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Images: Psychotherapy for Plankton

For her Ph.D. research in the MIT/WHOI Joint Program in Oceanography, Erin Bertrand has studied how marine phytoplankton get, use, and compete for nutrients in the ocean. She used a new approach for marine geochemistry: proteomics. (Photo by Mak Saito, Woods Hole Oceanographic Institution)
Diatoms like this one, Amphiprora sp., are one of several major types of marine phytoplankton. These microorganisms that live near the ocean surface and convert carbon dioxide into organic carbon via photosynthesis. They produce much of the oxygen we breathe and are the base of the marine food chain. They also play an important role in drawing heat-trapping carbon dioxide from the atmosphere into the ocean. (Photo by Dawn Moran, Woods Hole Oceanographic Institution)
In 2009, graduate student Erin Bertrand joined colleagues on an expedition to Antarctica to prospect for unknown marine phytoplankton in the remote Southern Ocean. (Photo by Mak Saito, Woods Hole Oceanographic Institution)
Bertrand's Ph.D. advisor is WHOI marine biogeochemist Mak Saito. His lab group has been working to advance techniques using proteomics to study critical proteins in the marine environment. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)

Bacteria and unicellular marine plants called diatoms depend on each other for some essential nutrients, but they also compete for other nutrients. So life gets complicated in the chemical soup of the ocean.

Diatoms convert carbon dioxide into organic carbon via photosynthesis.

Bacteria need organic carbon, but many cannot make it. So they rely on diatoms, which release organic carbon into the ocean when they grow, are lysed (broken apart), or are eaten by zooplankton.

Diatoms require vitamin B12, but they cannot make it. Bacteria do make it, so diatoms rely on bacteria, which release B12 into the ocean when they grow, are lysed, or are eaten.

Diatoms and bacteria both need iron, which is a scarce commodity in the ocean. So they compete for it. If bacteria use up the iron, diatoms won’t have enough to make organic carbon; if diatoms use up the iron, bacteria won’t have it to make vitamin B12.

(Illustration by Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution)
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