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New research furthers understanding of phytoplankton’s role in global carbon cycle

Diatoms are microscopic algae that come in all different shapes and sizes. (photo courtesy of Adrian Marchetti)

Did you know that about 50 percent of the oxygen in our atmosphere, the oxygen in every second breath we take, is produced by tiny microscopic organisms in the sea called phytoplankton?

A new paper published online in The ISME Journal (part of the Nature publishing group) by UNC-Chapel Hill marine scientists could have far-reaching implications for how marine diatoms (a major group of phytoplankton) may contribute to ocean carbon cycling.

The lead author is Adrian Marchetti, an assistant professor of marine sciences in UNC’s College of Arts and Sciences. Co-authors include undergraduate researcher Dylan Catlett and graduate student Kelsey Ellis from UNC, Brian M. Hopkinson from the University of Georgia and Nicolas Cassar from Duke University.

The Marchetti lab at UNC investigates how phytoplankton are affected by their environment and, in turn, influence ocean biogeochemistry and ecosystem dynamics. Particular interests include studying trace metals, such as iron, that are essential for the nutrition of phytoplankton and predicting the effects of future climate changes on phytoplankton distribution and abundance.

The diatom Pseudo-nitzschia granii isolated from the North Pacific Ocean that contains proteorhodopsin. (photo courtesy of Adrian Marchetti)

Diatoms stick to the shallows because they get their energy from the sun using photosynthesis. But some diatoms don’t have to rely on photosynthesis alone, like trees and other land plants have to do. Recently, some diatoms have been found to also possess a gene that produces a protein called proteorhodopsin. Previous research suggests this protein is used in some bacteria as an alternative way of getting energy from the sun, but this is the first time is has been identified in diatoms.

“However, little was known about whether diatoms actually use this protein in the same manner, or when they do so instead of using photosynthesis,” Marchetti said.

The new study demonstrates for the first time that a particular diatom from the open ocean significantly increases its protein use under low-iron conditions. Most other diatoms known to possess this protein live in chronically low-iron areas of the ocean — providing an explanation for how they are able to subsist and sometimes thrive in areas where other diatoms cannot.

Which diatoms are able to flourish in specific regions of the ocean has far-ranging impacts on the environment, from changing the amount of diatom “food” available to other organisms to altering carbon dioxide levels in our atmosphere.

Adrian Marchetti standing in front of a Rosette water sampling device while on a cruise in the North Pacific Ocean, an iron-limited region. (photo courtesy of Adrian Marchetti)

“Through climate change and ocean acidification, the chemistry of the oceans is changing, which may influence the availability of nutrients, including micronutrients such as iron,” Marchetti said. “Our findings suggest that energy synthesis by the protein proteorhodopsin could be more common if iron-limited regions expand in future oceans (as is predicted) — so this has implications for how phytoplankton are influencing the global carbon cycle.”

A portion of the research was funded by the National Science Foundation.

Read the paper online

Read a Natural History magazine article about graduate student Kelsey Ellis’ fascination with and research on diatoms.

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