CEB Committe on Evolutionary Biology

Seaweed State of the Union

Courtney Stepien's research explores how evolutionary history and functional diversity of seaweed communities affects community response to changing environmental conditions. Courtney is advised by Cathy Pfister.

Original article from UChiago's ScienceLife:

A University of Chicago researcher has discovered that the type of carbon that seaweeds harness can impact the structure of ecological communities within coastal zones, a finding with wide implications—seaweeds could act as a barometer to measure how healthy coastal ecosystems are affected by climate change, and in particular ocean acidification. The study is published in the Journal of Ecology on October 20th, 2015.

 “This establishes a baseline of what seaweed in coastal communities look like right now and hopefully that will be valuable as we move forward as we learn more about how acidification and climate change affect the oceans,” said study author Courtney Stepien, a graduate student in the Committee on Evolutionary Biology.

Aquatic plants are crucial components to maintaining coastal ecosystems, which are important habitats for wildlife, leisure spots for millions of people and even important economic centers (if you’ve ever enjoyed nibbling on sushi or relished ice cream on a steamy summer day, for example, then you’ve participated in the seaweed economy).

One of the biggest environmental changes affecting the health of seaweeds today is ocean acidification, a process similar to soda carbonation. Carbon dioxide (CO2) in the air gets absorbed by water, forming an acid (which is why carbonated sodas are so acidic). As levels of atmospheric CO2 increase, more CO2 is being absorbed by ocean waters than ever before.

Some seaweeds have become specialized to use other forms of carbon besides CO2—specifically, bicarbonate, an electrolyte that forms naturally as CO2 reacts with water. For seaweeds, the ability to utilize bicarbonate been hypothesized as an evolutionary advantage, but this could change as ocean acidification increases CO2 levels in the oceans. And some species may not be able to cope with those changes.

Stepien began exploring this question on intertidal zones—areas that are above water at low tides and underwater at high tides—in the Pacific Northwest, but soon realized that the patterns of carbon usage she identified were part of a global trend. An analysis of 76 previously published studies provided her with a global matrix of seaweed data. 

Scientists can measure whether aquatic plants use CO2 or bicarbonate for their carbon sources, as each chemicals’ carbon atoms are slightly different; bicarbonate is considered ‘heavy’ while CO2 is ‘light’.

Stepien discovered that seaweeds at the poles are made of lighter carbon building blocks; they use more CO2. Tropical seaweeds are made of heavier carbon building blocks, and they utilize bicarbonate. Not only does latitude seem to impact carbon usage, but also areas with very large temperature fluctuations have the ‘heavy’ signature of bicarbonate usage.

How the tropical seaweeds will react to changing carbon resources that result from ocean acidification could be an early indicator of a shift in the makeup of coastal ecosystems. Will they be able to convert to using the extra CO2 or will there be large-scale changes to the species that make up local communities? Stepien’s work will provide a much-needed state of the field for these important research questions to proceed in the future.

“We don’t know much about how aquatic plants are contributing to the global carbon cycle,” Stepien muses. “That’s going to be really important as climate change becomes more and more important of an issue to address.”


The study, “Impacts of geography, taxonomy and functional group on inorganic carbon use patterns in marine macrophytes,” was supported by the National Science Foundation (DGE1144082, DEB1311286, OCE0928232, DEB0919240) and the National Institutes of Health (T32 GM007197).