Scientists Detect Shift to Ocean Microbes that are “Flexible Feeders” in the North Atlantic

Ecologists long thought that there were two options for how organisms could get the energy they need to survive: through photosynthesis, using light and inorganic nutrients like carbon dioxide, or through absorbing organic nutrients and consuming “prey” directly. But, in recent years, scientists have learned that much of the microscopic life in the ocean is actually mixotrophic, falling on a spectrum between those two ends with the ability to shift feeding habits as needed.

A team of researchers led by Bigelow Laboratory examined almost 60 years of data on the community of microscopic plankton in the North Atlantic. In a new study recently published in Frontiers of Marine Science, they provide the first basin-level view of how the distribution of mixotrophs varies spatially and seasonally, showing a distinct increase in their proportion across the region in recent decades. Appreciating these patterns will help scientists understand the impact of mixotrophy on nutrient cycling and predict how marine life will respond to changing conditions in the future.

“Our results showed that mixotrophs are more prevalent when it’s warmer or when nutrients are less abundant — things that are expected to happen more often and for longer parts of the year with climate change,” said Karen Stamieszkin, a research scientist at Bigelow Laboratory and the study’s lead author. “Because they’re flexible in terms of how they get food, mixotrophs seem to possess resilience to changing conditions.”

Scientists have hypothesized that mixotrophs, compared to organisms that only use a single strategy to feed, will dominate in an ecosystem when a necessary resource, like light or nutrients, is scarce. Experiments have confirmed this competitive advantage, but there’s been limited research on what environmental conditions favor mixotrophs on a global scale.

To fill this gap, the researchers turned to the Continuous Plankton Recorder — a long-running survey of plankton populations across the entire northern half of the North Atlantic. Since 1958, CPR scientists have identified and counted organisms sampled from across the region in a consistent way, giving scientists over six decades of comparable data. The study’s authors combined the plankton survey with oceanographic information to track how the proportion of organisms capable of being mixotrophic varied across different areas, seasons, conditions, and years.

“The CPR data has been collected the same way for so long that it’s incredibly useful for tracking changes over time within the dataset,” Stamieszkin said. “It makes research like this, where we’re looking for the impacts of climate change on plankton, possible in the North Atlantic.”

The study confirmed that, as ecologists hypothesized, the proportion of mixotrophs was generally higher when temperatures were warmer or nutrient levels or light was lower. But, as temperatures have risen, the proportion of mixotrophs throughout the year has increased, especially far from shore where there’s less mixing in the water column and fewer nutrient inputs from land. In the early 1990s, there was a sharp, statistically significant increase, and the proportion of mixotrophs has remained unusually high since then. Stamieszkin described the pattern as “a potential signature of climate change.”

By identifying these kinds of large-scale patterns, the researchers can better prioritize future directions for research in this relatively new area. Stamieszkin and her collaborators are now running experiments to help scientists understand the triggers for mixotroph feeding and how flexible these organisms are. That could help inform a method for detecting mixotrophic activity directly in the field rather than focusing, as they did in this study, on organisms simply capable of mixotrophy.

“With our experiments, we are trying to figure out what these organisms are actually doing at any given moment,” Stamieszkin said. “If we can understand what drives a cell to feed or photosynthesize at particular times, we can start to understand how different species, and the food web as a whole, are going to respond to changes in the future.”

Photo 1: A scanning electron microscope image of Michaelsaria elgans, a type of microscopic plankton that can be found in the study area and is known by scientists to be capable of mixotrophy. (credit: Colin Fischer)

Photo 2: The Continuous Plankton Recorder (credit: Marine Biological Association)