Diatoms are single-celled algae that take many intricate shapes. The Pseudo-nitzschia multiseries used in the study are simple rods that carry out photosynthesis throughout the world’s oceans. Photo: California Academy of Sciences / Flickr

Diatoms are single-celled algae that take many intricate shapes. The Pseudo-nitzschia multiseries used in the study are simple rods that carry out photosynthesis throughout the world’s oceans. Photo: California Academy of Sciences / Flickr

University of Washington oceanographers have found that diatoms – the intricately patterned single-celled algae that exist throughout the world’s oceans – grow faster in the presence of bacteria that release a growth hormone known to benefit land plants. The study, published online May 27 in Nature, uses genetic and molecular tools to discover what controls marine ecosystems.

“These very small organisms are interacting with their environment, but they’re also interacting with other organisms,” said co-author Ginger Armbrust, a UW professor of oceanography. “In my mind, in order to understand how future ecosystems will work, we need to understand how these organisms that are the basis of the marine food web interact with one another.”

Armbrust’s research group has long studied diatoms, microalgae that carry out more of the planet’s photosynthesis than all the terrestrial rainforests combined. Lab members began this project by looking at which bacteria were found in all samples of Pseudo-nitzschia multiseries, a common coastal diatom collected from five places throughout the northern Pacific and Atlantic Oceans. Next they cured the water samples of all bacteria living in the seawater, and found that the diatoms did not reproduce as well.

Co-author Shady Amin, a former UW postdoctoral researcher now on the faculty at New York University Abu Dhabi, added the bacteria common to all five samples back one at a time. One type, Sulfitobacter, sped up the growth dramatically when added back at a high enough concentration.

The authors showed that these bacteria exchange material with the diatoms while in turn producing auxin, a well-known hormone made by microbes living around the roots of land plants. “The back-and-forth exchange of materials between these tiny creatures resembles an ongoing dialogue between two living organisms that culminates in the production of auxin,” Amin said. “It was so fascinating that we wondered if we could see this behavior elsewhere.”

So next, the researchers went to sea. Having shown what happens in the lab, they collected other ocean samples and found the same growth hormone. Then they used new genetic techniques to detect the activity of marine microbes that would never survive the transition to the lab. The same interaction was taking place, especially along coasts, but between different organisms that cannot be transferred to the lab.

She predicts that more such interactions will help to explain how ocean waters become or stay productive, or how the base of the marine food web might shift in a changing climate. “A lot of the high-powered tools that look at the function of individual cells were developed in the medical world,” Armbrust said. “Now that we can apply them to the ocean, we are starting to pull the curtains back on how this hidden world works.”

The research was funded by the National Science Foundation and the Gordon and Betty Moore Foundation.