Flexible Feeding May Be the Norm, Not the Exception

Anyone who has taken a basic ecology class has probably learned that there are two kinds of organisms: those that can photosynthesize, using the sun’s light and inorganic nutrients like carbon dioxide to grow and those that can’t produce their own food and rely on organic nutrients from the environment to survive. Increasingly, though, ecologists are learning how false that dichotomy is.

“The dogma was always that there’s autotrophs and heterotrophs,” said Nicole Poulton, a Senior Research Scientist at Bigelow Laboratory for Ocean Sciences. “In reality, there’s an entire spectrum between the two.”

Mixotrophs are the organisms that fall on that spectrum. They can switch between photosynthesis and other mechanisms for getting the energy and nutrients they need. Within the broad category of mixotrophs, of course, there are competing definitions, essential nuances, and different categories. For example, there are phagotrophs that can literally engulf their food and osmotrophs that absorb dissolved organic nutrients from the air or water. There are those that specialize in photosynthesis but consume prey when needed, and vice versa.

Interest in mixotrophy has grown in recent years. Research Scientist Karen Stamieszkin said that that’s because of new work suggesting both how common it is and how the behavior may make ecosystems more resilient by giving phytoplankton, the base of the marine food web, more diverse means of survival.

“It’s like an insurance policy,” said Stamieszkin. “It creates a buffer for them to be able to wait until there’s more optimal conditions.”

Stamieszkin and Poulton are among the scientists at Bigelow Laboratory that turned their attention to mixotrophy, illuminating just how widespread and important the phenomenon is in the ocean.

There are researchers looking at the molecular basis of mixotrophy, trying to identify the specific genes that give an organism the ability to change its feeding mechanism. Others, like Stamieszkin, have tried to understand the ecosystem implications of mixotrophy, using decades-long time series of plankton to understand how the distribution of mixotrophic organisms is changing seasonally and annually.

Last month, a team of researchers, including Poulton and Senior Research Scientist Barney Balch, published a study in Science Advances on the impact of coccolithophores’ mixotrophy on the global carbon cycle. Their work showed that this widespread type of phytoplankton can survive in low-light conditions by directly absorbing dissolved organic carbon in the water — complicating our understanding of how carbon is pumped from the surface to the seafloor.

This month, Bigelow Laboratory is hosting the third and final workshop of a mixotrophy working group funded through the Ocean Carbon and Biogeochemistry Program, a collaborative effort of NASA and the National Science Foundation.

“The goal of the working group was to bring together an interdisciplinary team of people interested in mixotrophy to identify key priority areas and methods to help the field take the next steps,” said Stamieszkin, one of the working groups leaders.

The collaboration recently published a paper in the Journal of Plankton Research that discusses priority research areas, and Stamieszkin said they’ll publish another in the coming months on what tools are needed to study these topics.

“Improvements in methodology and research are driving recent advances in our understanding,” Poulton said, highlighting flow cytometry and stable isotopes as two examples. “As we see advancements in the tools, it’s allowing us to look at these questions in more detail and better understand the importance of these organisms.”

With those advances, scientists can begin to better understand not just where mixotrophic organisms are, but what they’re actually doing. For example, Poulton and Stamieszkin have recently kicked off a new project that should provide insight into how mixotrophs acquire the nutrients they need.

Working with Nicole Millette, an assistant professor at the Virginia Institute of Marine Sciences, the team is looking at eight species of plankton that are known to rely primarily on photosynthesis but have the ability to eat prey like bacteria when needed. The researchers are going to identify what factors — such as the availability of nutrients and light — limit how fast each species can grow. Then, they’ll crank that factor way down, to see how each organism changes its feeding habits in response and how much carbon each is getting from photosynthesis versus predation.

The researchers are also hoping to illuminate how the size of each organism influences those changes. If there is a relationship between the two, then size, which is relatively easy to measure, could be used as a proxy for mixotrophy in models of ocean activity.

Preliminary work on the project started last fall. Mason Trottier, a rising senior at Maine Maritime Academy, kicked off the experiments while working with Poulton through Bigelow Laboratory’s Sea Change Semester. Trottier tested how a single species of dinoflagellate changed its feeding habits in response to limited light and nutrient availability.

As expected, the organism began consuming more prey directly as they lowered the lights. They didn’t see much of a response to reduced nutrient availability, but Poulton said that those results have helped them refine how they’ll manipulate nutrient levels in the next round of experiments.

Trottier is continuing to help with the current project by working with Millette this summer, and he’s even planning to do his senior capstone project on mixotrophy. Just a year ago, though, Trottier said he had never even heard of mixotrophy, reflecting how poorly understood the topic remains.

“We’re learning that mixotrophy in general is the norm in the ocean, not the exception,” Stamieszkin said. “It’s really important to understand that because of its huge implications for how we think of the ocean ecosystem functioning.”

Photo 1: A scanning electron microscope image of Calciopappus rigidus, a type of coccolithophore that has developed adaptations to enable mixotrophy. Colin Fischer

Photo 2: A microscope image of Heterocapsa triquetra, a mixotrophic species of dinoflagellate examined by semester student Mason Trottier in the lab of Senior Research Scientist Nicole Poulton. Mason Trottier