This strange organism from the depth could hold secrets about the origins of complex life on Earth

Illustration of a eukaryotic cell. They evolved from single-celled organisms about 2 billion years ago. | Shutterstock

A mysterious microbe discovered in the depths of the Pacific Ocean could hold the secrets of the evolution of the first muticellular life forms. Indeed, according to a new study, the little “tentacle organism” could tell us about what allowed life to become more complex during the early stages of evolution.

Long before complex living things existed, the world was home to simple single-celled organisms, archaea and bacteria. Between 2 and 1.8 billion years ago, these microorganisms began to evolve, leading to the emergence of more complex life forms, called eukaryotes.

The field of eukaryotes today includes humans, animals, plants and fungi. But this incredible evolution from aquatic life to life out of the water, then to walking - and for more advanced beings, thought and feeling, is still poorly understood by scientists.

The Asgard archaea, potential ancestors of eukaryotes

In a previous study, biologists hypothesized that an archival group called Asgard archaea (the archaea of ​​Asgård) regrouped the much sought-after ancestors of eukaryotes because they contain genes similar to their complex counterparts.

To analyze what these microbes looked like and how this transition could have happened, a group of Japanese researchers then spent a decade collecting and analyzing the mud from the bottom of Omine Ridge, off the coast of Japan. The results of the study were published on January 15 in the journal Nature.

The team stored the mud samples (and the microorganisms found there) in a special bioreactor, in a laboratory environment that mimics the conditions of the deep sea in which they were found.

Years later, they began to isolate the microorganisms in samples. The researchers' original goal was to find methane-consuming microbes that might be able to clean up the wastewater, according to the New York Times . But when they discovered that their samples contained an unknown strain of Asgard archaea, they decided to analyze it and grow it in the laboratory.

These images obtained by scanning electron microscopy show (A) an isolated archaea, (B) several cells developing together in the laboratory archaea, and (C&D) with tentacle-like protrusions, which develop towards the end of their growth. Credits: Japan Agency for Marine and Land Science and Technology (JAMSTEC)

Prometheoarchaeum syntrophicum , a new strain of Asgard archaea

They named the newly found strain of Asgard archaea “ Prometheoarchaeum syntrophicum “, after the Greek god Prometheus, who is said to have created humans out of mud, according to mythology.

The researchers found that these archaea were grown relatively slowly, doubling only every 14 to 25 days. Their analysis then confirmed that P. syntrophicum had a large number of genes similar to those of eukaryotes. In fact, these genes held the instructions to create certain proteins present inside these microbes. However, as expected, proteins did not create organelle-like structures like those found inside eukaryotes.

They also found that the microbes had long, branched, tentacle-like protrusions on the outside that could be used to catch passing bacteria. Indeed, the team discovered that microbes tended to stick to other bacteria in the petri dishes.

An elegant hypothesis explaining the changes in P. syntrophicum

The authors offer an interesting hypothesis for what happened in these ancient waters, which they explain in a recently published video (available at the end of the article): about 2.7 billion years ago, oxygen a started to accumulate on Earth. But having lived so long in a world without oxygen, this element would prove to be toxic for P. syntrophicum .

So P. syntrophicum may have developed a new adaptation: a way of forming bonds with oxygen-tolerant bacteria. These bacteria would have provided P. syntrophicum with the vitamins and compounds necessary to survive, while in turn feeding on archaeal waste.

As oxygen levels increased even more, P. syntrophicum could have become more aggressive, uprooting passing bacteria with its long tentacle-like structures and integrating them. Inside P. syntrophicum , this bacterium would eventually have evolved into an energy-producing organelle, the key to eukaryotic survival: the mitochondria.

The success of the team in the culture of Prometheoarchaeum , after efforts spanning more than a decade, “represents a huge breakthrough for microbiology”, writen by an accompanying editorial Christa Schleper and Filipa L. Sousa, two researchers from the University of Vienna, who did not participate in the study. " It opens the way to the use of molecular and imaging techniques to further elucidate the metabolism of Promethéoarché and the role of eukaryotic signature proteins in biology of Archean cells " they added.


Isolation of an archaeon at the prokaryote–eukaryote interface

Hiroyuki Imachi, Masaru K. Nobu, Nozomi Nakahara, Yuki Morono, Miyuki Ogawara, Yoshihiro Takaki, Yoshinori Takano, Katsuyuki Uematsu, Tetsuro Ikuta, Motoo Ito, Yohei Matsui, Masayuki Miyazaki, Kazuyoshi Murata, Yumi Saito, Sanae Sakai, Chihong Song, Eiji Tasumi, Yuko Yamanaka, Takashi Yamaguchi, Yoichi Kamagata, Hideyuki Tamaki & Ken Takai

Nature volume 577, pages519–525(2020)

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