Scientists Provide a New View of the Microbial Drama at Play in Southern Ocean Phytoplankton Blooms

A team of ocean researchers has uncovered a series of complicated relationships among marine microbes, and these relationships appear to have critical consequences for coastal Southern Ocean food webs.

Phytoplankton (single celled photosynthetic algae) and bacteria, microscopic organisms that live suspended in seawater and form the base of marine food webs, underpin the success of more iconic creatures such as penguins, seals, and orca whales. The Southern Ocean around Antarctica is home to massive phytoplankton populations, and scientists have long considered their growth to be controlled largely by the availability of iron and light. Now, thanks to a new study published in the Proceedings of the National Academy of Sciences (PNAS), the picture looks much more complicated, and a bit like a theatrical drama, packed with tension and intrigue, but starring microbes.

In order to examine these complex interactions in the field, scientists from the J. Craig Venter Institute (JCVI), Scripps Institution of Oceanography at UC San Diego, the University of Southern California (USC), and the Virginia Institute of Marine Sciences (VIMS) ventured to the U.S. Antarctic Program’s McMurdo Station, located on the southern tip of Ross Island.  Via helicopter, they conducted expeditions over the frozen sea to the ice edge where they carefully collected water samples from the sunlit surface and returned them to the Crary lab at McMurdo, in order to perform experiments. The researchers learned that although the water appeared teeming with a particular type of phytoplankton called diatoms, the diatoms displayed cellular signatures of malnourishment. Unlike most regions of the global ocean which do not contain sufficient nitrogen or phosphorous for sustained phytoplankton growth, diatoms in the remote waters of McMurdo Sound, adjacent to the Ross Sea, were starving from lack of iron and deficiency of vitamin B12, just like the vitamin and mineral supplements you might take with breakfast in the morning. 

Additionally, the researchers identified a range of different bacteria growing in the water.  What wasn’t apparen, however, was what role the various types of bacteria might play in driving or alleviating the starvation exhibited by the diatoms. Were they competing for the scarce resources and exacerbating the starvation conditions? Or were they somehow cooperating to effectively share these resources?  By looking at how these microbial groups changed the expression of their genes in response to shifts in micronutrient availability, the researchers could begin to untangle these questions.

Erin Bertrand, a former JCVI and Scripps researcher, now an assistant professor at Dalhousie University in Halifax, Nova Scotia, was lead author of the study.

“Just like humans, phytoplankton require vitamins, including vitamin B12, to survive,” Bertrand said.  “We’ve shown that the phytoplankton in McMurdo Sound acquire this precious resource from a very specific group of bacteria. Those bacteria, in turn, appear to depend directly on phytoplankton to supply them with food and energy.”

But here is where it gets messy. A different group of bacteria, also relying on the phytoplankton for food and energy, appear to compete with these photosynthesizers (i.e., diatoms) for the precious vitamin, and all three groups of microbes are competing for iron, which, due to the extreme remoteness of the Southern Ocean, is a scarce and consequently invaluable resource.  The result is a new picture of a precariously balanced system, full of microbial, soap opera-like, drama.  

The team further confirmed that a large portion of the B12 supply in the Southern Ocean appears to be produced by a particular group of gammaproteobacteria bacteria belonging to the Oceanospirllaceae. This aspect of the study was facilitated by collaboration with researchers at the University of Rhode Island (URI) and the Marine Biological Laboratory who have been conducting studies on bacteria in and around the Amundsen Sea, another region of the Southern Ocean.

 “Through a combination of field manipulation experiments and next generation sequencing, we’ve obtained a new view of the microbial interactions underpinning a highly productive ecosystem,” said senior author Andrew Allen, a joint associate professor at Scripps and JCVI.  “Although oceanographers have long recognized that iron fertilization in the Southern Ocean will drive phytoplankton blooms, it is now evident that particular groups of bacteria, perhaps specifically cultivated by the phytoplankton, are important for regulating the magnitude of the blooms as well as sustaining them through supply of the critically limiting micronutrient vitamin B12. I think this study also illustrates the eye-opening sensitivity of marine phytoplankton and bacteria to very minor additions of scarce micronutrients over very short, hourly time scales.”

“This study suggests that these marine ecosystems are naturally poised to respond swiftly to changes in availability of these nutrients,” Bertrand added. “It’s likely that these states of resource boom and bust are a common, perhaps critical, feature of this remote environment.”

With this new understanding of the nature of these interactions and the careful balance of competitive and cooperative behaviors that exist in this key ecosystem, researchers can work towards predicting how these relationships might change in the future, as temperatures begin to rise in the Southern Ocean.

This research is funded in part by the Gordon and Betty Moore Foundation through Grant GBMF3828 to Scripps, A.E. Allen, and the National Science Foundation, Antarctic Sciences Section Grant 1043671 to JCVI, A.E. Allen and 1103503 to E. M. Bertrand.

About J. Craig Venter Institute

The JCVI is a not-for-profit research institute in Rockville, MD and La Jolla, CA dedicated to the advancement of the science of genomics; the understanding of its implications for society; and communication of those results to the scientific community, the public, and policymakers. Founded by J. Craig Venter, Ph.D., the JCVI is home to approximately 200 scientists and staff with expertise in human and evolutionary biology, genetics, bioinformatics/informatics, information technology, high-throughput DNA sequencing, genomic and environmental policy research, and public education in science and science policy. The JCVI is a 501 (c)(3) organization. For additional information, please visit http://www.JCVI.org.

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