Award: OPP-1644073

Phytoplankton are microscopic algae that can form large blooms along some of Antarctica?s coastlines during the southern hemisphere?s spring and summer seasons. These blooms can play a significant role in lowering global atmospheric carbon dioxide levels due to their high rates of photosynthesis. Phytoplankton species composition is important as rates of carbon fixed can vary among different species. In the past couple of decades glaciologists have documented enhanced rates of large icebergs calving off the western Antarctic coastal region. In the Amundsen Sea region of western Antarctica, recent increases in the rate of glacial ice melt has resulted in enhanced fresh water and trace metal additions such as iron to the coastal environment. Iron is a nutrient that can stimulate large phytoplankton blooms in Antarctic coastal waters. Mounting evidence suggests that as glaciers move toward Antarctic coastal waters the scouring and erosion of the land under the ice sheet at the grounding line increases the levels of iron entering Antarctic coastal waters. Previously we published data suggesting that both iron and vitamin B12 may play a role in stimulating phytoplankton growth in the Ross Sea. Our present study investigated the potential for both iron additions from the glacial melt water and vitamin B12 (cobalamin) additions from coastal bacterial populations to stimulate phytoplankton growth and influence species composition in the Amundsen Sea. Recent research has demonstrated the importance of some Antarctic bacterial species to supply vitamin B12 to other bacteria and algal species that cannot synthesize the vitamin. It is becoming clear that bacterial-algal interactions are important in structuring the microbial community composition of Antarctic coastal waters. It is well known that phytoplankton species composition can be a major factor determining the role of the Southern Ocean on impacting nutrient cycles and global climate. We observed that the growth of diatom populations in the Amundsen Sea region was preferentially limited by both iron and vitamin B12 compared to the predominantly iron-limited growth of Phaeocystis antarctica, another important phytoplankton species in the Southern Ocean. Experiments performed during our expedition complemented and supported our laboratory studies indicating that iron additions can cause a shift from single cells of Phaeocystis antarctica to a different life stage that results in the formation of visible colonies containing hundreds of cells. These colonies have the potential to house bacteria that can provide the algal cells with vitamin B12 as well as sequester iron from seawater and store it inside the colonies. But additional research is required to test this hypothesis. During the expedition we collected hydrographic data along the western Antarctic coastline. This data was published in the journal Nature in a collaborative effort with Italian physical oceanography researchers. The salinity data from our expedition along with data from previous expeditions revealed that the freshening of Ross Sea waters that has been occurring since the 1990?s changed abruptly during the 2014-2018 time period. We believe that glacial meltwater has altered the physical circulation of ocean currents along the western Antarctic coast. This hypothesis will be tested by physical oceanographers on future expeditions. The project fostered international collaborations between Italian students, post-docs and researchers from the University of Napoli and Stazione Zoologica Dohrn with USA students and researchers. Specifically, Italian post-doctoral associate (Pasquale Castagno) and Ph.D. student (Francesco Bolinesi) participated on the expedition (at no cost to NSF) and interacted with graduate students and technicians from the College of Charleston, Stanford University, Rutgers University, Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution. Last Modified: 12/30/2020 Submitted by: Giacomo R Ditullio