Award: OPP-1643684

Phytoplankton blooms in the coastal waters of the Ross Sea, Antarctica are typically dominated by either diatoms or Phaeocystis antarctica (a flagellated algae that often can form large colonies in a gelatinous matrix). This project examined the influence of the cobalt-containing vitamin B12, which can be an important co-limiting micronutrient in the Ross Sea, on ecosystem dynamics in this region. The supply of an essential vitamin coupled with the ability to grow at lower iron concentrations may put Phaeocystis at a competitive advantage over diatoms. Because Phaeocystis cells can fix more carbon than diatoms and Phaeocystis are not grazed as efficiently as diatoms, the project contributed to refining understanding of carbon dynamics in the region as well as the basis of the food web webs. Such understanding also has the potential to help refine predictive ecological models for the region. On the 2018 CICLOPS expedition to Amundsen Sea we observed primary vitamin B12 limitation of the phytoplankton community. This was induced by glacial iron input that shifted the ecosystem from primary iron limitation to this cobalt-containing vitamin. Further evidence supported this finding, including rapid B12 uptake in the upper photic zone, and high iron and other metals in this region, and rapid cobalt depletion in incubation studies. Laboratory studies of B12 limited Phaeocystis antarctica's proteome observed adaptive biochemical responses that allow it to thrive relative to other phytoplankton species. Together these results are consistent with mutualistic symbioses between bacteria and Phaeocystis antarctica that provide colonial cells a mechanism for alleviating chronic vitamin B12 co-limitation effects thereby conferring them with a competitive advantage over diatom communities. Last Modified: 01/16/2021 Submitted by: Mak A Saito