Earth’s Earliest Lifeforms

Viruses may be the missing piece of the puzzle that could help explain how soft microbial mats transition into hard stromatolites that are prevalent in such places as Shark Bay and the Pilbara in Australia.

Stromatolites at Shark Bay, Western Australia. Image credit: Paul Harrison / CC BY-SA 3.0.

Stromatolites at Shark Bay, Western Australia. Image credit: Paul Harrison / CC BY-SA 3.0.

Stromatolites are geobiological systems formed by complex microbial communities, and fossilized stromatolites provide a record of some of the oldest life on Earth.

Microbial mats are precursors of extant stromatolites. However, the mechanisms of transition from mat to stromatolite are controversial and are still not well understood.

To fully recognize the profound impact that these ecosystems have had on the evolution of the biosphere requires an understanding of modern lithification mechanisms and how they relate to the geological record.

“Stromatolites are one of the oldest known microbial ecosystems, dating back some 3.7 billion years,” said Dr. Brendan Burns, a researcher in the Australian Centre for Astrobiology and the School of Biotechnology and Biomolecular Sciences at the University of New South Wales.

“They are pervasive in the fossil record and are some of our earliest examples of life on Earth.”

“The microbial mats that created them were predominantly made up of cyanobacteria, which used photosynthesis — like plants do — to turn sunlight into energy, while producing so much oxygen over time they changed the early Earth’s atmosphere to make it habitable for complex life. You could say we owe our very existence to these living rocks.”

Dr. Burns and colleagues wanted to understand the mechanism behind microbial mats lithifying into stromatolites, not only because so little is known about the process, but because of what this could add to our knowledge about life on Earth — and possibly other planets.

They postulate that microbial mat transition from soft cells to rock is enhanced by interactions with viruses.

“We propose viruses may have a direct or indirect impact on microbial metabolisms that govern the transition from microbial mat to stromatolite,” Dr. Burns said.

“In the direct impact scenario, viruses infiltrate the nucleus of the cyanobacteria and influence the host metabolism, inserting and removing genes that increase the fitness of the virus and the host at the same time.”

“This, in turn, increases survival of the microbial mat and selects for genes that potentially influence carbonate precipitation — basically the process of microbes pouring the concrete to make their stromatolite apartment blocks.”

In the indirect scenario, in a process known as viral lysis, viruses invade living cells and trigger the disintegration of their membranes and release of contents.

“We think viral lysis may release material that promotes metabolism of organisms which results in mineral precipitation and eventual stromatolite formation,” Dr. Burns said.

“We want to be able to identify what viruses are actually involved and see if we can then manipulate potential virus-host interaction to find out whether or not they can, in fact, change some of the metabolisms that might result in stromatolite formation.”

The team’s paper was published in the journal Trends in Microbiology.

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Richard Allen White III et al. 2021. Between a Rock and a Soft Place: The Role of Viruses in Lithification of Modern Microbial Mats. Trends in Microbiology 29 (3): 204-213; doi: 10.1016/j.tim.2020.06.004

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