Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - TRACES (Tracing ancient microbial cells embedded in silica)

Teaser

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...

Summary

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. The question will be answered whether the preservation of an ecosystem is biased towards a certain group of microorganisms or to specific favourable environmental conditions. Critical nano-scale differences will be determined between microfossils and abiologic artefacts in hydrothermal settings. The knowledge gained from this research can also be applied to the younger fossil record because it provides a firm basis for any taphonomic study of organic remains in mineral matrices. Overall this work will lead to a better understanding of the evolution of life.

Overall objectives:

TRACES is studying the critical transformations that occur when representative groups of microorganisms are subjected to artificial silicification and thermal alteration. At incremental steps during these experiments the (sub)micron-scale changes in structure and composition of organic cell walls are monitored. This will be compared with fossilized life in diagenetic hot spring sinters and metamorphosed Precambrian chert deposits. The combined work will lead to a dynamic model for microfossil transformation in progressively altered silica-matrices. The critical question will be answered whether certain types of microorganisms are more likely to be preserved than others. In addition, the critical nano-scale structural differences will be determined between abiologic artefacts – such as carbon coatings on botryoidal quartz or adsorbed carbon on silica biomorphs – and true microfossils in hydrothermal cherts. This will provide a solid scientific basis for tracing life in the oldest, most altered part of the rock record.

Work performed

TRACES started in May 2015. The first months of the project were focused on recruitment of personnel, and project and field work planning. Work packages 1, 2 (postdoctoral researcher Jian Gong, MSc. Student Myriam Agnel) and work package 4 (Ph.D. student Joti Rouillard, M.Sc. student Selmia Esselma) were then initiated.

The aim of WP-1 is to study the compositional variation of key microbial cell wall types as they are subjected to artificial silicification, and to compare these characteristics with those of silicified microbial communities in natural hot spring sinters. In order to achieve this, we have successfully cultured various cyanobacterial and algal cell types and have worked out different protocols of silicification. These steps of silica entombment are now being studied using time-lapse optical microscopy, Raman spectroscopy, SEM and TEM. In the summer of 2016 and spring of 2017 we successfully carried out field work in Iceland and Chile respectively, and obtained natural silica sinters that contain entombed cyanobacteria. We are currently comparing these natural samples with our artificially produced silica-entombed cyanobacteria. These detailed observations have generated insights into the many factors that influence the initial preservation of microbial cells in silica deposits.

The aim of WP-2, which started in 2016, is to study the changes in structure and composition of silicified microorganisms at different stages of artificial diagenesis, and to compare these characteristics with those of altered microbial communities in recrystallized hot spring sinters. In order to achieve this, we have carried out long duration alteration experiments, in dedicated high-P,T autoclaves, of natural silica sinter samples from Chile as well as artificially silicified cultures of cyanobacteria from WP-1. Our results show how specific components of cyanobacteria were preferentially preserved during progressive steps of silica recrystallization from opal-A to opal-CT. Also, for comparison, we have studied a vertical sinter profile from Iceland, that represents the diagenetic transition from opal-A to opal-CT.

The aim of WP-4 is to study the structure and composition of artificially metamorphosed organic carbon-enriched silica biomorphs, and compare these characteristics with those of artificially metamorphosed silica-entombed microorganisms and natural organic microstructures in Archean hydrothermal cherts. In order to achieve this, we have generated a wide variety of silica-carbonate biomorphs under a range of chemical conditions and have studied them in detail using optical microscopy and SEM. For instance we were able to generate spherical or filamentous structures that strongly resemble microbial cells. In the summer of 2016 we also successfully carried out field work in the Western Australia, and collected a suite of hydrothermally-influenced Archean cherts. Throughout 2017 we have studied the characteristics of putative microfossils and carbonaceous materials in several of these samples, using Raman spectroscopy, SEM, and TEM. This work is critical for microfossil identification in the oldest rock record on Earth.

Final results

The proposed work will generate a wealth of information on the reactions and interfaces between organic compounds and silica matrices. It is thus envisioned that some of the structures produced in the experiments could lead to new bio-inspired hybrid materials.