Ask-a-Scientist, Part III: All About Phytoplankton

Thanks to Mr. Perry’s seventh-grade science classes at Rippowam Cisqua School in Bedford, New York for sending us some questions. Mr. Perry was my science teacher in seventh grade and it was great to be able to connect with some students at my old school!

Are coccolithophores considered animals, plants, or something else? Do phytoplankton photosynthesize like plants?

Most phytoplankton, including coccolithophores, are considered marine plants. The word “phytoplankton” comes from the Greek words phyto, or “plant”, and plankton, or “made to wander.” Just like plants on land do, coccolithophores take in sunlight, carbon dioxide, and other nutrients, and photosynthesize to turn those ingredients into energy. Like land plants, they also produce oxygen — scientists estimate that about 50% of earth’s oxygen is generated by marine photosynthesizers. In addition to the plant-like phytoplankton, there are other types, such as bacteria and protists. There are also mixotrophs, which are plankton that get some of their energy from photosynthesizing but eat other plankton too!

I know that there are some phytoplankton that glow in the dark! Why is that?

There are some types of phytoplankton that glow in the dark! This is known as bioluminescence — and it’s actually the same chemical reaction that helps fireflies light up at night. Generally speaking, plankton glow as a way to scare away predators. Most bioluminescence is caused by phytoplankton called dinoflagellates, which we’ve seen lots of on this trip in our microscopes and FlowCam. We’ve also seen bioluminescence in action a few times! On a good night, if you go up to the bow of the ship and look at the wake, you can see tons of phytoplankton sparkling in the water. It’s definitely one of my favorite sights at sea.

A crowd of people is standing at the back of a ship, bathed in red light. Behind them, the wake of the ship is glowing blue with bioluminescence.
Bioluminescence can appear in the wake of ships as we churn through the phytoplankton in the water. In this photo from my last cruise, the bioluminescence appeared as a ghostly blue glow following behind us!

Compared to the size of a human cell, how big are the phytoplankton you see?

Emiliania huxleyi, sometimes called “E hux.” for short, is the most common type of coccolithophore found in the ocean. They’re generally about 5 micrometers, or “microns”, in diameter — about the same size as a human’s red blood cell. The largest coccolithophores can grow to about 100 microns in diameter, which is about the size of a bone marrow cell.

How do coccolithophores build their shells?

Coccolithophores take in their nutrients from the seawater surrounding them and then precipitate out the calcium carbonate that forms their shells in a part of the cell called the golgi apparatus. (Precipitation in the chemistry sense means for solids — in this case, calcium carbonate — to come out of the solution that they were dissolved in.) Coccolithophores build their shells piece by piece, one plate at a time, and they can produce a new plate about once every two hours. There’s a very cool time-lapse video of a coccolithophore building the plates of its shell here.

A black-and-white microscope image of a coccolithophore building its shell. A single plate can be seen on the round cell body.
Coccolithophores build their shells plate-by-plate. In this microscope image, the coccolithophore in the center is adorned with a single plate that it has just made.

Why do coccolithophores and diatoms live in different climates?

Coccolithophores and diatoms do thrive in different places, but it’s less about the temperatures of the waters and more about the types of nutrients available. The further south you go, the more silicon is available for diatoms to build their “frustules,” or glass shells, out of. So yes, they live in colder waters than coccolithophores do, but it’s because they need the silicon-rich waters found near Antarctica. In contrast, coccolithophores can thrive in relatively low-nutrient and low-iron environments, where there isn’t enough silicon for diatoms to live — and where the waters are warmer.

Have you ever found phytoplankton outside of its typical temperature environment? If so, do you know why?

We have, on this very cruise! When we were down south, we found some E hux. in water that was about 1 °C. Barney Balch says this is “almost unheard of” — usually, E hux. populations are dwindling by the time you reach water temperatures of about 4 °C and completely gone by 2 °C waters. We don’t know what made these coccolithophores grow as far south as they did — one of the many interesting and surprising findings of our cruise!

Giuliana Viglione is a journalist and science communicator who has joined Senior Research Scientist Barney Balch’s research cruise to study the impact of coccolithophores in the Southern Ocean. On board the ship, she’ll be helping the team carry out experiments, document the research cruise, and conduct educational outreach with students across the country. She can be reached at

Ask-a-Scientist, Part III: All About Phytoplankton
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