Spotted salamanders and green algae make for an odd couple, but they share a close bond that begins when the algae start growing in the salamander’s egg–and enter the developing embryo's tissues and cells, a type of relationship known as endosymbiosis. Just because it’s an intimate arrangement, though, doesn’t mean it goes smoothly for both parties, a new study by Museum scientists shows. While salamanders seem to take the connection in stride, it leaves algae cells struggling to get by.
© AMNH/E. Chapman
This rare relationship between two very different species—which you can learn about in this episode of the Museum’s Shelf Life web series—has intrigued scientists for decades. That’s in part because, while algae are known to form similar relationships with invertebrates like corals, this is the only algae endosymbiosis that involves a vertebrate species.
“Science shows us the many ways that life is interconnected, especially on the microscopic level, where we see how many organisms depend on close contact with or internalization of other species for food, defense, or reproduction,” said lead author John Burns, a postdoctoral researcher in the Museum’s Division of Invertebrate Zoology. “But the relationship between this particular alga and salamander is very unusual.”
© R. Hangarter
The alga species Oophila amblystomatis grows in the egg cases of the spotted salamander Ambystoma maculatum, a strange pairing that is visible to the naked eye thanks to the green hue it gives the salamanders’ eggs.
“This is really such a strange arrangement. It would be like having a bunch of green algae in a womb,” said study co-author Ryan Kerney, an assistant professor at Gettysburg College. “What we set out to look at now is the kind of molecular change that happens when the salamander cells and green algae cells are together.”
© R. Hangarter
The findings of the new study, published today in the journal eLife, suggest that algae inside salamander cells are stressed and change the way they make energy. Instead of using light energy to produce food to support the salamander host, as happens in coral-algae interactions, the algae in salamander cells struggle to adapt to their new environment. It’s not clear if or how the algae benefit from this arrangement.
In stark contrast, affected salamander cells appear to recognize the alga as foreign, but show no signs of stress. The researchers found that the salamanders overexpress several genes that might suppress an immune response, suggesting that the host cell experience is neutral or beneficial.
© AMNH/E. Chapman
“We are learning that these two fundamentally different cells are changing each other dramatically, and this might be relevant for other symbiotic systems, including human and parasitic bacteria relationships,” said study co-author Eunsoo Kim, an assistant curator in the Museum’s Division of Invertebrate Zoology.
Too learn more about the special relationship between these two species, and how Museum researchers are trying to understand it, check out this episode of Shelf Life.
JOHN BURNS (Postdoctoral Researcher, Division of Invertebrate Zoology): In the springtime, after the first warm rain, that’s when the salamanders start getting on the move.
And they lay all these beautiful little egg masses out in all the ponds across the East Coast.
So, we go out there in our waders, collect egg masses, and then set up different ways of studying them in the lab.
My name is John Burns and I’m a post-doctoral researcher in the Division of Invertebrate Zoology.
[SHELF LIFE TITLE SEQUENCE]
RYAN KERNEY (Research Associate, Department of Herpetology): Deep inside this bowl there are cells that are interacting between the world of algae and the world of salamanders. And so we want to get in on that action and figure out what’s going on.
I’m Ryan Kerney. I’m an assistant professor of biology at Gettysburg College and a research associate here at the American Museum of Natural History.
EUNSOO KIM: (Associate Curator, Division of Invertebrate Zoology): We’re really interested in a symbiosis that exists between the salamander embryos and an algae.
Symbiosis refers to the living together of two or more different organisms.
I’m Eunsoo Kim. I’m an assistant curator of microbial diversity and systematics in the Division of Invertebrate Zoology.
So, you can find algae in most of the aquatic environments like pond water in Central Park. You just put a drop of water on the microscope slide and take a look at them and you will find at least some of them swimming.
Here at the Museum, we maintain about 80 live cultures of microorganisms, including algae. And about half of them are unique to our collection.
In the case of salamander and green algae, during the embryonic stage, algal cells live basically inside of the egg.
BURNS: The association between the spotted salamander and this green algae was first described in 1888 by a naturalist-scientist named Henry Orr. He basically just started wondering what is going on that the salamanders allow all these green algae to live inside their egg case.
It would be like having a bunch of green algae inside of the womb, which is totally bizarre.
And then over time, people started to study it. So, they found that when the sun is out, the algae are producing oxygen inside the egg mass. So, if you try to kill the algae or you keep them in the dark, the salamanders will be smaller, and they might have a poorer chance of survival post-hatching without the benefit of the extra oxygen.
KIM: The algae may benefit because of these nitrogen that’s a waste product for the salamander, but that can be utilized for their cells’ metabolism.
BURNS: We’re pretty sure that the algae are able to grow to high densities and they get some nutrients that they would not be able to get outside in the pond.
KERNEY: Our initial question was how is it that a beneficial microbe— this algae—is able to get through the big jelly that surrounds the embryos and the individual egg capsules that the embryos live in. We wanted to look at this association a little bit more closely through the microscope.
The algae, like all photosynthetic life, has these pigments that fluoresce. That means that if we shine light on them of one wavelength, they emit light of a different wavelength.
And basically on a lark, I looked at a later stage embryo with the fluorescent microscope to see if there was any sign of algae, you know, persisting near the time of hatching. And it was really surprising to see that there’s algal cells embedded inside the embryo itself, inside the tissues.
Then we wanted to look at the inside of cells. We were able to see that the algae is actually going inside the cells, the individual cells of the embryo. Which was a real high-five moment around the microscope to see this cool interaction that was really unexpected.
That cell within a cell relationship—like a Russian matryoshka doll—occurs in giant clams. It occurs in corals. It occurs in sea slugs. But there are no other real known examples of a symbiont entering into the cells of a vertebrate.
KIM: The question that we are currently tackling is what kind of molecular change is happening when these salamander cells and green algal cells are together.
BURNS: We took cells from four different groups—salamander cells that have algae, salamander cells that didn’t have algae, the algae cells that were living inside the salamander cells, and the algae living in the egg capsules.
And what we did was we basically just counted how many of each type of RNA there were and then we compared them to one another.
We actually are starting to know how these two species are adapting to each other. Like, you know, for example, the algae actually, when they get inside the salamander cells, it’s a super nutrient-rich environment. So, they turn off the genes associated with importing inorganic nutrients. Because they don’t need them anymore. They’re flooded with them.
KIM: We are learning that these two fundamentally different cells, they're really changing each other very dramatically. And this change may be relevant for other symbiotic systems, including like human and parasitic bacteria relationships.
KERNEY: Researchers are increasingly interested in how the microorganisms that are inside and on our bodies interact with us throughout the course of our lives and affect our physiology.
So, this project really is in keeping with the sort of research attention of today. Will it potentially show us some new way about how life on Earth interacts with one another? That’s what we really hope for. And to just be part of that conversation is extremely exciting right now.