF-SIMS analyses of the sample surfaces reproducibly revealed numerous organic compounds producing ions with masses as high as 325 Da within the fracture fillings. Molecular mappings indicated an indigenous origin of these organic substances from fossil biofilms, as they showed a very specific distribution, exactly lining the boundary between calcite and fluorite phases.

CONCLUSIONS

(i) The enormous trace and rare earth element enrichments highlight the efficiency of G. ferruginea mats in removing metals from the supplying water and point to the potential utility of these microbial systems for the recycling of precious trace metals and radionuclides. The distinctive trace and REE patterns observed may be utilized as "inorganic biosignatures" for iron oxidizing microbial systems in paleoenvironmental studies.

(ii) The results highlight the existence of fossil deep biosphere remains in the Äspö Diorite and demonstrate the utility of ToF-SIMS for the spatially resolved analyses of organic biosignatures in geobiological materials.

ACKNOWLEDGMENTS

We are grateful to Emmeli Johansson, Magnus Kronberg, Teresita Morales and the SKB Chemistry Lab staff for technical, logistic and analytical support at the Äspö HRL. Erwin Schiffzyk is acknowledged for his assistance with the ICP-OES measurements. Our study received financial support from the German Research Foundation (DFG) and the Swedish Governmental Agency for Innovation Systems (VINNOVA).