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Secrets of the diatom

Speed read
  • Diatoms produce 20% of the world’s oxygen and are critical to the functioning of marine ecosystems
  • Genetic sequencing reveals new paths in the evolution of diatoms and their closest relatives
  • Adaptability of diatom species over time gives clues to how species might survive climate change

If you haven’t heard of diatoms, it’s time you did. These single-celled algae produce 20 percent of the world’s oxygen, accounting for one of every five breaths you take. Diatoms are also the base of the ocean’s food webs, producing the nutrient-rich omega-3s in fish oils.

<strong>Made of glass.</strong> Diatoms are a single-celled algae that produce 20 percent of the world’s oxygen. Making their cell walls out of glass, the 200,000+ diatom species form breathtakingly diverse geometric patterns. Courtesy Wipeter <a href='https://creativecommons.org/licenses/by-sa/3.0/deed.en'>(CC BY-SA 3.0)</a>But the most astonishing fact about diatoms may be their ability to make cell walls out of silica (glass), forming breathtakingly diverse geometric patterns. By producing enzymes that pull dissolved silica out of water, diatoms drive the recycling of silica, which makes up a quarter of the earth’s crust.

Andrew Alverson, a biologist at the University of Arkansas, first became fascinated with diatoms by seeing what they look like under the microscope.

“When you see all the different varieties and forms, it’s really captivating,” recalls Alverson. “While an undergraduate, I was motivated by their beauty and diversity. They’re absolutely striking.”

Now, Alverson is using high-performance computing (HPC) to study the genetics of diatoms, in the hope of better understanding their adaptability in relation to climate change.

A superstar on the tree of life

Phylogenetic trees demonstrate the genealogical relationships between all living things on earth, past and present. Some branches on this tree of life contain hundreds of thousands of species, while others represent only a handful. Scientists want to know why some groups are vastly more successful than others.

<strong>Advances in genetic sequencing technology</strong> have led scientists to revise the structure of the diatom’s phylogenetic tree. They’ve discovered that many species that share physical attributes aren’t necessarily related. Courtesy Andy Alverson. With an estimated 200,000 species, diatoms are a superstar phylogenetic lineage. Their closest living relatives are a group of only 20 species. Having started from the same point way back in time, diatoms outperformed these closest relatives by 10,000 percent! So what’s their secret?

Alverson is combining genetic sequencing with HPC to find out.

“In the past, when we were just looking at diatoms under the microscope, we were getting a very different impression of their relationships than now when we can sequence the DNA, which is in many ways a more objective way to do it,” says Alverson.

This new objectivity has led to profound revisions in the structure of the diatom tree of life.

“People often assume that when two organisms look alike, that they’re related to each other,” says Alverson. “But one of the things we find when we build these phylogenetic trees based on DNA sequence data is that things that share a lot of similar attributes aren’t necessarily related to one another. There’s been a lot of convergent evolution.”

Convergent evolution is the process by which organisms from distantly related species independently evolve similar traits while adapting to similar environments.

“For example, distantly related lineages of electric fish have evolved electric organs in exactly the same way. That tells you there’s one common solution, and you can then ask detailed questions about the actual genetic changes that are driving it,” Alverson says.

These new perspectives are only possible thanks to the recent accelerated growth in genetic sequencing technology and computing power. In the early 2000s, it took five years to sequence just four genes for 70 species. Now Alverson can easily sequence 10,000 genes for hundreds of species. 

“DNA sequencing technologies have evolved over the past 10-15 years, allowing us to ask questions in much more powerful ways,” says Alverson. “Absolutely everything we do is on HPC, from assembling genomes to all the comparative analyses.”

Forecasting climate change

One particularly interesting aspect of diatoms is their ability to evolve from living in saltwater environments to freshwater and back again.

“That’s a really difficult barrier to cross, but they've done it many times,” says Alverson. “They are very adaptive little creatures.”

<strong>Adaptability.</strong> Diatoms have evolved to live in both saltwater and freshwater environments multiple times over the course of their history. This ability to adapt to different habitats may help scientists forecast how other species will cope with climate change. Moving from one habitat to another has been a boon for diatoms in terms of increasing their diversification. “It also tells us that adapting from marine to fresh water is a solvable evolutionary problem,” Alverson says.

Alverson’s team can even pinpoint the branches on the tree where diatoms crossed back and forth from one environment to the other. That knowledge can then be used as a forecasting tool for how species might adapt to climate change.

“We know glaciers are melting at the poles and oceans are getting much more rainfall, causing a huge influx of fresh water into the oceans,” says Alverson. This increase in freshwater is a potential threat to marine species. “Our diatom research tells us that adapting to low salinity is a solvable problem, but the question is how quickly this can happen.”  

Alverson encourages students to study diatoms because they're absolutely critical to the functioning of marine ecosystems, and they're the base of the food chain. Also, as a relatively understudied group of organisms, there’s much to learn and many questions to answer.

“Diatoms are so important, and almost everything we discover is new,” he says. “And, that's really exciting.”

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