By Kevin Krajick
Green algae are typically regarded as purely photosynthetic organisms, making a living by soaking in sunlight and carbon dioxide. But new research suggests that they also may eat bacteria on a scale previously unsuspected. The work, led by scientists at the American Museum of Natural History and Columbia University’s Lamont-Doherty Earth Observatory, found that at least five strains of single-celled green algae will consume live bacteria when they are hungry. The study was published this week in The ISME Journal.
“We’ve come to understand that there are potentially a number of species of green algae that also can eat bacteria when the conditions are right,” said Eunsoo Kim, an associate curator at the American Museum of Natural History and one of the study’s corresponding authors. In 2013, Kim and her colleagues were the first to provide definitive proof that green algae sometimes eat bacteria. They were able to show this by studying an alga from the genus Cymbomonas. But while some in the field viewed this behavior as a rare exception, Kim’s lab continued to explore whether other types of green algae use both photosynthesis and phagocytosis (cell-eating) to power themselves. This was difficult to confirm until the research team came up with a new experimental approach led by Nicholas Bock, a graduate student at Lamont-Doherty, and Museum postdoctoral researcher Sophie Charvet.
The researchers conducted feeding experiments with live bacteria that they labeled with a nontoxic green fluorescent dye. They combined the bacteria with five different strains of unicellular green algae called prasinophytes. They then analyzed the mixture by running it through a flow cytometer, which helps scientists analyze cell properties in solution. The cytometer measured increasing levels of green fluorescence in the algal cells over time, suggesting that the algae were consuming the glowing bacteria. To confirm this, the researchers used high-precision microscopy to pinpoint the origin of the fluorescence to the interior of the algal cells.
In the process, the team discovered that the algae were finicky eaters. They ate only live bacteria, rejecting dead specimens; and, they preferred to eat bacteria mainly when levels of other nutrients were low.
Green algae are found around the world, and help form the foundation of the aquatic food web. Along with other photosynthetic organisms like cyanobacteria, diatoms and dinoflagellates, which all fall under the umbrella term phytoplankton, green algae function as a sort of biological carbon pump, consuming carbon dioxide on a scale equivalent to trees and other land plants in terrestrial ecosystems. Because of their vast presence in the oceans and their interactions with the atmosphere, the new findings have wide implications for both biological oceanography and climate.
“For decades, scientists have been able to send satellites up and get optical data to infer global distributions of phytoplankton via chlorophyll measurement,” said Bock, the study’s lead author, who conducted the work at Lamont-Doherty under biological oceanographer Solange Duhamel. (Duhamel is now at the University of Arizona, Tucson.) “Through that, we’ve come to understand that phytoplankton are vitally important for carbon cycling,” he said. But he said, “the assumption in all of this is that all that chlorophyll just represents photosynthesis. There’s no easy way to detect if they’re eating other cells. Our findings highlight that the story is actually more complex.”
In parallel to the experiments led by Bock and Charvet, a gene-based prediction model formulated by John Burns from the American Museum of Natural History and the Bigelow Laboratory for Ocean Sciences agreed with the experimental results, and suggested that phagocytosis may be quite widespread among the green algal tree of life.
Other authors of this work include Yangtsho Gyaltshen from the American Museum of Natural History and Andrey Rozenberg from the Israel Institute of Technology.
Adapted from a press release by the American Museum of Natural History.