Asgard archaea (Asgardarchaeota) are considered to be the closest known relatives of eukaryotes, organisms (including animals, plants and fungi) whose cells contain a nucleus and other membrane-bound organelles. Their genomes contain hundreds of eukaryotic proteins, which inspired hypotheses on the evolution of the eukaryotic cell. In new research, scientists from the University of Vienna, ETH Zürich and the Ludwig-Maximilans-Universität München were able to cultivate an Asgard archaeon called ‘Candidatus Lokiarchaeum ossiferum’ and characterize its cellular architecture using cryo-electron tomography.
Scanning electron micrograph of a ‘Candidatus Lokiarchaeum ossiferum’ cell showing the long and complex cell protrusions. Image credit: Thiago Rodrigues-Oliveira, University of Vienna.
“Soon after the discovery of archaea as a separate lineage besides bacteria, molecular and phylogenetic studies suggested that there is a deep common evolutionary descent between archaea and eukaryotes,” said University of Vienna researcher Thiago Rodrigues-Oliveira and colleagues.
“However, only recently has the discovery of the first Lokiarchaeia and the wider superphylum of Asgardarchaeota corroborated a distinct relationship and a possible direct emergence of eukaryotic cells from archaea.”
“In fact, eukaryotes form a direct sister group to Asgard archaea or even arise within this phylum in most phylogenomic analyses.”
In the research, the scientists succeeded in cultivating a representative of Asgard archaea in high concentrations.
Named ‘Candidatus Lokiarchaeum ossiferum,’ this microorganism was isolated from a shallow sediment of a small estuarine canal that regularly receives water from the Mediterranean near the coast of Piran, Slovenia.
It thrives anaerobically at 20 degrees Celsius on organic carbon sources, divides every 7-14 days, reaches cell densities of up to 50 million cells per ml, and has a significantly larger genome compared with ‘Candidatus Prometheoarchaeum syntrophicum,’ the single previously cultivated Asgard strain.
“It was very tricky and laborious to obtain this extremely sensitive organism in a stable culture in the laboratory,” Dr. Rodrigues-Oliveira said.
Using a modern cryo-electron microscope, the authors imaged shock-frozen cells of ‘Candidatus Lokiarchaeum ossiferum.’
“This method enables a three-dimensional insight into the internal cellular structures,” said Dr. Martin Pilhofer, a researcher at ETH Zurich.
“The cells consist of round cell bodies with thin, sometimes very long cell extensions,” added Dr. Florian Wollweber, also from ETH Zurich.
“These tentacle-like structures sometimes even seem to connect different cell bodies with each other.”
According to the team, the cells of ‘Candidatus Lokiarchaeum ossiferum’ contain an extensive network of actin filaments thought to be unique to eukaryotic cells.
This suggests that extensive cytoskeletal structures arose in archaea before the appearance of the first eukaryotes and fuels evolutionary theories around this important and spectacular event in the history of life.
“Our new organism, ‘Candidatus Lokiarchaeum ossiferum,’ has great potential to provide further groundbreaking insights into the early evolution of eukaryotes,” said Dr. Christa Schleper, a researcher at the University of Vienna.
“It has taken six long years to obtain a stable and highly enriched culture, but now we can use this experience to perform many biochemical studies and to cultivate other Asgard archaea as well.”
The research is described in a paper in the journal Nature.
_____
T. Rodrigues-Oliveira et al. Actin cytoskeleton and complex cell architecture in an Asgard archaeon. Nature, published online December 21, 2022; doi: 10.1038/s41586-022-05550-y
