Highlights
- Lamont-Doherty and Woods Hole researchers have identified previously hard-to-detect small molecules released by phytoplankton that help power microbial life in the upper ocean.
- These compounds can account for up to 23% of the dissolved organic carbon released by phytoplankton, underscoring their important role in the ocean’s carbon cycle.
- Different phytoplankton species release distinct mixes of chemicals, helping shape which bacteria thrive in different parts of the ocean.
- Identifying these “chemical currencies” could improve models of how marine microbes move carbon through the ocean and respond to changes like warming and acidification.
This article was adapted from a press release by the Woods Hole Oceanographic Institution.
A new study, led by researchers at Columbia University and Woods Hole Oceanographic Institution (WHOI), identifies a diverse set of molecules released by marine phytoplankton that fuel microbial life and help drive Earth’s carbon cycle. While scientists know that carbon is moved through an invisible network of phytoplankton and other microbes in the surface ocean, the specific compounds have long been a mystery. These compounds are small, chemically difficult to detect in salty seawater, and are rapidly consumed by other organisms almost as soon as they are produced.
Phytoplankton, a type of microscopic organism, take in carbon dioxide and convert it into organic carbon through photosynthesis, like plants. Each year, this process moves many tens of billions of tons of carbon through the sunlit surface ocean and contributes to the oxygen in the air we breathe. These massive natural carbon flows highlight the central role the surface ocean plays in regulating Earth’s carbon cycle.
“For this study, we placed six phytoplankton species representing major groups of marine phytoplankton under controlled conditions. They had the nutrients and light they needed to grow,” said Yuting Zhu, co-lead author of the study and former WHOI postdoctoral investigator, now with Old Dominion University. “Using a chemical-tagging method developed at WHOI, we were able to quantify the composition of biologically available small molecules released by globally abundant microorganisms.”
These compounds accounted for up to 23% of the dissolved organic carbon that phytoplankton released and may support a substantial share of microbial metabolism in the global ocean.
However, many bacteria are metabolic specialists, or picky eaters. The study found that different phytoplankton species release distinct combinations of metabolites, including carbon compounds also containing nitrogen, phosphorus, and sulfur. Because bacteria vary in which molecules they can consume, the chemical “menu” produced by phytoplankton helps determine which microbial communities thrive in different parts of the ocean.
“The findings help illuminate a long-standing mystery about the composition of the ‘chemical currencies’ that are moved by microbes in the surface ocean,” said microbial oceanographer Sonya Dyhrman, a researcher at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and professor of Earth and environmental sciences. “I think of it as a microbial carbon economy. By identifying the currencies produced by phytoplankton, scientists can begin to build more realistic representations of how marine microbial communities cycle billions of tons of carbon.”
To explore the broader implications, the team, also including researchers from the Massachusetts Institute of Technology and Marine Biological Laboratory, combined laboratory measurements with global ecosystem modeling. Their results suggest that phytoplankton-derived metabolites could supply up to 5 percent of the daily carbon needs of SAR11, one of the most abundant groups of bacteria in the surface ocean.
“Combining the ecological and chemical approaches here allowed us to view the system through a new lens,” said co-lead author Hanna Anderson, a researcher at Lamont and PhD candidate in Earth and environmental sciences at Columbia. “Thinking synthetically about how these carbon substrates can mediate interactions between phytoplankton and heterotrophs, which in turn cycle this carbon within the marine food web.”
The research was conducted as part of the National Science Foundation-funded Center for Chemical Currencies of a Microbial Planet, a science and technology center that investigates how small molecules govern interactions among microorganisms across Earth’s ecosystems.
“Understanding these exchanges is critical because a huge portion of Earth’s carbon cycle passes through this microbial system, but we still don’t fully understand it,” said the center’s director and co-author of the study, WHOI senior scientist Elizabeth Kujawinski. “If we understand what molecules phytoplankton release and what molecules bacteria can take up, we can start building models of how these organisms interact. We think of the surface ocean as a network, where phytoplankton and bacteria are connected by molecules—some compounds feed many different bacteria, while others only support a few.”
Future studies will investigate how environmental conditions such as nutrient limitation, temperature changes, and ocean acidification alter the molecules that phytoplankton release and how microbial communities respond to those “chemical currencies.”
