The fossilization of microorganisms in sedimentary environments

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To be recognized as evidence of life, microbial fossils must contain chemical and structural attributes that are unique indicators of microbial cells or cellular processes. In spite of the diversity of potential biosignatures, it remains enormously challenging to demonstrate the biogenicity of microfossils and microbial sedimentary structures. There is, however, a growing consensus that microorganisms contribute greatly to the development of sedimentary deposits. I propose to focus in particular on iron-rich sedimentary rocks to look for evidence that microorganisms had a key role in their formation. Iron is an essential element for all life, which has evolved an intimate dependency on iron's unique chemistry. For the iron-oxidizing and iron-reducing bacteria, however, iron plays an additional key role in respiration. The change in iron's chemical state consequent to respiration leads to the formation of new mineral phases. Reactive iron may also be generated indirectly through the activity of sulfate-reducing bacteria. Regardless of the source, it appears that under some circumstances iron acts to preserve the bacterial cell wall, leading to entombment and eventual fossilization. I propose to investigate the role of iron and sulfate respiration in the preservation of bacteria and the genesis of microbial fossils, together with the graduate and undergraduate students that my NSERC Discovery grant will support.