For 2,000 years, humans have relied on compasses to sail across the oceans. We’re not alone as a variety of marine creatures, including sharks, sea turtles and seabirds, tap into the Earth’s magnetic field to help them traverse the seas.
But perhaps no creature navigates better than magnetotactic bacteria. Each sports magnetic iron particles that help guide them through aquatic environments. As a result, these miniscule microbes are literally equipped with internal, nano-sized compasses.
According to Courtney Wagner, a geobiologist and postdoctoral fellow at the National Museum of Natural History, some ancient microbes also crafted their own compasses. These microbes include magnetoactic bacteria and other unidentified organisms. Wagner studies magnetofossils that have been found sporadically in the fossil record over the past 85 million years or so. While some of them are “giant” compared to the magnetofossils made by magnetotactic bacteria, they’re still infinitesimally small. But with the help of large magnets and powerful microscopes, Wagner and her colleagues have observed a variety of magnetofossil shapes — some resemble needles while others are rounded like bullets or taper like arrowheads.
These tiny fossils likely helped guide these mysterious microbes around their marine ecosystem. But today, these fossils allow Wagner and her colleagues to explore how climate change impacted ancient oceans and potentially offer clues for navigating future oceanic changes.
Before encountering your research, I had never heard of magnetofossils. What about these microscopic iron fossils captures your interests as a researcher?
You’re not alone—even most geologists and paleontologists I talk to have never heard of these things. 99% of the people that I’ve talked to have never heard of these critters. They’re like, ‘What, a microbe that makes magnets?’ And they’re perfect little magnetic particles that we can’t even make synthetically. They do it better than we can!
In graduate school, I mostly worked on the magnetofossils from this warming event called the Paleocene-Eocene Thermal Maximum that happened 56 million years ago. It’s this classic global warming event that happened in Earth’s history that is one of our main analogs for modern climate change.
The iron particles that modern magnetotactic bacteria have are so tiny—I think that’s just incredible. Compared to these critters and their iron particles, giant magnetofossils are kind of like dinosaur bones because we don’t know exactly what made them and we don’t know if they’re around today. These giant ones are a micron in length, so maybe 1/100 the width of your hair. Still really small, but to us it’s like ‘Oh it’s huge!’
And some of them just have really wacky shapes. Some of them look like spindles, some of them have these arrowhead or leaf-like shapes, and then there’s giant bullets and giant needle shapes. I just think their size and their shapes are really interesting.
Do you have any theories about what types of animals these iron fossils came from?
The giant ones could’ve been really, really large bacteria. But most likely it was some type of protist, which is a microorganism bigger than bacteria that can actually eat bacteria.
The giant ones are so big that we’re trying to figure out why something would go to all that energy to make that [the iron magnets]. What kind of advantage would making a giant magnetofossil give an organism?
We think some of them may have been used for navigation like modern bacteria. And that’s why we think they were made by some sort of protist to help them easily navigate towards different nutrient pools or organic matter. That’s our best guess but an alternative hypothesis is that they’re some kind of metabolic battery source.
While their identity remains enigmatic, are you able to tell how these tiny organisms fared during periods of extreme climate change?
We think that the extreme responses to warming events like the PETM created a more preferred environment for these critters. Some folks have actually found them outside these global warming events, but they’re not in the same abundance. We're still working on figuring out the specific environmental conditions, but our best guess is that some combination of increased iron supply and organic matter during global warming events were what these types of organisms required to proliferate.
For my postdoc, we’re looking at a lot of other rapid warming events in geologic history when we know there was a lot of iron and probably organic matter, so theoretically if they existed at this time, maybe we will find them. Hopefully—fingers crossed!
Do you think any similar animals are currently living in our oceans?
There’s no reason why they can’t exist today unless they went extinct of course. If there were similar critters living in the ocean today, I would look for them in places like the Gulf of Mexico because you’re getting lots of nutrients, oxygen minimum zones, and tropical storms that mix everything up throughout the water column. Modern magnetotactic bacteria have specific oxygen concentrations that they can live in, and we think it’s similar with the giant magnetofossils.
What can studying these magnetic fossils teach us about how modern climate change could be impacting marine ecosystems?
We’ve only known about them since 2008 and people haven’t really been looking specifically at what they could mean environmentally. We really are pioneering this still.
It’s also hard to tell from stuff going on in the bottom of the ocean whether it’s connected to larger scale changes in climate. But the time intervals in which these magnetofossils seem to flourish in the fossil record seem to be over these geologically short time intervals during transitional warming events.
But we can learn more about the different stages of ocean chemistry during warming events and get a sense for things like changing oxygen content, seasonal nutrient cycling, iron supply, organic matter — things that would help us qualify ocean health. The conditions where these magnetofossils thrived were probably not great for other organisms like fish. So the existence of similar organisms in certain areas might warn of big impacts on the fishing industry.
The theme of my research career so far has been developing magnetofossils as biomarkers and I hope they can become, for lack of a better word, more mainstream like diatoms (microscopic bits of algae) and pollen. And magnetofossils may even have some advantages over other environmental proxies. For example, the measurements that give us this environmental context from magnetofossils are non-destructive. For the magnetic measurements, you literally only need a little piece of rock and you can get all of this information. They’re quick, pretty cost-effective and very robust.
I really hope that we can add magnetofossils to the toolbox of environmental indicators.
This interview has been edited for length and clarity.
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