Photo Credit: Tom Kleindinst/WHOI

Erin Bertrand, right, studies how marine phytoplankton get, use and compete for nutrients in the ocean. She worked with her advisor, WHOI marine biogeochemist Mak Saito, left, whose lab group has been working to advance techniques using proteomics to study critical proteins in the marine environment.

Scientists have revealed a key cog in the biochemical machinery that allows marine algae at the base of the oceanic food web to thrive. They have discovered a previously unknown protein in algae that grabs an essential but scarce nutrient out of seawater – vitamin B12.

Many algae, as well as land-dwelling animals, including humans, require B12, but they cannot make it and must either acquire it from the environment or eat food that contains B12. Only certain single-celled bacteria and archaea have the ability to synthesize B12, which is also known as cobalamin.

Photo Credit: Chris Dupont/J. Craig Venter Institute

An epifluorescence micrograph of a marine alga indicates that the cobalamin acquisition protein 1 (tagged with a light green fluorescent label) is associated with the outside of the cell wall, where it appears to facilitate bringing in vitamin B12 from the ocean into the cell. Red indicates cholorophyll.

Photo Credit: Erin Bertrand/Woods Hole Oceanographic Institution and J. Craig Venter Institute

Cobalamin acquistion protein 1 (CBA1) appears to operate by binding vitamin B12 in the ocean and bringing it into algal cells, where the vitamin is needed to create another enzyme essential for growth, methionine synthase (MetH).

Studying algal cultures and seawater samples from the Southern Ocean off Antarctica, a team of researchers from Woods Hole Oceanographic Institution (WHOI) and the J. Craig Venter Institute found a protein they described as “the B12 claw.” Stationed at the algae’s cell walls, the protein appears to operate by binding B12 in the ocean and helping to bring it into the cell. When B12 supplies are scarce, algae compensate by producing more of the protein, officially known as cobalamin acquisition protein 1, or CBA1.

The team reported their findings May 31 in Proceedings of the National Academy of Sciences. Erin Bertrand , a graduate student in the MIT/WHOI Joint Program in Oceanography , was the lead author on the study.

To discover CBA1, Bertrand and her advisor, WHOI biogeochemist Mak Saito , used an approach now common in biomedical research but only recently applied to marine science: proteomics, the study of the proteins organisms make to function in their environment and respond to changing conditions.

Among thousands of other proteins present in the algae, or phytoplankton , they identified the novel CBA1 protein when it increased in abundance when the algae were starved of vitamin B12. They then worked with colleagues at the Venter Institute to demonstrate CBA1’s function and its presence in the oceans.

Discovery of CBA1 illuminates a small but vital piece of the fundamental metabolic machinery that allows phytoplankton to grow. The free-floating marine algae are a critical source of food for shrimplike krill and other zooplankton. They also play a key role in Earth’s climate, drawing huge amounts of carbon dioxide, a greenhouse gas, from the air via photosynthesis, incorporating carbon into their bodies.

Some of the carbon ends up sinking to the ocean depths when phytoplankton die, effectively removing carbon from reentering the atmosphere for millennia.

The discovery also opens the door for industrial or therapeutic applications. Since CBA1 is essential for phytoplankton growth, it could provide clues to how to promote growth of algae used to manufacture biofuels. Learning to manipulate the B12 biochemical pathways of beneficial or detrimental microbes could eventually lead to antibiotic or antifungal medicines.

In addition to Saito, co-authors of the papers are Andrew Allen, Christopher Dupont, Trina Norden-Krichmar, Jing Bai and Ruben Valas of the Venter Institute. The research was funded by the National Science Foundation as part of the International Polar Year 2007-2008 . The Gordon and Betty Moore Foundation’s Marine Microbial Initiative program also provided funding.

NSF-funded research in this story: Mak Saito, Woods Hole Oceanographic Institution, Award No. 0732665 ; and Andrew Allen and Jonathan Badger, J. Craig Venter Institute, Award No. 0732822 .