Mineralized

Based on our analyses and a comparison of our findings with modern hyperthermophilic biofilm remnants, we propose that the threads and curved films are the mineralized remains of microbial biofilms. We have also found that this

Extracellular Polymeric Substances Eps

Figure 11. Examples of remnants of modern hyperthermophilic biofilms collected from near-boiling hot springs, Yellowstone National Park, USA. The biofilm remnants consist primarily of extracellular polymeric substances (EPS) that give enough coherency and flexibility to the biofilm matrix to allow it to remain attached at several places to the mineral substratum even after some amount of contraction and fluid-movement induced disruption. (a) Note the attachment points of the extracellular biofilm matrix on the substratum. The flexible biofilm matrix has also curled back around itself along some of its edges. (b) Biofilm matrix attached to underlying substratum with several convergent and distinct "threads" characterized by a range of thicknesses. For this particular microbial community, the EPS was attached at numerous points by polymers that bifurcate and become more numerous toward the center of the biofilm. (c) An SEM micrograph reveals the same distinct stretched, torn, curved-film characteristics displayed by the ancient biofilm remnants discovered in the hydrothermal veins in the Siljan Impact Structure.

Figure 11. Examples of remnants of modern hyperthermophilic biofilms collected from near-boiling hot springs, Yellowstone National Park, USA. The biofilm remnants consist primarily of extracellular polymeric substances (EPS) that give enough coherency and flexibility to the biofilm matrix to allow it to remain attached at several places to the mineral substratum even after some amount of contraction and fluid-movement induced disruption. (a) Note the attachment points of the extracellular biofilm matrix on the substratum. The flexible biofilm matrix has also curled back around itself along some of its edges. (b) Biofilm matrix attached to underlying substratum with several convergent and distinct "threads" characterized by a range of thicknesses. For this particular microbial community, the EPS was attached at numerous points by polymers that bifurcate and become more numerous toward the center of the biofilm. (c) An SEM micrograph reveals the same distinct stretched, torn, curved-film characteristics displayed by the ancient biofilm remnants discovered in the hydrothermal veins in the Siljan Impact Structure.

particular combination of specimen preparation and analysis techniques, which includes specimen etching, FE-SEM, and Rutherford backscattering analyses, is a particularly effective means by which to reveal microbial biosignatures preserved within hydrothermal minerals that precipitated as fracture filling mineral assemblages. Though the curved and flexible biofilm-like remnants appear to have formed and become preserved in situ, the filamentous-like remains appear to have been transported some distance, as evidenced by the twisted curvi-linear nature of the thread-like objects that have been twisted around one another. In any case, the fact that the objects are embedded in the hydrothermal minerals that fill the calcite veins underscores the importance of impact-related hydrothermal systems as possible paleobiological repositories of subsurface biofilm remnants.

As noted below, we found no evidence that would indicate that the biofilm-like objects were produced abiologically (e.g., Garcia-Ruiz et al., 2002; Holm and Charlou, 2001) or that they represent calcified insoluble residues of oil.

Our hypothesis, that impact-related hydrothermal deposits are important paleo-biological repositories that will preserve evidence of allocthonous and autocthonous microbial biofilm remains, is supported by the following key findings:

1. The features are structurally and texturally complex, and display morphological attributes known only to be produced by living microbial communities. Experiments designed to produce biological-like objects abiologically have not resulted in the production of curved films with flexible structures and multiple attachment points such as those presented here (J.M. Garcia-Riuz, personal communication, 2005). The bifurcation of threads at attachment points, the curved and flexible morphology of the films, the elliptical perforations in films stretched between attachment points, the morphological fidelity of the objects exposed by etching; all of these attributes support our hypothesis that the features are remnants of objects produced by living microbial communities that thrived as thin microbial biofilms on mineral precipitates within the impact-induced fractures.

2. The mode of occurrence and the partly embedded nature (e.g., Fig. 5a) of the features indicate that they formed while the hydrothermal system was active. No mineral alteration or secondary infilling of the fracture post-hydrothermal fluid activity has occurred (Hode et al., 2003), nor is there any evidence of such biogenic-like features in the control (non-etched) sample, which would be consistent with the accumulation of such features having grown on the surface of the samples (i.e., in the field or the laboratory). The fact that the biogenic-like features were partly-to-fully mineralized (calcite or pyrite) indicates that the features originated in a reducing and actively mineralizing environment, which eliminates the possibility that the biogenic-like features were produced by modern endoliths.

3. RBS analysis indicated that the calcified biogenic-like features are enriched in carbon relative to the surrounding mineral matrix. The fact that enhanced concentrations of carbon were found in association with objects that display morphological characteristics like those of biofilm remnants is only consistent with a biological origin for the features. The anoxic and reducing character of the hydrothermal fluids that circulated through fractures in the impact structure (i.e. evidenced by the mineral assemblage within the veins) would have enhanced the preservation of organic remains that eventually converted to the kerogenate breakdown products of a biofilm matrix during mineralization.

4. The geological setting, while the hydrothermal system was active, could have supported a hyperthermophilic community such as those known to produce extensive biofilms on mineral surfaces in hot springs (e.g., Cady and Farmer, 1996). The 90-110°C fluid was anoxic, reducing, and rich in organics, and reduced sulphur phases (Hode et al., 2003). It may also have supported an ancient subsurface hyperthermophilic community. Isotopic evidence suggests that Pb and Zn were leached from surrounding granitic rocks (Johansson, 1984) and deposited as galena and sphalerite as they entered the limestone. Together with ferrous iron, these reduced metal ions may have acted as electron donors. Molecular hydrogen could also have served as an electron donor for a subsurface chemolithoautotrophic microbial community (e.g., Stevens and McKinley, 1995). Modern subsurface hydrothermal systems that display similar chemical and physical properties are known to support chemolithoau-totrophic communities (e.g. Takai et al., 2002; McKinley et al., 2000; Stevens and McKinley, 1995).

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