Researchers analyzing the DNA of a sediment core taken from the bottom of the Arctic ocean in 2010, have discovered a rather surprising fact about it.
A prior unknown organism coming from the same peculiar domain of microbes known as Archaea seemed to have genomic features linked with a completely different species, called Eukaryota.
They dubbed the finding Lokiarchaeota, but then doubt appeared to cover the discovery, as researchers asked themselves whether the organism could have actually been contaminated by something else in the core.
Now, due to the studies performed by Japanese scientists, those questions can be eliminated. For the first time ever, the team has isolated Lokiarchaeota and grown it in a laboratory. This means that researchers could study and interact with the living organism closely, which could help them find the very first ancestors of humans on Earth.
Te tree of life is basically split into three different domains. One of those is taken by bacteria or single-celled microbes that do not have a core, or organelles bound by membranes, and move around by waving structures similar to the hair, known as flagella.
Another domain is occupied by eukaryotes, which are organisms whose cells have a core and membranes. This category includes the humans, animals, plants, and algae. The third section is taken by archaea. These are similar to bacteria because they do now have nuclei and membrane-bound organelles, and move with the help of flagella. However, there are numerous differences between these two domains. They split; differently, their cell walls are constructed of different stuff, and their RNA is sufficiently different to separate them on the phylogenetic tree.
Then, Lokiarchaeota was discovered, along with other archaea species that showed eukaryotic features. These were dubbed Thorarchaeota, Odinarchaeota, and Heimdallarchaeota. Jointly, they are called the Asgard archaea, and some researchers believe they could be the very source of eukaryotic life, probably after an Asgard-like archaeon consumed a bacterium.
However, this is hard to tell without analyzing the organisms in isolated specifics. The Japanese scientists have collected a sediment core from the floor of the Nankai Trough, 2,533 meters (8,310 feet below sea level, back in 2006.
The Origins of Life on Earth
The team grew the samples for five years, in a methane-fed continuous-flow bioreactor system created to mirror the settings of a deep-sea methane vent. The microbes multiplied, and the team places samples from the bioreactor in glass tubes with food to keep them growing. After a year, the organisms began developing a very dim population of Lokiarchaeota. “Repeated subcultures … gradually enriched the archaeon with extremely slow growth rate and low cell yield,” the researchers wrote in their paper.
“The culture consistently had 30-60 days of lag phase and required over three months to reach full growth [..] Variation of cultivation temperatures, and substrate combinations and concentrations did not significantly improve the lag phase, growth rate or cell yield.”
They made numerous peculiar discoveries. The first is that Prometheoarchaeum would only multiply when two or three other microbes were present – the archaeon Methanogenium and the bacterium Halodesulfovibrio. When Prometheoarchaeum splits amino acids into the nutrients, it produces hydrogen, which the other microbes consume.
Overall, the archaea has a commensal relationship with other microbes, which means the growth of one specimen or both relies on what the other eats. Later, when the microbe was analyzed under an electron microscope, it showed a weird shape for an archaeon: long tentacles springing from its body, within which the other microbes huddled.
The scientists theorized that when oxygen levels started rising on Earth, this organism could have swapped to a relationship with bacteria that used oxygen, growing the possibilities of survival, and creating a new path, the one towards the eukaryotic life.
“This is a monumental paper that reflects a tremendous amount of work and perseverance,” evolutionary microbiologist Thijs Ettema of Wageningen University, who wasn’t associated with the paper, told Nature in August 2019.
“It’s a major step forward in understanding this important lineage.”