Sailing and studying the bottom of the ocean food chain

GSO oceanographer studies microscopic organisms in world’s oceans

Elizabeth Rau

On left, Kerry Whittaker, a lecturer in marine ecology at Bates College
and faculty chair at Costal Studies for Girls, a semester school in
Freeport, Maine, and Tatiana Rynearson, an oceanographer at the
University of Rhode Island’s Graduate School of Oceanography.
Photo courtesy of Tatiana Rynearson

You can’t see them with the naked eye, but they’re all over the ocean: diatoms, single-celled organisms that drift on currents.

These microscopic creatures are key to the planet’s health. 

They sit at the base of the food chain, feeding everything from zooplankton to fish. 

Through photosynthesis diatoms also regulate the air people breathe, and they regulate the global climate.

Yet they remain a mystery to many scientists.

Tatiana Rynearson, an oceanographer at the University of Rhode Island’s Graduate School of Oceanography, and Kerry Whittaker, who received her doctorate from GSO in 2014, are contributing their knowledge about diatoms through a paper published by the Proceedings of the National Academy of Sciences.

Their study explores the diversity of diatoms in the ocean, and what oceanic conditions support the evolution of these organisms. Whittaker is a lecturer in marine ecology at Bates College and faculty chair at Costal Studies for Girls, a semester school in Freeport, Maine. Rynearson has been teaching and doing research at GSO for 12 years.

Diatoms are enormously diverse, with an estimated 200,000 species. The physical and ecological processes that support this diversity are widely debated by oceanographers, evolutionary biologists and microbial ecologists.

Whittaker and Rynearson, who was Whittaker’s adviser at GSO, explored the diversity of the diatom species, Thalassiosira rotula. To accomplish this, the team collected cells of this one species from around the world.

They reached out to researchers from 19 places across hemispheres and ocean basins, from the coast of Germany and South Africa to Tasmania and Puget Sound in Washington state. The researchers took seawater samples and sent them to GSO by Federal Express five times during the year.

In the lab, individual cells of Thalassiosira rotula were isolated from the samples and cultured. These samples were then “genetically fingerprinted’’ to examine if they were related—a technique often used by forensic scientists.

The team concluded that the species has a remarkable ability to travel throughout the ocean, influencing “high genetic connectivity among global populations,” say Rynearson and Whittaker.

On land, geographic distance limits the genetic relationship between things. For example, two trees located close together tend to be more genetically related than two trees separated by a great distance.

Surprisingly, the team’s data show that geographic distance might not limit relatedness or “genetic connectivity” of diatoms, possibly due to the potential of diatoms to disperse widely as they drift with the tides and currents.

Instead of geographic distance being the primary regulator of relatedness between diatom populations, the team found that environmental conditions play a much stronger role.

For example, diatom populations sampled from similar marine environments tended to be more closely related than populations from different environments.

So what is one of the environmental factors that was most important in explaining the distribution of diversity within the species? The answer is temperature, a factor that is changing rapidly in the ocean in response to climate change.

Understanding genetic diversity and how it is regulated in these important diatoms has implications for studies that try to predict how a changing environment will influence the productivity of diatoms, say Rynearson and Whittaker.

“The bottom line is that we now have a global-scale view of diversity in diatoms, and it’s fundamentally different than what we see on land,’’ says Rynearson. “The mechanism on land is distance but in the ocean—and for diatoms—it appears to be the environment. In particular, it’s the temperature, and temperature is changing rapidly in response to climate change.’’

“This work provides a global snapshot of diversity that reveals the importance of environmental conditions in driving the evolution of these ecologically influential organisms,’’ says Whittaker. “In the context of a rapidly changing ocean environment, this insight is important for predicting the impact of climate change on primary producers in the ocean, particularly diatoms.”