B12--an essential vitamin for land-dwelling animals, including humans--also turns out to be an essential ingredient for growing marine plants that are critical to the ocean food web and Earth's climate, scientists have found.
The presence or absence of B12 in the ocean plays a vital and previously overlooked role in determining where, how much, and what kinds of microscopic algae (called phytoplankton) will bloom in the sea, according to a study published in the May issue of the journal Limnology and Oceanography.
These photosynthesizing plants, in turn, have a critical impact on Earth's climate: They draw huge amounts of carbon dioxide, a greenhouse gas, from the air, incorporating carbon into their bodies. When they die or are eaten, carbon is transferred to the ocean depths, where it cannot re-enter the atmosphere.
B12 contains the metal cobalt and can be synthesized only by certain singled-celled bacteria and archaea. Humans, animals, and many algae require B12 to manufacture essential proteins, but they cannot make it and must either acquire it from the environment or eat food that contains B12, said the study's lead authors, Erin Bertrand and Mak Saito. The two biogeochemists at the Woods Hole Oceanographic Institution wondered whether the vitamin was also important in the ocean, where B12 and cobalt are both found in exceedingly low concentrations.
Bertrand, Saito, and colleagues collected water samples from three locales in the highly fertile Ross Sea off Antarctica during an expedition in 2005 aboard the icebreaker Nathaniel B. Palmer. To one set of samples, they added B12 and iron (another essential nutrient for plant growth); to a second set, they added just iron; and to a third, they added neither. Samples stimulated with both iron and B12showed significantly higher concentrations of plant life in general and greater concentrations of a particular type of marine algae called diatoms.
"The possibility that a vitamin could substantially influence phytoplankton growth and community composition in the marine environment is a novel and exciting finding," the study's authors wrote.
The finding underscores the complexities of the marine food web and raises questions about the delicately balanced ecosystem's vulnerabilities to changing climate. It also sheds light on the sources and cycling of vitamin B12 and cobalt in the ocean, especially in the Southern Ocean around Antarctica, where the only nearby continent--a standard source of metal particles blown into the sea--is largely ice-covered.
Nevertheless, polar regions harbor some of the most extensive phytoplankton blooms in the world and are believed to play a significant role in exporting carbon to the deep ocean. In the Ross Sea, spectacular spring blooms of marine algae called Phaeocystis antarctica dissipate by summer and are followed by blooms of diatoms.
The scientists' experiments--showing more diatom growth with the addition of B12--indicate that Phaeocystis may have a competitive advantage over diatoms in the Ross Sea's springtime. The sea contains bacteria and archaea that make B12, but their populations are low, particularly in the spring, and so B12 supplies are limited.
Phaeocystis effectively monopolize the B12 supply by forming colonies cemented by sticky mucous that attracts B12 -making bacteria, Bertrand and Saito theorize. In a symbiotic relationship, the algae get their required vitamin and the bacteria get a steady supply of carbon made by the plants. When Phaeocystis dies off and the bacteria are eaten or decomposed, B12 is released once again to the ocean and is available to be used by diatoms.
Any disruption in the timing or abundances of these microbial populations has ramifications on the ecosystem and the climate, the scientists said. For example, Phaeocystis antarctica in the Ross Sea takes up more carbon dioxide than diatoms, so if the marine community shifts to diatoms, the Ross Sea would likely remove less carbon dioxide from the atmosphere. Unlike diatoms, Phaeocystis also produce a compound called dimethylsulfioniopriopionate, or DMSP, which is released into the air and helps produce clouds that block solar radiation.
Polar oceans do not have large bacterial populations to produce B12, making the vitamin a critical factor influencing the food web, the cycling of carbon in the ocean, and the climate, Bertrand and Saito said. At the same time, climate changes could affect the availability of B12 by causing changes in ocean temperatures, bacterial populations, and other factors. The ozone hole produced in the austral spring above Antarctica could also induce a cascade of effects by allowing more penetration of ultraviolet radiation that is known to degrade B12, they said.
The National Science Foundation Office of Polar Program provided funding for the 2005 Controls on Ross Sea Algal Community Structure (CORSACS) expedition. Erin Bertrand was a Carl and Pacha Peterson Summer Research Fellow at WHOI and is now a graduate student in the MIT/WHOI Joint Program.
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