endosymbiotic

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Eastern emerald elysia (Elysia chlorotica)

Elysia chlorotica is a small-to-medium-sized species of green sea slug, a marine opisthobranch gastropod mollusc. This sea slug superficially resembles a nudibranch, yet it does not belong to that clade of gastropods. Instead it is a member of the clade Sacoglossa, the sap-sucking sea slugs. Some members of this group use chloroplasts from the algae they eat; a phenomenon known as kleptoplasty. Elysia chlorotica is one of the “solar-powered sea slugs”, utilizing solar energy via chloroplasts from its algal food. It lives in a subcellular endosymbiotic relationship with chloroplasts of the marine heterokont alga Vaucheria litorea. They can be found along the east coast of the United States and as far north as Nova Scotia, Canada.

photo credits: New Scientist, elescrutinio, bizzarrobazar

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These are Scaly-foot snail (Chrysomallon squamiferum), an iconic vent endemic gastropod known only from the Indian ocean deep-sea hydrothermal vents, at more than 2500 metres deep.

This unique snail species lives just beside black smokers that are churning out superheated water exceeding 350°C. Has also harnessed the power of chemosynthesis, housing endosymbiotic bacteria in an enlarged part of its gut. This produces the energy it needs. it has a food factory inside its body and doesn’t even need to feed! This is likely the reason it can grow to about 45mm in size, when most of its close relatives without endosymbionts are only 15mm or smaller.

Endosymbiosis - The Appearance of the Eukaryotes

Image: (CC BY-NC 4.0)  Virtual Fossil Museum

A Long time ago (maybe more than 3 billion years ago in the Archaean Eon) in a land far, far away, a primitive eukarayotic cell ate a primitive photosynthetic prokaryotic cell, hijacked it’s metabolic machinery, and this is why we humans, and all animals breath oxygen. Part of that machinary is the mitochondrial DNA in each of your cells. The story is a lot more complicated, so see The Endosymbiotic Origin of Domain Eukaryota.

Image: (CC BY-NC 4.0)  Virtual Fossil Museum

Now, I hope you realize that each cell in your body carries mitochondrial DNA from your mother. If you sequence the DNA, there is similarity to purple-aerobic bacteria. So, your most distant ancestors are single cells and you are a chimera containing essentially bacterial DNA – and you’ve maybe been thinking you descended from apes.

Endosymbiotic Theory

The simple question of the origins of the mitochondria and chloroplast takes us all the way back to the origins of complex organisms.

Unlike the other organelles, which originated inside the plasma membrane, the mitochondria and chloroplast were once primitive but independent prokaryotic cells, most likely a type of bacteria. The endosymbiotic theory proposes that ancient prokaryotes engulfed each other around 1.5 billion years ago. Although this would usually result in the unfortunate engulfed cell becoming a meal for the host cell, for some reason, sometimes the digestion failed. Eventually, the swallowed cells were allowed to just hang out there, because the host cells realised they were beneficial.

To understand why, we have to understand how organisms are classified based on their metabolism. There are two main types: autotrophs, which make their own food, and heterotrophs, which rely on energy made by other organisms. Within each category are phototrophs, which derive their energy from light, and chemotrophs, which derive their energy from chemical compounds. This table makes it pretty clear:

(Image Credit: Pearson Education)

Mitochondria are aerobic heterotrophs, meaning they create energy using oxygen, and chloroplasts are photoautotrophs, meaning they produce energy from light. When the mitochondria was stuck in the host cell’s cytoplasm, surrounded by other bits of half digested food, they had plenty of fuel to take in for themselves and convert it into cellular energy. Soon they must have been so bursting with energy that some leaked out, allowing the host cell to use it for their own cellular activities.

Having an houseguest that churns out energy would have been a huge advantage for the host cell, since it wouldn’t have to go to the effort of making its own. The mitochondria basically acted like an installed battery, and in return, it was given protection by the host cell. Something very similar would have happened with chloroplasts, though chloroplasts would have created energy from light rather than half-digested molecules. As the host and guest cell benefit from each other, the partnership is called a symbiotic relationship.

But eventually, the two came to depend on one another—the host cell stopped producing much energy by itself, and the guest cell stropped producing genes for protection. The relationship became more and more dependent, until the mitochondria and the chloroplast simply become part of the eukaryotic cell.

There’s significant evidence supporting this theory. Firstly, the mitochondria and chloroplast each have two membranes, one enclosing the other, as if they had one originally and then were given another when the host cell engulfed them.

(Image source)

Both also have their own DNA, which is circular in nature like prokaryotic DNA instead of the linear DNA of eukaryotes. They use it to produce proteins and enzymes. They also divide by binary fission (splitting one parent cell into two identical daughter cells) just like bacteria, and they don’t use vesicles to send or receive substances from other organelles.

It’s like the Endomembrane system is a bunch of kids who have grown up together, while the mitochondria and chloroplast are those weirdos who have just moved into the neighbourhood and like, absorb sunlight and radiate energy and all kinds of X-men-esque junk.

Further resources: Animation or read about Lynn Margulis, who first proposed Endosymbiosis

Lynn Margulis (1938-2011) was an evolutionary theorist and science author, the first modern proponent of the significance of symbiosis in evolution. Her research fundamentally transformed and established the understanding of the evolution of cells with nuclei. Her work was seen as controversial and was widely rejected for years, until genetic evidence proved it definitively.

Her 1966 paper, “On the Origin of Mitosing Cells”, was rejected for publication by 15 scientific journals initially, but today it is considered a landmark in endosymbiotic theory. She also proposed the Gaia theory, which sees Earth as a self-regulating system.

working title: Better Together; The Story of Endosymbiotic Theory And How Eukaryotic Cells Came To Be

think of it as a metaphor for your relationships

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