In the Paleoproterozoic era, about two billion years ago, eukaryotes appeared on Earth – a phylogenetic line of all living creatures that have cells with a nucleus. These single-celled creatures lived on a star very different from the one known to everyone. It was a planet with red waters punctuated by patches of exposed land. This very specific color has been associated with an event called the “Great Oxidation” which resulted in a spike in atmospheric oxygen due to the proliferation of cyanobacteria that produce this molecule through photosynthesis.
Most living creatures of that time could not consume oxygen, so it acted like a poison on the vast majority of living things, leading to mass extinction and at the same time turning the ocean red, oxidizing the iron particles that were there. Eukaryotes were perfectly adapted to this new environment as they could consume oxygen to produce CO2, but their exact origins are shrouded in mystery. However, many recent discoveries have shed light on several plausible scenarios.
In a study published in the journal Nature, researchers from the universities of Zurich, Switzerland, and Vienna, Austria, painted a portrait of one of the protagonists of the two scenarios that received the most recognition from the scientific community. They both have the same protagonists, archaea and bacteria, and come to the same conclusion: the emergence of a complex cell with a nucleus and organelles. In this publication, the researchers focused on the archaeal species Candidatus Lokiarchaeum ossiferum, which are part of the supertype Asgard.
2 scenarios with the same conclusion: eukaryogenesis
In the Paleoproterozoic era, about two billion years ago, eukaryotes appeared on Earth – a phylogenetic line of all living creatures that have cells with a nucleus. These single-celled creatures lived on a star very different from the one known to everyone. It was a planet with red waters punctuated by patches of exposed land. This very specific color has been associated with an event called the “Great Oxidation” which resulted in a spike in atmospheric oxygen due to the proliferation of cyanobacteria that produce this molecule through photosynthesis.
Most living creatures of that time could not consume oxygen, so it acted like a poison on the vast majority of living things, leading to mass extinction and at the same time turning the ocean red, oxidizing the iron particles that were there. Eukaryotes were perfectly adapted to this new environment as they could consume oxygen to produce CO2, but their exact origins are shrouded in mystery. However, many recent discoveries have shed light on several plausible scenarios.
In a study published in the journal Nature, researchers from the universities of Zurich, Switzerland, and Vienna, Austria, painted a portrait of one of the protagonists of the two scenarios that received the most recognition from the scientific community. They both have the same protagonists, archaea and bacteria, and come to the same conclusion: the emergence of a complex cell with a nucleus and organelles. In this publication, the researchers focused on the archaeal species Candidatus Lokiarchaeum ossiferum, which are part of the supertype Asgard.
2 scenarios with the same conclusion: eukaryogenesis
The living world is divided into two super-kingdoms: prokaryotes, consisting of bacteria and archaea, and eukaryotes. However, archaea are genetically much closer to eukaryotes than to bacteria, and among them it is the asgardians that are closest to cells with a nucleus on the phylogenetic tree. Recent work concerning the origin of eukaryotes goes even further and states that they evolved from the ingestion of bacteria by archaea, probably belonging to the supraphylum Asgard, and that eukaryotes must have evolved within the archaean kingdom itself.
The scenarios describing this phenomenon are named: the “inside out” model and the E3 hypothesis (entanglement – absorption – internalization). In the “inside out” model, bacteria found on the surface of ancestral archaea would be gradually taken up by the ancestral archaea. This would form structures external to the cell membrane called “bubbles” to feed the bacteria. Little by little, by expanding their outer shell, the archaea completely captured the bacteria inside their body. They then evolved into organelles such as mitochondria, which are the energy factories of eukaryotic cells. Over time, the remains of an archaeal organism found in its ancient inner membrane developed into a nucleus, giving rise to the modern eukaryotic cell structure.
In the E3 scenario, things happen a little differently: it is the extending protrusions of the archaea that entangle the bacterium in order to take advantage of its ability to consume toxic oxygen and turn it into nutrients for the eukaryotic ancestor. This symbiosis then turned into endosymbiosis as the archaea completely engulfed the bacterium. This evolved into an organelle, and the rest of the archaeal organism in its inner membrane became the nucleus of a primitive eukaryotic cell.
Detailed analysis of archaea frozen with liquid nitrogen
By observing Candidatus Lokiarchaeum ossiferum in detail, the researchers were able to better understand how this endosymbiosis phenomenon could have occurred almost two billion years ago. To get a detailed picture of this organism, scientists had to be patient and use high-precision instruments. This research has gone the furthest in describing the Asgardian superphylum organism. “Candidatus Lokiarchaeum ossiferum develops in much larger numbers than previously studied species, so we were able to study its cellular structure in detail using electron cryotomography and other microscopy methods,” Florian Wollweber, co-author of the study, explains Science et Avenir. and specialist in cellular evolution at the Institute for Molecular Biology and Biophysics at the University of Zurich.
Electronic cryotomography allows researchers to obtain high-quality 3D images of the organisms they observe by freezing them with liquid nitrogen. Scientists can then observe these microscopic ice structures from all angles. It was with this precise technology that scientists were able to discover the common ground between eukaryotes and Asgardian archaea. Both possess a cytoskeleton composed of actin, called lokiactin in the Asgardian archaea. This cell scaffold allows eukaryotes to perform many key cellular movements, such as mitosis, the division of a mother cell into two daughter cells, or the emission of filopodia, deformations of the cell membrane that allow cells to move.
In Candidatus Lokiarchaeum ossiferum, the researchers noticed that lokiactin allows archaea to form long projections, extensions of the outer membrane of prokaryotes, similar to those proposed by the inside-out and E3 models. The scientists also note that the genes encoding lokiactin have been found in all species of Asgardian archaea studied to date.
Asgardian archaea have rekindled the tree of life debate 🦠 But do they have eukaryotic traits? YES!! 🤩 Check out our new @Nature article by @F_Wollweber and @xujw_thu in collaboration with @Archaea_Vienna’s Schleper Lab. https://t.co/dJDCFKXpn1
🧵1/4 pic.twitter.com/egZXjNru1Z— Pilhofer Lab (@PilhoferLab) December 21, 2022
A tweet from the Candidatus Lokiarchaeum ossiferum lab showing detailed observations of this archaea.
A discovery that calls for new
This new discovery required a particularly long cultivation time, as did all experiments with new species of Asgardian archaea. “It took six long years to get a stable and very rich culture of this archaea species, but now we can use this experience to conduct many biochemical studies and cultivate other Asgardian archaea,” writes enthusiastically Florian Wollweber in the journal of the University of Vienna.
While this study is particularly accurate, it only scratched the tip of the iceberg—the emergence of eukaryotes. Scientists will have to study the archaea of Asgard from all sides in order to unravel the mystery of eukaryogenesis. “Our paper represents the first description of the basic cellular processes of Candidatus Lokiarchaeum ossiferum, but there are still many questions regarding its cellular physiology. For example: how its cells divide, how its shape changes are organized, and how this archaea behaves with other organisms in a culture medium,” explains Florian Wollweber for Sciences et Avenir.
Although his team is focused on research on Candidatus Lokiarchaeum ossiferum, the researcher believes that new data regarding the superphylum Asgard may shed more light on the origin of the group of cells in which multicellularity developed. “We can hope that different species of Asgard can be studied with the same level of detail. Some lineages of this supertype are even closer to the ancestors of eukaryotes, and it would be interesting to see if they show even more complex structure,” the researcher enthusiastically notes. specialist.