Imagine that you are not watching  the microcosmos right now.

Instead, imagine you are living in the world as  it existed around one billion years ago,   and you are the ancestor of this red algae. Inside of you lies the DNA that you have  painstakingly acquired from your ancestors.

It contains the instructions for everything from the walls of your cellular   body to the components of your organelles. That DNA has changed and accumulated through  randomness and selection over billions of years,   and you are the masterpiece  sculpted by that process. Except you are about to lose  a quarter of your genes.

The history of red algae can be told  in part through what they have gained,   lost, and given away over billions of years. It begins with a tale that is familiar  to many of us in the microcosmos. A long, long, long time ago, a eukaryote consumed   a cyanobacterium, only to convert it into a  photosynthetic organelle called a plastid.

This process is known as endosymbiosis, and  the organisms descended from that eukaryote   are now known as the Archaeplastida,  which is made up of three groups. Green algae and land plants are  considered one of those groups,   and then there’s the other group  of algae called the glaucophytes. And then there are the red  algae, called rhodophyta.

Like their Archaeplastid counterparts,  red algae have chlorophyll. But they also possess other pigments that help  with photosynthesis and also often turn them red. Now when you hear the word “algae,” your mind might   first conjure up images of  the greener Archaeplastids.

But red algae is actually  widespread and influential. For starters, if you eat nori,  you are actually eating red algae. And red algae’s impact goes far  beyond that.

As we've just said,   red algae got their photosynthetic  plastids through endosymbiosis. But they also essentially gave their  plastids to others through endosymbiosis. Many organisms, like diatoms, can trace the  origins of their photosynthetic capabilities   to an ancestor that consumed and converted  a red algae plastid into its own machinery.

So red algae have a legacy that  spans far beyond their own actions. They show up embedded in the ways  that other organisms are able to live,   in every bit of food they derive from the sun  and the chemical byproducts that sustain us all. But for all that red algae were able to  gain and share through endosymbiosis,   their history is also marked  by the things they’ve lost.

Like their genes. One of the things that is really striking about red   algae is they don’t seem to have that  many genes compared to other eukaryotes. And more weirdly, it seems like they  actually once did have more genes… they just lost them.

It’s hard to pick apart exactly  how red algae lost their genes. And we can’t say if it happened very suddenly,   or if it was more a gradual chipping  away at their genomes. But based on the species  they’re able to study today,   scientists think there were at least  two major phases of genome reduction.

The first likely involved some old lineage of  red algae, which likely lost about 25% of their genes. And in this process, they lost  a key feature: the flagellum,   that thread-like structure that many  eukaryotes use to drive themselves around. And there are some interesting implications  to losing the flagellum, especially for   an aquatic organism like red algae that would  probably want to be able to swim to locations   that allow them better access to light,  or to find other algae to reproduce with.

But perhaps that ancestral red algae  was living in conditions where the   energy required to make flagella and  operate them just wasn't worth it. Maybe it just thrived without the flagellum,  and there never was a reason to keep it. In that case, that ancestral red algae’s  loss seems to have been justified.

Because red algae are thriving without  their flagella, and without any number   of other pathways that were lost with those genes. And in fact, in the second phase, another red  algae would reduce its genome by another 25%. The descendants of that algae became  members of a group known as Cyanidiophytina.

The algae within this group— and particularly those from  the class Cyanidiophyceae— are particularly well-studied  because they are extremophiles,   meaning they are found in environments  that are incredibly difficult to survive in. Heat? Acid?

Metal? Salt? All things we might tolerate  in a very specific range,   but these red algae extremophiles are not phased.

Some can survive in acidic environments  whose pH values range from 0 to 4. For context, the least painful end of that  scale would be like living in acid rain. The most painful?

Like battery acid. And as far as temperature goes,   there are species that can survive at 56  degrees Celsius, or 132 degrees Fahrenheit. So the reason we’re not actually featuring any   Cyanidiophytes is that they tend to be  found in places like volcanoes or hot springs.

No, our red algae are their tamer cousins,  found living along a Spanish beach. But how could the Cyanidiophytes have   survived such extreme conditions when  they have lost so much of their genome? Surely those lost genes were useful for something?

Maybe. But it turns out the  cyanidiophytes had a back-up plan: they could get the genes they  needed from their neighbors. There’s a process scientists have  been unraveling over the past few   decades called horizontal gene transfer, where  genetic material gets moved from one species to another.

And in general, we know it as a  process that happens in prokaryotes,   driven by viruses or other vectors that are able  to transfer DNA from one organism to another. And prokaryotes are well-suited to not  only take in genetic material this way,   but also to quickly embed those changes in  their population as they divide asexually. Eukaryotes seemed unlikely to  engage in horizontal gene transfer,   if only because we couldn’t  readily find examples of it.

But when scientists studied the  genome of Cyanidiophyceae algae,   they found hundreds of genes in them that seemed  to come from bacteria, and that seemed to help   with various functions like surviving  high temperatures or salty conditions. And so the same group of organisms that  lost so much of itself also found a way   to gain so much of others, adopting the genes of  other organisms into their own way of life. And the most exciting thing is how  much there is still to learn.

We still really don’t know that  much about how horizontal gene   transfer shows up in the history of eukaryotes. We don’t know whether red algae have gone  through other major phases of genome reduction. We know simply that nature’s boundaries are  porous, and its identities are multi-faceted.

Thank you for coming on this journey with us as  we explore the unseen world that surrounds us. The people on your screen right now, they are  the people who allow us to spend so much time   gathering beautiful footage and also beautiful  and wonderful and bizarre stories of our world. And then to combine them for you to look  at here on Journey to the Microcosmos.

If you would like to be one of those  people, there's a way to do that. You just go to Patreon.com/JourneytoMicro. If you'd like to see more from our  Master of Microscopes, James Weiss,   you can check out Jam and Germs on Instagram.

And if you want to see more from us, there's  always a subscribe button, somewhere nearby.