Finding A New Link In The Food Chain

Feb 7, 2019

The line between plant and animal is not a firm one when it comes to plankton, the tiny organisms at the base of the food chain in Alaska’s oceans. It is their complexity that may help explain why our seas support so many fish and marine mammals. KYUK’s Johanna Eurich takes a look at research on mixotrophes in the Gulf of Alaska.

There has always been a bit of a mystery about how the small quantity of plankton bloom in the Gulf of Alaska can support the abundant marine life found there. Susanne Storm from Western Washington University thinks that she may have discovered part of the answer: mixotrophes. Storm has found that the single-celled world of plankton is complicated. She points to the way some tiny zooplankton eat phytoplankton, the tiny single-celled plants at the bottom of marine food webs.

“If they are a mixotrophic species,” Stome says, “when they eat phytoplankton, they digest the cell except for the chloroplasts, and they sequester the chloroplasts in their own cytoplasm, and they can use those for photosynthesis.”

These one-celled marine animals can now use that chlorophyll like a plant to feed off the sun and the carbon in the ocean. It’s as if they have converted the inside of their bodies into a garden. Storm has also found the equivalent of a single-celled “venus flytrap” floating around in the Gulf. These are phytoplankton, which are plants, that eat bacteria and tiny micro-zooplankton for protein.

“Others have started to survey many, many of the photosynthetic dinoflagelates in the world and we know that most of them, and probably all of them, can eat.”

But the eating doesn’t stop there. She has even discovered mixotrophes eating other mixotrophes. So what does all this eating and keeping chloroplasts alive do to the amount of food available further up the food chain? The answer, according to Storm, is a lot. The evidence that she and colleagues have found is that there is chlorophyll in the marine system long after the spring plankton bloom is over, and most of it is in those tiny floating gardens inside the bodies of mixotrophic zooplankton.

“In summer,” she says. “it was almost half of it, and at maximum all of it.”

In the fall she found that mixotrophic species were more than three-quarters of the plankton species in parts of the Gulf. Mixotrophes are everywhere and in crucial parts of the food web.

“It affects everything,” she asserted, based on modeling work. “So I actually think this might be really, really important in the Northern Gulf of Alaska in terms of how we get so much higher trophic level production out of what is actually, honestly, a fairly modest primary production base.”

Storm was presenting new information at the Alaska Marine Science Symposium. Because these tiny mixotrophic microorganisms typically have passed through or been destroyed by plankton nets, many scientists did not know much about them.

“It’s a whole universe, and you would not see most of these things at all with a net.”

The plankton Storm studies came from water collected 10 meters down in the Gulf of Alaska. Using various microscopic techniques, she and colleagues studied their anatomy. Along the way, the team identified many more new plankton species.