This episode is sponsored by Manukora Honey.

It’s hard to count how many times we have encountered diatoms on Journey to the Microcosmos. We’ve talked about the beautiful stained glass homes they build from silica in the ocean.

We’ve visited with diatoms that make little neighborhoods for themselves out of tubes. And we’ve highlighted how their golden chloroplasts and plentiful numbers make them responsible for a fifth of all the photosynthesis done on Earth. But you know who isn’t contributing to that photosynthesis despite being a diatom?

This guy. Now, it might be a little harsh for us to blame this particular diatom for not being engaged in photosynthesis at the moment. It is, of course, a little occupied with the fact that it is being consumed in the tendrils of a naked foraminifera.

For James, our master of microscopes, this was an incredible sight because foraminifera are usually shelled, and it’s very hard to record them feeding. But yes, we understand that for the diatom, this might not be one of its finer moments. When that diatom was not being eaten, it looked a bit more like this.

This diatom came as part of a recent batch of samples that James received from the Baltic Sea. As he was sifting through them under the microscope, he noticed a diatom that looked like this. And what stood out to him immediately was the lack of color.

On its own, colorlessness in an algae shouldn’t be that weird for a microscopist to see. If you’re watching algae under the microscope for a long time— like several hours— the light from the microscope will bleach the pigments in the microbe, ultimately taking the color of their chloroplasts away. But a bleached algae is also a dying algae.

And the supposedly bleached diatom that James saw was not dying at all. In fact, as you can see here, it was gliding around like a proper diatom, using a mucus that it secretes through a slit called the raphe. And the more James looked, the more he kept finding colorless diatoms, traveling around like little ghostly versions of the diatoms we are more familiar with.

Let’s just take a moment to step back and observe our own master of microscopes through what he is seeing. This is the person who patiently samples the same pond for years, poring over samples for hours each day and tracking down rare ciliates recorded in only a few old texts. For the rest of us on the Journey to the Microcosmos team, it’s hard to imagine anything in the unicellular world would be a true surprise to someone like James.

But that just goes to show how deep the mysteries of the microcosmos go. And for James, those mysteries took him to the internet. He googled “colorless diatoms,” and that’s how he learned that colorless diatoms are a very real thing.

They are also, however, a seemingly uncommon thing. We know of somewhere around 12,000 species of diatoms, but estimates suggest there could be as many as 200,000 species out there. And so far, only a handful of those species seem to be colorless.

These colorless diatoms are also known as apochlorotic diatoms. The first one was discovered in 1854 and named Synedra putrida. And since then, every few decades, scientists seem to find a few more species.

And while these species might be uncommon in the grand scheme of diatoms, scientists who have found them say that they’re not particularly uncommon in their given habitats. They can be found on nutrient-rich waters, and particularly on the surface of decaying seaweed. For James, this tracks with the fact that his samples came from the highly eutrophic shores of the Baltic Sea.

Their tendency towards these habitats also makes sense when you remember that they lack the photosynthetic capabilities of other diatoms. They can’t make their own food, so they need to make sure they live in a place that is full of the nutrients they need to survive. To understand how apochlorotic diatoms became colorless, we first have to understand how diatoms became so colorful to begin with.

A very, very, very, very long time ago, a eukaryote consumed a cyanobacterium and instead of converting it into food, the eukaryote retained the cyanobacterium as a photosynthetic organelle called a plastid. That event is what we call a primary endosymbiosis, and it spawned an array of photosynthetic eukaryotes… including red algae. And eventually— but still a very, very, very long time ago— that red algae was consumed by another eukaryote that converted the red algae into its own plastid.

That event is what we call a secondary endosymbiosis, and through evolution, it eventually brought us the colorful array of diatoms that we so love to look at. Recent genetic analysis suggests that after that point, some species of diatom lost their ability to do photosynthesis and that led to the creation of new, non-photosynthetic species. And at least as far as science can see right now, that loss has happened only twice.

The interesting thing is that these apochlorotic diatoms do still have plastids— those organelles that made diatoms photosynthetic to begin with. It’s just that their plastids are colorless, lacking chlorophyll and other components necessary for photosynthesis. And yet their plastids are still at work, able to produce other compounds essential for the survival of the diatom.

It’s hard to know what exactly sparked the loss of these diatoms’ ability to do photosynthesis. They aren’t the only organisms that have experienced that particular evolutionary loss. There are species of green and red algae, along with plenty of other eukaryotes, that have gone through this process as well.

But while we’re not going to speculate about the past, we do want to make a few guesses about the future. Because this question struck James as he was looking at the colorless diatoms: could we be facing a future where we see more of them? As we keep dumping nutrient-rich wastewater into habitats that weren’t originally meant for them, maybe we’re also setting up a world where colorless diatoms become more and more abundant.

This question isn’t a judgment on what’s good or right for the world. It’s about how we may or may not shape the future of an organism that has shaped our own past and present, and the questions built on each prior answer. If colorless diatoms do become more plentiful, how does that affect the trajectory of diatom evolution at large?

And how does that affect the creatures who live in close proximity with them? And how does that affect the ones who don’t? And so on, and so forth.

But like we said, this is all speculation. We don’t know the future of colorless diatoms. And we’ve made enough episodes about diatoms to know that despite the seeming rigidity of their individual bodies, they are overall, quite difficult to predict and even more fascinating to question.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thanks again to Manukora Honey for sponsoring this episode. This rare, single-origin honey is only produced in New Zealand and the bees that produce it collect nectar exclusively from the native Mānuka flowers that flower for just 2 to 6 weeks.

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Also a shout out to all the people whose names are on the screen right now. They are our Patreon patrons and they make it possible for us to continue looking into the bizarre mysteries that we are constantly surrounded by. And if you'd like to become one of them, you can always go to Patreon.com/JourneytoMicro.

If you want to see more from our Master of Microscopes, James Weiss, you can check out Jams & Germs on Instagram. And if you want to see more from us, there's probably a subscribe button somewhere nearby.