Lokiarchaeon Genome Analysis Supports Hydrogen Hypothesis

[BillTre]

Comparative genomic analysis (Free pdf download here) shows that Lokiarchaeon (the mysterious closest presumed relative to the archaeon host (known only by its genome sequence collected from an environment), which contained the endosymbiont bacteria (that became the mitochondria), had hydrogen dependent metabolism (in agreement with predictions of the hydrogen hypothesis (free pdf of original paper here)). The pre-mitochondrial bacterial cell made hydrogen, so a mutually beneficial win-win relationship could be established.

• It has genes for the complete energy production pathway characteristic of a hydrogen dependent, strictly anaerobic, probably autotrophic, archaeon, as predicted by the Hydrogen hypothesis that the host cell was a hydrogen dependent archaeon.

They also question the interpretation that Lokiarchaeon's genome indicates it had phagocytic abilities.

This argument has to do with whether a cell has to be phagocytic (able to deform its membrane and engulf large food particles) into enclosed vesicles inside the cell to get an endosynbiont like a mitochondria.
This behavior (phagocytosis) would probably require the evolution of:

• New cell and membrane proteins to interact with the cytoskeleton, in order to pull it around the membrane and make the different shapes required of engulfment.

No known bacteria or archaea have been found with these characteristics.

• New molecularly defined region of the cell to define the digestive vesicles. Thus, the proper proteins or membrane components can be efficiently be directed to their proper locations in the cell (digestive vesicle in this case)). The stuff bound for the digestive vesicle will need an address to get there efficiently.
  Some bacteria seem to have different regions of membrane.

They argue:

• The genes used to support possible phagocytic cellular activities, are used during cell division in living archaea. Cell division could also involve shaping of the cell membrane as it pinches off and divides.
  
• The forming of a symbiont does not require the host be phagocytic. There are examples of non-phagocytic bacteria (or archaea) with other bacteria (or archaea) living inside them.

They conclude that Lokiarchaeon is probably typical in size for an archaea, does not not have nucleus or dynamic cytoskeleton, and will probably have a cell wall.

 

[Ygggdrasil]

In 2017, researchers identified a larger superphylum of archaea related to Lokiarchaeota (see https://www.physicsforums.com/threa...he-evolution-of-complex-cellular-life.899972/). I wonder if these results apply to the wider Asgard superphylum. This is an important question because it is not as if the Lokiarchaeota have stopped evolving since eukaryogenesis occurred. In the intervening ~2 billion years, the Lokiarchaeota will have undergone some evolution and might have been forced to occupy new niches as eukaryotes expanded to occupy niches previously held by Lokiarchaeota and other similar organisms.

If Lokiarchaeota is indeed an obligate anaerobe as suggested by the Sousa et al. paper, it raises the question of how it would have encountered the (presumably aerobic) bacterium that would become the mitochondria.

 

[BillTre]

If Lokiarchaeota is indeed an obligate anaerobe as suggested by the Sousa et al. paper, it raises the question of how it would have encountered the (presumably aerobic) bacterium that would become the mitochondria.

Can't say I thought of that.
I guess they might could have possibly met at the interface between the aerobic and anaerobic environments. Once they became host and symbiont, presumably they they would be free to reside fully in an aerobic environment.