Hungry Green Algae Will Eat Live Bacteria, New Study Shows

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Scientist Eunsoo Kim sits at a lab table, wearing a lab coat and looking into a microscope. Museum Curator Eunsoo Kim studies how green algae seeks out energy through cell-eating, or phagocytosis.

In 2013, Museum Curator Eunsoo Kim and colleagues were the first to provide definitive proof that green algae also eat bacteria, showing alga sought out energy from gobbling up other organisms in addition to converting light into food through photosynthesis.

Now, Kim and researchers from Columbia University and the University of Arizona have published new work suggesting that phagocytosis (cell-eating) is likely more widespread than previously thought.

“Traditionally, we think of green algae as being purely photosynthetic organisms, producing their food by soaking in sunlight,” said Kim, an associate curator in the Museum's Department of Invertebrate Zoology and one of the corresponding authors of the study, which is published today in The ISME Journal. “But 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. And we’ve also found out just how finicky they are as eaters.”

Following the 2013 study, Kim’s lab continued to look for other types of green algae that use both photosynthesis and phagocytosis to power themselves. This was a difficult task until the research team devised a new experimental approach focused on live bacteria labeled with a non-toxic fluorescent dye.

Pterosperma cristatum with 3 different flourescent views.
Light and epifluorescence microscopy images of Pterosperma cristatum. The prasinophyte cells were fed with fluorescently labeled bacteria. The characteristic shape of the cells with their flagella (black arrows) were captured with the differential interference contrast optic (a). Red shows autofluorescence from the chloroplasts (b). The green spots localized close to the base of the flagella (white arrow heads), observed with green fluorescence, indicate ingestion of fluorescently labeled prey (c). An overlay of green and red fluorescence images indicates the relative localization of the feeding compartment within the cells (d). Scale bar: 10 µm.
N. Bock & E. Kim

They then combined the bacteria with five strains of unicellular green algae called prasinophytes inside a flow cytometer, which helps scientists analyze cell properties in solution. The flow cytometer measured increasing levels of green fluorescence in the algal cells over time, suggesting that the algae were consuming the glowing bacteria. To confirm that ingestion was actually occurring, they peered inside of the algal cells with high-precision microscopy. 

The researchers uncovered two particular quirks about the finicky eaters: the algal strains they tested only ate live bacteria, and they ate more when the levels of other nutrients were low. These findings have large implications for the environmental study of green algae.

“Traditionally when people study bacterial ingestion by algae in the oceans for environmental samples, they use fluorescently labeled bacteria that have been killed in the labeling process,” said Museum postdoctoral researcher Sophie Charvet. “At least for the five algal strains we had in culture, they preferentially feed on the live bacteria and seem to be snubbing the killed bacteria. This means that the impact of algae on bacterial communities in their natural environment has possibly been underestimated drastically because of the methods used.”

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 are given 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.

“For decades, scientists have been able to send satellites up and get optical data to infer global distributions of phytoplankton via chlorophyll measurement,” said Nicholas Bock, a graduate student at Columbia University’s Lamont-Doherty Earth Observatory, who conducted the work at Columbia under Solange Duhamel, now at the University of Arizona, Tucson. “Through that, we’ve come to understand that phytoplankton are vitally important for carbon cycling. The assumption in all of this is that all that chlorophyll just represents photosynthesis. It doesn’t account for the mixotrophy piece because there’s no easy way to detect [via satellite] if they’re eating other cells. Our findings highlight that the story is actually more complex.”

The study also included the development of a gene-based prediction model by John Burns, a Museum research associate and researcher at the Bigelow Laboratory for Ocean Sciences. The predictions agreed with the experimental results and suggested that the behavior is even more widespread among the green algal tree of life.