Small organism, big answers?

Louise Elliott on efforts to study the origin of living things as we know them.

Louise Elliott
2nd March 2020
A research group from the Japan Agency for Marine-Earth Science and Technology in Yokosuka, led by Hiroyuki Imachi, have grown a new lineage of archaea that could provide insight into how eukaryotic cells evolved.

Archaea are single cellular organisms that are similar to bacteria in size and shape but are evolutionarily distinct. Initially they were thought to be a form of bacteria, but isolation of their genes and investigation of their metabolic pathways showed that they were more closely related to eukaryotes. A 2015 study by Spang et al., discovered an Asgard archaea: Lokiarchaeota. Their research found that genetically, the Asgard phylum are the closest prokaryotes (lacking a distinct nucleus) to eukaryotes.

A leading model of how eukaryotes developed from prokaryotes is the symbiogenic model. Simply, it is when two prokaryotic cells merge and work together. The idea that this is how eukaryotes began to evolve is supported by the fact that mitochondria and chloroplasts have no distinct nuclei but have their own form of genetic systems and DNA; suggesting they may have once been prokaryotic. Imachi et al. wanted to learn more about the evolution of eukaryotes by cultivating an Asgard archaea specimen that could be studied.

After 12 years of work, group have successfully grown a new lineage of the Asgard phylum; which they have named Prometheoarchaeum syntrophicum. This process was long and extremely complex. The first major challenge was collecting a sample that would contain these ancient eukaryotic ancestors. These microorganisms thrive in cold, oxygen-poor extreme environments, such as those found in the deep-sea mud. The group dove 2,500 meters under the pacific in a small submarine to the bottom of Japan’s Nankai Trough to collect a sample of mud. But this was just the beginning.

These microbes, because they live in such extreme environments, are very high maintenance in the lab; to successfully culture these archaea the team had to re-create these environments. Initially they used a bioreactor continually pumping methane, followed by a culture with a wide range of peptides, amino acids and other nutrients including baby milk powder! They also had to administer antibiotics to the samples to stop any contamination from bacteria. Eventually, after an enormous amount of trial and error, they have successfully cultured Prometheoarchaeum syntrophicum. Sequencing the DNA and high-resolution imaging provided not only huge insight into the functions and evolutionary relationships of this archaea, but also supported the findings of Spang et al from 2015. Although this research has been revolutionary for the study and culturing of ancient archaea, caution should still be had concerning what it can teach us about the evolution of eukaryotes. 

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