Searching for the Asgard

Eukaryogenesis — The Origin of Complex Life
For much of the planet’s early years, the Ocean was a primordial stew of single-celled prokaryotes, and two distinct prokaryotic domains emerged: the Bacteria and the Archaea. Both domains lack a cell nucleus, mitochondria, and other membrane-bound organelles.

Cyanobacteria — a microbe capable of photosynthesis — began to proliferate  2.5 billion years ago, subsequently enriching both the Ocean and the atmosphere with oxygen. This is known as The Great Oxygenation Event. 

Soon after this transformative event, a new cell structure, the eukaryotic cell, evolved. This structure is more complex than that of the prokaryotes, containing a nucleus, mitochondria, and membrane-bound organelles within a cell wall. These cells function much like an animal body does, with each internal structure responsible for a different essential function, such as digestion, waste removal, and reproduction. Eukaryotic cells can also reproduce sexually, combining genetic material to create new organisms. Over time, they evolved into diverse organisms, including plants, fish, fungi, and humans. 

Scientists hypothesize that for the eukaryotic cell to exist, an archaea and a bacterium must have combined to form a symbiotic relationship — a mutually beneficial relationship where both organisms benefit from the interaction. Billions of years ago, an archaeon ingested a bacterium, but rather than consuming it for energy, it housed the bacterium, protecting it while the bacterium produced energy. 

Several lines of evidence have shown that the consumed bacteria ultimately evolved into mitochondria as we know them today, creating the first eukaryotic cell. The eukaryote would then go on to ingest more microbes and fuse with them in a process known as endosymbiosis, ultimately resulting in increasingly complex organisms.

Scientists have also identified the likely candidates that formed this first symbiotic relationship: a proteobacterium and an Asgard archaea. 

While the organisms that began the symbiotic relationship have been identified, the relationship has not yet been observed in nature. Dr. Brett Baker and the expedition science team hypothesize that Asgard may still be living in symbiotic relationships with bacteria on our planet, providing insights into their ancient ancestors and the evolution of complex life. 

Asgard Archaea 
Ten years ago, a group of researchers exploring the Loki’s Castle hydrothermal vent field off Greenland collected a very special sediment sample. Within that sample, they identified the DNA for a new type of archaea, which they named Lokiarchaeota, after the location it was found and the Norse trickster god. Lokiarchaeota’s DNA was strikingly similar to that of eukaryotes, making them the closest living microbial relatives to complex organisms.

Scientists have subsequently discovered more microbial relatives of Lokiarchaea, naming them after various Norse gods, including Thor, Odin, and Heimdall. Together, these archaea form the Asgard phylum, and they inhabit a wide range of environments, from mud at river mouths to thermal springs in Yellowstone National Park. 

Asgard archaea are challenging to detect and only a few have been successfully photographed. Scientists know they exist because they can detect their DNA signatures in samples; however, fewer than a handful of Lokiarchaea have been successfully grown in a laboratory. 

Baker and the team will sample water and sediment from the mouth of the Rio de la Plata out to the continental shelf of Uruguay, sequencing the DNA held within each sample to detect Asgard. Their goal is to identify potential symbiotic bacteria within Asgard Archaea and develop novel pathways to discover new archaeal species. The team is especially intent on finding a type of Heimdallarchaea, known as the Hodarchaeales, named after the Norse God of Darkness, Hod. 

Hodarchaeales is the most closely related to eukaryotes; its genome suggests that, like eukaryotes, it digests carbon and metabolizes oxygen. Scientists will work to confirm whether hodarchaeales metabolize oxygen and whether they have symbiotic bacteria, or if they have evolved the ability to consume oxygen without a symbiotic relationship. Either answer to the question would provide essential insights into eukaryogenesis. Hodarchaeales are most commonly found in estuaries, where rivers flow into the ocean, likely because these environments are rich in carbon. Since hodarchaeales seem to prefer estuaries, the mouth of the Rio de la Plata is an ideal environment for looking for them.  

Building a microbial library
They will collect water and sediment samples from the mouth of the Río de la Plata out to the Uruguayan continental shelf using the ship’s CTD & Rosette, gravity corer, and ROV SuBastian. Detecting Asgard archaea requires specialized tools, specifically those for sequencing DNA and sorting microscopic cells. The science team will bring three small DNA sequencers called nanopores to R/V Falkor (too). In addition to the nanopores, the team will use a flow cytometer, a device that uses fluidics, optics, and electronics to analyze the characteristics of individual cells in a fluid stream. This will enable scientists to identify which cells are present in the samples and select candidates for genetic sequencing. 

The team will also use R/V Falkor (too)’s high-performance computing system to upload the newly sequenced DNA to a global database of Asgard genomes, allowing them to identify Hod archaea, as well as potential new species and candidates for bacterial symbionts. Shoreside collaborators from labs around the world will be able to investigate the microbial genomes in real-time alongside the science team. 

In addition to sequencing DNA, the team will attempt to grow Hod archaea in incubation experiments, which has not yet been successfully done. Cells from freshly gathered samples will increase their chances of cultivation. Finally, the team will photograph some of the Asgard archaea, either on the ship or back in the lab when the expedition returns, gathering novel insight into their cell structure and physical characteristics. 

Ultimately, they hope to detect an Asgard archaea with a bacterial symbiont, as they suspect the relationship is still present in nature. By discovering this relationship, they will unlock answers to one of life’s greatest mysteries, gaining essential insight into how complex life developed on our planet and potentially how to detect life elsewhere.