The Oldest Fossils Of Methanogenic Bacteria Discovered (Paleontology)

By analyzing the rocks present in the Unesco site known as the Barberton Green Rock Belt, in South Africa, one of the oldest geological structures in the world, a team of researchers led by Barbara Cavalazzi of the University of Bologna has discovered the oldest remains fossils of methanogenic archaea – microorganisms that lived 3.42 billion years ago within a system of hydrothermal venules

There is a place on our planet where one of the oldest geological complexes in the world is located. A place where the rocks present, of volcanic and sedimentary origin, tell how the Earth was between 3.2 and 3.6 billion years ago. We are talking about the Barberton Green Rock Belt : an area located in South Africa, included in the Unesco World Heritage List since 2018 – the list of World Heritage Sites.

It is an environment with unique chemical-physical characteristics, and therefore ideal for astrobiology studies, aimed at searching for life forms that existed long ago, when the Earth was very young. And it is precisely by conducting similar studies that an international team of researchers led by the University of Bologna, analyzing rock samples taken on the site, discovered the oldest remains ever identified of methanogenic bacteria , ancestral microorganisms capable of converting molecular hydrogen and carbon dioxide into methane through a process known as  methanogenesis . A discovery that expands the frontiers of potentially habitable environments on early Earth and other planets, such as Mars.

According to what is reported in the article describing the discovery, published yesterday in Science Advances , it would be archaea that lived 3.42 billion years ago in a system of venules carved into the rock by the activity of hydrothermal springs – environments in which the interaction of the seabed water with hot water heated by volcanic activities creates suitable conditions for the life of various microorganisms.

“We found exceptionally well-preserved evidence of microbial fossils that appear to have thrived along the walls of cavities created by the warm water of hydrothermal systems present a few meters below the seabed,” explains Barbara Cavalazzi , geologist and astrobiologist at the University of Bologna. and first author of the study. “These subsurface habitats, heated by volcanic activity, have probably hosted some of the earliest microbial ecosystems on Earth, and this is the oldest example we have found to date.”

Image of the outcrop from which the rock sample examined in the study was taken. Credits: Cavalazzi et al., 2021

That these are precisely fossilized remains of bacteria are suggested by the chemical analyzes that the researchers conducted on filamentous structures, similar to a biofilm , found in two thin layers of the rocks examined. The results show that the filaments include most of the structural elements necessary for life: a carbon-rich outer coating, consistent with a cell wall  or with what remains of the extracellular polymeric substance , substances secreted by microorganisms in their environment and considered the Fundamental component that determines the physico-chemical properties of a biofilm; and a chemically distinct core structure which may be condensed cytoplasmic matter .

That they are, more specifically, methanogenic archaea is indicated by the presence in the filaments of organic compounds of nickel, a cofactor of enzymes involved in the metabolism of methane, the organic molecule which, as mentioned, these microorganisms produce and use as a source of “food”. The concentrations of nickel measured in the filaments, the researchers explain, are similar to those found in the methanogens that now populate some extreme environments of our planet.

Filamentous microfossils observed under an optical microscope. Credits: B. Cavalazzi

The specific combination of the site where they were observed, their morphological complexity in three dimensions, their kerogenic nature (kerogen is a mixture of chemical compounds produced by the decomposition of organic matter), the spectroscopically observed ultrastructures and their specific metal signature -organic, the researchers add, exclude that they may be abiotic pseudofossils , that is structures produced by natural physical and chemical processes, therefore of a non-biological nature, which however resemble fossils of primitive life forms.

“We know that archaea can be fossilized, however we have few direct examples,” concludes Cavalazzi. “Our findings could extend, for the first time, the Archaea fossil record to the time when life emerged on Earth.”

Featured image: The Barberton Belt of Green Rocks, South Africa. Credits: A. Hofmann.

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Provided by INAF

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