ABOVE: A microscope image of various diatoms living in Antarctica’s sea ice NOAA, GORDON T. TAYLOR

New research challenges a longstanding assumption about how unicellular organisms communicate and coordinate their behavior with one another. According to the study, published in Science Advances on Wednesday (December 15), at least one species of diatom can send and receive signals that previously went undetected by scientists.

Scientists assumed diatoms, which are single-celled phytoplankton, could only signal and communicate with one another by secreting infochemicals. The new study suggests that the pelagic diatom Pseudo-nitzschia delicatissima can also communicate with others through red and infrared autofluorescence triggered by exposure to sunlight. When exposed to lights of the right frequencies, diatoms in a lab synchronized their behavior, aligning vertically in the water and wobbling in time with one another, suggesting that they’re capable of coordinated social behavior.

To learn more, The Scientist spoke with Idan Tuval, a physicist-turned-microbial ecologist at the Mediterranean Institute for Advanced Studies, about how he and his team made the discovery and where the field can go from here.

The ScientistTell me more about your background and what led you into the field of fluorescent diatoms, of all things.

Idan Tuval: Okay, so I’m a physicist. I actually started as a theoretician long ago, and then slowly drifted into addressing questions of the world, and the interface with biology just happened naturally—I was looking at fluid mechanics, and then chaos, and mixing, and things like that, which just happened to be everywhere in biology. . . . In particular, I slowly drifted towards the behavior of microorganisms and how physical constraints are involved in the behavior and the development and evolution of life forms. I have been, since then, trying to combine experiments and theory as much as possible. After more and more interaction with my colleagues here, I got into marine microorganisms. Diatoms are one of the most interesting ones around, so it just happened that I ended up looking at them.

TS:  Where did the initial idea that prompted this research on whether these microorganisms fluoresce and act socially come from?

IT: It came from fieldwork that some of my colleagues here have been doing. We’re helping them develop a technique to address questions related to the structure of phytoplankton communities in situ. We’re using a laser diffractometer to look at size distributions in the ocean. Basically, what that gives us as an output is sizes of what you have in the volume of interest. They were getting water samples at the time and, in some cases, that got into large blooms of pennate diatoms. We figured that we can use some signature out of this metadata to analyze the orientation of particles in situ and try to understand how these diatoms are oriented and distributed, not only in terms of position but in terms of orientation with respect to the vertical in the water column. So we managed to develop a technique to do that. When we applied it to some field data, it clearly showed that in many cases, these diatoms were not just randomly oriented, but instead they tended to be very clearly aligned under low shear conditions or calm waters. They were basically sinking, oriented vertically.

These are not flagellators . . . they can glide around, but they don’t move much. They are basically suspended in the water column, sinking without any clear propulsion mechanism. So how do they find each other, in particular, for things like sexual reproduction? That’s a big question.

We did microscopy, tracking, looking at orientation in the lab. And we published a paper on encounter rates [among the diatoms] based on this particular orientation. Looking more carefully at the data, we realized after publishing that paper that there was something more happening there. It was that they were not just sinking, but they were actually oscillating around the axis while they were sinking. So again, we wondered about the consequences of this little wobbling. We wanted to characterize it and understand if it has any influence or any consequence on the life of the community or the population that they have.

There was some info in the literature about diatoms having encoded in the genome the genes for some photoreceptors in the red and infrared band. There was no clear indication of why they would have that. We’re talking about organisms that live down in the water column most of the time—they live in a very bluish environment; no red light around. . . . We just thought that maybe—we wanted to test these hypotheses, of course—they can actually sense each other.

They have chlorophyll autofluorescence. Chlorophyll autofluorescence would fluoresce mostly in the red. They give red light signals from out of the cells, and they are sinking. And they are wobbling, oscillating around, while they are sinking. That might actually give you a sign that this stems from a signal from the fluorescence inside. So can they sense that, and does it have a consequence on how they behave? That’s where the idea came from.

It’s telling you that there’s a clear phenomenon—that has not been taken into account at all—that allows cells to sense each other and react. And that’s a game changer.

—Idan Tuval, Mediterranean Institute for Advanced Studies

TS: How do you test for that response to autofluorescence?

IT: So we were using the same device in the lab, a diffractometer, with a technique that we developed before that can give us an idea of how cells are oriented in a volume. And what we did to test for the effect of red light—of the light it will emit—was to mimic that communication externally by applying to the sample the red light and oscillating the light close enough to their wobbling frequency. We were able to scan on different frequencies and check out their response, and see where we were getting [a] collective response in terms of coherent oscillatory motion. We found that for frequencies very close to the natural frequency, they were basically synchronizing to the external [light]. So we could, in a way, whisper to them in a way that they can sense, and they will respond to that whispering by actually oscillating together in sync.

TS: A lot of things blink in the water—how do you know for sure that this is used for communication? I know you see the response, but defining social behavior among unicellular organisms seems difficult. Do you have any ideas about the purpose of that signaling and why they respond in the way that they do?

IT: You’re completely right. It depends on how you define communication. Here, we are talking about a very basic level. We are talking about the fact that they have the two key components of any communication, which is the fact that they can emit a signal and they can receive that signal. The cells can emit a very clear oscillatory signal, a fluorescent signal, at a very clear frequency, and they do respond to it. They have the necessary tools to sense it, and they modify behavior based on that signal, meaning they synchronize the wobbling.

Now, the consequences of that, of the “why,” what we can say from what we measured is that they do synchronize their wobbling. If that’s useful for anything, that is still to be understood. Meaning we can hypothesize that wobbling in sync will increase cell-to-cell interactions and encounters. But we cannot, at the moment, pinpoint an exact reason why that communication is happening. There is communication there, and they do respond collectively. But yeah, that’s as far [as] we can go at this point.

TS: This suggests that there is some sort of light-triggered cooperative behavior among microorganisms where scientists previously assumed there was none. What does it mean for the field that you’ve challenged or started to [challenge] that assumption? Is there any obvious next step for future research now that you’ve uncovered this phenomenon?

IT: Communication in microorganisms is a well-known phenomenon. But . . . most microbial communication is thought to happen through chemical signaling. That is the basis of a lot of things [that] have been very well described in very different contexts. Now, what we are seeing here is communication through light in organisms that don’t have well-developed light sensing organelles. For the emitting organelles, we are talking about very basic ingredients. We’re talking about just the natural autofluorescence of chlorophyll, which is there for most photosynthetic microorganisms. So if there is a photo response that links behavior to emission of light based on this very simple mechanism, that’s a huge change in the field.

See “Clouds and Rain Carry a Menagerie of Photosynthetic Microbes

It’s telling you that there’s a clear phenomenon—that has not been taken into account at all—that allows cells to sense each other and react. And that’s a game changer.

There are plenty of things to develop and discover in the field, [such as] if other organisms will do something similar. We have characterized the photo response in terms of lab conditions, but then there’s studying exactly which conditions they will encounter and what that will imply for their interaction. We need to look into that. Does it have any effect on reproduction? On other aspects of the life cycle? That all needs to be looked at in detail.

TSWhat’s next for you? Have you continued exploring this in a way you can talk about today?

IT: We have a number of things going on at the moment related to this. One was to explore this mechanism in other organisms. We have just worked with one species here in this paper. We have tried to extend that to other species in different areas, collaborating with people in France and in other places, collecting and isolating strains from different places to see how widespread this phenomenon is.

On the other hand, we have tried to dig more into the molecular basis of the responses. And for that we have some ongoing collaborations looking at mutants of the different diatoms, in which you can knock out some of the photoreceptors and pinpoint the factor better–going into the molecular mechanisms, and then the cascade that is behind the photo response.

And then we have another avenue trying to link this to sexual reproduction. So again, light is known to play a role in sexual reproduction in diatoms. We don’t know if light also plays a role [in reproduction] through this photo response that we have found here.

Editor’s note: This interview has been edited for brevity.