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Tracing ancient microbial cells embedded in silica

Periodic Reporting for period 4 - TRACES (Tracing ancient microbial cells embedded in silica)

Reporting period: 2019-11-01 to 2020-09-30

Problem being addressed:

Reconstructing the nature and habitat of early life is a difficult task that strongly depends on the study of rare microfossils in the ancient rock record. The preservation of such organic structures critically depends on rapid entombment in a mineral matrix. Throughout most of Earth’s history the oceans were silica-supersaturated, leading to precipitation of opal deposits that incorporated superbly preserved microbial cells. As we trace this record of life back in deep time, however, three important obstacles are encountered; 1) microorganisms lack sufficient morphologic complexity to be easily distinguished from each other and from certain abiologic microstructures, 2) the ancient rock record has been subjected to increased pressures and temperatures causing variable degradation of different types of microorganism, and 3) early habitats of life were dominated by hydrothermal processes that can generate abiologic organic microstructures. The goal of TRACES is to determine whether key types of fossilized microbial life can be distinguished from each other, and from abiological artifacts, in the oldest, most altered part of the rock record.

Importance for society:

TRACES will establish the experimental basis required for the unambiguous identification of microfossils in silica deposits, which is the key to reconstructing the early history of life on Earth. Overall this work will lead to a better understanding of the evolution of life.

Overall objectives:

TRACES is determining whether key types of fossilized microbial life can be distinguished from each other, and from abiogenic features, in progressively altered silica deposits. The entire range from recent hot spring silica sinters to diagenetic silica deposits to ancient hydrothermally altered and metamorphosed chert deposit are studied. This will provide a solid scientific basis for tracing life in the oldest, most altered part of the rock record.
The project consisted of 4 work packages that were designed to study the transformations that microorganisms undergo when they are subjected to silicification (WP-1), diagenesis (WP-2), metamorphism (WP-3) and hydrothermal processes (WP-4). Throughout the project laboratory alteration experiments were carried out, microbial communities in natural hot spring environments were studied, and microfossils in Proterozoic and Archean chert deposits were characterized. We made use of a wide range of in situ analytical techniques (including CLSM, Raman spectroscopy, SEM, TEM, synchrotron-based XRF, XANES, STXM, and micro-CT) to characterize our experimental and natural samples. Currently we are still working on several manuscripts of the various projects. We recently created a dedicated website ( that highlights all aspects of project TRACES, and contains links to all published papers. Below a few highlights of the 4 work packages are described.

Work packages 1 and 2 focused on silicification and early diagenesis of microbial communities in geothermal silica sinters. A large part of this research was based on extensive field work and drilling operations in the El Tatio geothermal field, Atacama Desert, Chile. In low-temperature zones we traced the progressive silica-entombment of individual sheathed filamentous cyanobacteria from the surface to the interior of an active geyser silica sinter, and subsequently recorded specific steps of degradation during artificial diagenesis in controlled autoclaves. In a second research component, field-based chemical depth profiling into living microbial mats enabled us to study the exact environmental conditions for silica-entombment and conversion to microbialite structures. The third research component focused on the preservation of microbial communities in high-temperature geothermal zones. For this purpose we studied biofilms, mats, and streamers occupying various niches in and along the edges of outflow channels of two geyser systems.

Work package 3 focused on progressive diagenesis and early metamorphism of microbial remains over short to long time spans (from thousands to billions of years). We carried out a core-drilling project in one of the 10-thousand-year old extinct geyser systems, and reconstructed the 3D-evolution of a hydrothermal system over several thousands of years. In the second research component we focused on the preservation of microfossils in Proterozoic cherts. The 1.0 Ga old Angmaat Formation, Baffin Island, Canada, has a complex diagenetic history, enabling comparison of low-temperature alteration effects on suites of microfossils. In-situ analytical techniques were combined to determine alteration steps of individual coccoidal cyanobacterial microfossils.

Work package 4 focused on life detection in the oldest, hydrothermally-influenced, silica deposits on Earth. In such rocks the organic traces of life are often ambiguous because there are many abiogenic microstructures with life-like morphologies - such as self-assembled ‘biomorph’ mineral aggregates and pore space networks – that mimic preserved microorganisms. In the first component of this work package we created silica-carbonate biomorphs in variable aqueous solutions and silica gels, and compared their morphologies with those of microbial cells. We then explored the use of statistical methods to distinguish populations of microfossils from populations of biomorphs and rock pore-networks, and then expanded this into a new quantitative approach for life detection.
The analytical and experimental research of work packages 1 and 2 is shedding light on nano-scale processes at the interface between organic materials and silica matrices, and is showing how only specific resistant components of cells are preserved and are contributing to the formation of biosignature fabrics in silica sinters (Gong et al., 2020). Our drilling project in an extinct geyser sinter, provided one of the first studies on 3D development of hydrothermal systems through time (Wilmeth et al., 2020; various papers in prep). This type coring technology has great potential for the future understanding of preservation of life in extreme environments.

The statistical methods for population morphometry that were developed during the project (Rouillard et al., Geobiology, 2020; Rouillard et al., Astrobiology, in press), provide an important new venue for life detection.